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• ASP.NET exception gives irrelevant stack trace on YSOD, very challenging!

  - by pootow

  Here is the YSOD: Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.Data.SqlClient.SqlException: Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding. Source Error: An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below. Stack Trace: [SqlException (0x80131904): Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding.] System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject) +428 System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection) +65 System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory) +117 System.Data.SqlClient.SqlConnection.Open() +122 ECommerce.PMethod.Sql.SqlConns.Open() +78 ECommerce.PMethod.Sql.SqlConns..ctor() +120 ECommerce.login.DatasInfo.Proc.UserCenter.IsLogin(String UserGUID, Int32 UserID) +49 ECommerce.login.Rules.Users.UserLogin.isLogin() +44 Config.isUserLogined() +5 Shopping_Shopping.Page_Load(Object sender, EventArgs e) +10 System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +14 System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +35 System.Web.UI.Control.OnLoad(EventArgs e) +99 System.Web.UI.Control.LoadRecursive() +50 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +627 [TypeInitializationException: The type initializer for 'ECommerce.ERP.DAL.DBConn' threw an exception.] ECommerce.ERP.DAL.DBConn.get_ConnString() +0 [ObjectDefinitionStoreException: Factory method 'System.String get_ConnString()' threw an Exception.] Spring.Objects.Factory.Support.SimpleInstantiationStrategy.Instantiate(RootObjectDefinition definition, String name, IObjectFactory factory, MethodInfo factoryMethod, Object[] arguments) +257 Spring.Objects.Factory.Support.ConstructorResolver.InstantiateUsingFactoryMethod(String name, RootObjectDefinition definition, Object[] arguments) +624 Spring.Objects.Factory.Support.AbstractAutowireCapableObjectFactory.InstantiateUsingFactoryMethod(String name, RootObjectDefinition definition, Object[] arguments) +60 Spring.Objects.Factory.Support.AbstractAutowireCapableObjectFactory.CreateObjectInstance(String objectName, RootObjectDefinition objectDefinition, Object[] arguments) +56 Spring.Objects.Factory.Support.AbstractAutowireCapableObjectFactory.InstantiateObject(String name, RootObjectDefinition definition, Object[] arguments, Boolean allowEagerCaching, Boolean suppressConfigure) +436 [ObjectCreationException: Error thrown by a dependency of object 'styleService' defined in 'assembly [ECommerce.Services.Impl, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null], resource [ECommerce.Services.Impl.AppContext.xml] line 56' : Initialization of object failed : Factory method 'System.String get_ConnString()' threw an Exception. while resolving 'constructor argument with name promotionservice' to 'promotionService' defined in 'assembly [ECommerce.Services.Impl, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null], resource [ECommerce.Services.Impl.AppContext.xml] line 31' while resolving 'constructor argument with name domainservice' to 'promotionDomainService' defined in 'assembly [ECommerce.Domain, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null], resource [ECommerce.Domain.AppContext.xml] line 20' while resolving 'constructor argument with name promotionrepos' to 'promotionRepos' defined in 'assembly [ECommerce.Data.AdoNet, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null], resource [ECommerce.Data.AdoNet.AppContext.xml] line 34' while resolving 'constructor argument with name connstr' to 'ECommerce.ERP.DAL.DBConn#389F399' defined in 'assembly [ECommerce.Data.AdoNet, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null], resource [ECommerce.Data.AdoNet.AppContext.xml] line 34'] Spring.Objects.Factory.Support.ObjectDefinitionValueResolver.ResolveReference(IObjectDefinition definition, String name, String argumentName, RuntimeObjectReference reference) +394 Spring.Objects.Factory.Support.ObjectDefinitionValueResolver.ResolvePropertyValue(String name, IObjectDefinition definition, String argumentName, Object argumentValue) +312 Spring.Objects.Factory.Support.ObjectDefinitionValueResolver.ResolveValueIfNecessary(String name, IObjectDefinition definition, String argumentName, Object argumentValue) +17 Spring.Objects.Factory.Support.ConstructorResolver.ResolveConstructorArguments(String objectName, RootObjectDefinition definition, ObjectWrapper wrapper, ConstructorArgumentValues cargs, ConstructorArgumentValues resolvedValues) +993 Spring.Objects.Factory.Support.ConstructorResolver.AutowireConstructor(String objectName, RootObjectDefinition rod, ConstructorInfo[] chosenCtors, Object[] explicitArgs) +171 Spring.Objects.Factory.Support.AbstractAutowireCapableObjectFactory.AutowireConstructor(String name, RootObjectDefinition definition, ConstructorInfo[] ctors, Object[] explicitArgs) +65 Spring.Objects.Factory.Support.AbstractAutowireCapableObjectFactory.CreateObjectInstance(String objectName, RootObjectDefinition objectDefinition, Object[] arguments) +161 Spring.Objects.Factory.Support.AbstractAutowireCapableObjectFactory.InstantiateObject(String name, RootObjectDefinition definition, Object[] arguments, Boolean allowEagerCaching, Boolean suppressConfigure) +636 Spring.Objects.Factory.Support.AbstractObjectFactory.CreateAndCacheSingletonInstance(String objectName, RootObjectDefinition objectDefinition, Object[] arguments) +174 Spring.Objects.Factory.Support.WebObjectFactory.CreateAndCacheSingletonInstance(String objectName, RootObjectDefinition objectDefinition, Object[] arguments) +150 Spring.Objects.Factory.Support.AbstractObjectFactory.GetObjectInternal(String name, Type requiredType, Object[] arguments, Boolean suppressConfigure) +990 Spring.Objects.Factory.Support.AbstractObjectFactory.GetObject(String name) +10 Spring.Context.Support.AbstractApplicationContext.GetObject(String name) +20 ECommerce.Common.ServiceLocator.GetService() +334 ECommerce.Mvc.Controllers.StylesController..ctor() +72 [TargetInvocationException: Exception has been thrown by the target of an invocation.] System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean noCheck, Boolean& canBeCached, RuntimeMethodHandle& ctor, Boolean& bNeedSecurityCheck) +0 System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean fillCache) +86 System.RuntimeType.CreateInstanceImpl(Boolean publicOnly, Boolean skipVisibilityChecks, Boolean fillCache) +230 System.Activator.CreateInstance(Type type, Boolean nonPublic) +67 System.Web.Mvc.DefaultControllerFactory.GetControllerInstance(RequestContext requestContext, Type controllerType) +80 [InvalidOperationException: An error occurred when trying to create a controller of type 'ECommerce.Mvc.Controllers.StylesController'. Make sure that the controller has a parameterless public constructor.] System.Web.Mvc.DefaultControllerFactory.GetControllerInstance(RequestContext requestContext, Type controllerType) +190 System.Web.Mvc.DefaultControllerFactory.CreateController(RequestContext requestContext, String controllerName) +68 System.Web.Mvc.MvcHandler.ProcessRequestInit(HttpContextBase httpContext, IController& controller, IControllerFactory& factory) +118 System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContextBase httpContext, AsyncCallback callback, Object state) +46 System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContext httpContext, AsyncCallback callback, Object state) +63 System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData) +13 System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +8677954 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155 Version Information: Microsoft .NET Framework Version:2.0.50727.3082; ASP.NET Version:2.0.50727.3082 Question is: the first stack trace is irrelevant to others, what happened? Any ideas? Let me make this more clear: a MVC page uses the spring part trying to load a lazy-init service which constructor wants a connection string through a static property like this: <object id="promotionRepos" type="ECommerce.Data.AdoNet.Promotions.PromotionRepos, ECommerce.Data.AdoNet" lazy-init="true"> <constructor-arg name="provider"> <null /> </constructor-arg> <constructor-arg name="connStr"> <object type="ECommerce.ERP.DAL.DBConn, ECommerce.ERP.DAL" factory-method="get_ConnString" /> </constructor-arg> <property name="RefreshInterval" value="00:00:10" /> </object> the timeout part is some what irrelevent to all others. see this in the first exception: Shopping_Shopping.Page_Load(Object sender, EventArgs e) +10 it's another page at all. And also, ECommerce.PMethod.Sql.SqlConns.Open() uses its own connection string, not the one loaded by spring, it's different module from diffrent team. And I am sure the connection string is correct. And, this ysod cames up randomly. Sometimes nothing is wrong, and sometimes, it appears. I thought there could be something wrong with my database or the network/firewall, I will check it later, but now I want understand this tricky stack trace.

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• Save object states in .data or attr - Performance vs CSS?

  - by Neysor

  In response to my answer yesterday about rotating an Image, Jamund told me to use .data() instead of .attr() First I thought that he is right, but then I thought about a bigger context... Is it always better to use .data() instead of .attr()? I looked in some other posts like what-is-better-data-or-attr or jquery-data-vs-attrdata The answers were not satisfactory for me... So I moved on and edited the example by adding CSS. I thought it might be useful to make a different Style on each image if it rotates. My style was the following: .rp[data-rotate="0"] { border:10px solid #FF0000; } .rp[data-rotate="90"] { border:10px solid #00FF00; } .rp[data-rotate="180"] { border:10px solid #0000FF; } .rp[data-rotate="270"] { border:10px solid #00FF00; } Because design and coding are often separated, it could be a nice feature to handle this in CSS instead of adding this functionality into JavaScript. Also in my case the data-rotate is like a special state which the image currently has. So in my opinion it make sense to represent it within the DOM. I also thought this could be a case where it is much better to save with .attr() then with .data(). Never mentioned before in one of the posts I read. But then i thought about performance. Which function is faster? I built my own test following: <!DOCTYPE HTML> <html> <head> <title>test</title> <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script> <script type="text/javascript"> function runfirst(dobj,dname){ console.log("runfirst "+dname); console.time(dname+"-attr"); for(i=0;i<10000;i++){ dobj.attr("data-test","a"+i); } console.timeEnd(dname+"-attr"); console.time(dname+"-data"); for(i=0;i<10000;i++){ dobj.data("data-test","a"+i); } console.timeEnd(dname+"-data"); } function runlast(dobj,dname){ console.log("runlast "+dname); console.time(dname+"-data"); for(i=0;i<10000;i++){ dobj.data("data-test","a"+i); } console.timeEnd(dname+"-data"); console.time(dname+"-attr"); for(i=0;i<10000;i++){ dobj.attr("data-test","a"+i); } console.timeEnd(dname+"-attr"); } $().ready(function() { runfirst($("#rp4"),"#rp4"); runfirst($("#rp3"),"#rp3"); runlast($("#rp2"),"#rp2"); runlast($("#rp1"),"#rp1"); }); </script> </head> <body> <div id="rp1">Testdiv 1</div> <div id="rp2" data-test="1">Testdiv 2</div> <div id="rp3">Testdiv 3</div> <div id="rp4" data-test="1">Testdiv 4</div> </body> </html> It should also show if there is a difference with a predefined data-test or not. One result was this: runfirst #rp4 #rp4-attr: 515ms #rp4-data: 268ms runfirst #rp3 #rp3-attr: 505ms #rp3-data: 264ms runlast #rp2 #rp2-data: 260ms #rp2-attr: 521ms runlast #rp1 #rp1-data: 284ms #rp1-attr: 525ms So the .attr() function did always need more time than the .data() function. This is an argument for .data() I thought. Because performance is always an argument! Then I wanted to post my results here with some questions, and in the act of writing I compared with the questions Stack Overflow showed me (similar titles) And true enough, there was one interesting post about performance I read it and run their example. And now I am confused! This test showed that .data() is slower then .attr() !?!! Why is that so? First I thought it is because of a different jQuery library so I edited it and saved the new one. But the result wasn't changing... So now my questions to you: Why are there some differences in the performance in these two examples? Would you prefer to use data- HTML5 attributes instead of data, if it represents a state? Although it wouldn't be needed at the time of coding? Why - Why not? Now depending on the performance: Would performance be an argument for you using .attr() instead of data, if it shows that .attr() is better? Although data is meant to be used for .data()? UPDATE 1: I did see that without overhead .data() is much faster. Misinterpreted the data :) But I'm more interested in my second question. :) Would you prefer to use data- HTML5 attributes instead of data, if it represents a state? Although it wouldn't be needed at the time of coding? Why - Why not? Are there some other reasons you can think of, to use .attr() and not .data()? e.g. interoperability? because .data() is jquery style and HTML Attributes can be read by all... UPDATE 2: As we see from T.J Crowder's speed test in his answer attr is much faster then data! which is again confusing me :) But please! Performance is an argument, but not the highest! So give answers to my other questions please too!

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• A ToDynamic() Extension Method For Fluent Reflection

  - by Dixin

  Recently I needed to demonstrate some code with reflection, but I felt it inconvenient and tedious. To simplify the reflection coding, I created a ToDynamic() extension method. The source code can be downloaded from here. Problem One example for complex reflection is in LINQ to SQL. The DataContext class has a property Privider, and this Provider has an Execute() method, which executes the query expression and returns the result. Assume this Execute() needs to be invoked to query SQL Server database, then the following code will be expected: using (NorthwindDataContext database = new NorthwindDataContext()) { // Constructs the query. IQueryable<Product> query = database.Products.Where(product => product.ProductID > 0) .OrderBy(product => product.ProductName) .Take(2); // Executes the query. Here reflection is required, // because Provider, Execute(), and ReturnValue are not public members. IEnumerable<Product> results = database.Provider.Execute(query.Expression).ReturnValue; // Processes the results. foreach (Product product in results) { Console.WriteLine("{0}, {1}", product.ProductID, product.ProductName); } } Of course, this code cannot compile. And, no one wants to write code like this. Again, this is just an example of complex reflection. using (NorthwindDataContext database = new NorthwindDataContext()) { // Constructs the query. IQueryable<Product> query = database.Products.Where(product => product.ProductID > 0) .OrderBy(product => product.ProductName) .Take(2); // database.Provider PropertyInfo providerProperty = database.GetType().GetProperty( "Provider", BindingFlags.NonPublic | BindingFlags.GetProperty | BindingFlags.Instance); object provider = providerProperty.GetValue(database, null); // database.Provider.Execute(query.Expression) // Here GetMethod() cannot be directly used, // because Execute() is a explicitly implemented interface method. Assembly assembly = Assembly.Load("System.Data.Linq"); Type providerType = assembly.GetTypes().SingleOrDefault( type => type.FullName == "System.Data.Linq.Provider.IProvider"); InterfaceMapping mapping = provider.GetType().GetInterfaceMap(providerType); MethodInfo executeMethod = mapping.InterfaceMethods.Single(method => method.Name == "Execute"); IExecuteResult executeResult = executeMethod.Invoke(provider, new object[] { query.Expression }) as IExecuteResult; // database.Provider.Execute(query.Expression).ReturnValue IEnumerable<Product> results = executeResult.ReturnValue as IEnumerable<Product>; // Processes the results. foreach (Product product in results) { Console.WriteLine("{0}, {1}", product.ProductID, product.ProductName); } } This may be not straight forward enough. So here a solution will implement fluent reflection with a ToDynamic() extension method: IEnumerable<Product> results = database.ToDynamic() // Starts fluent reflection. .Provider.Execute(query.Expression).ReturnValue; C# 4.0 dynamic In this kind of scenarios, it is easy to have dynamic in mind, which enables developer to write whatever code after a dot: using (NorthwindDataContext database = new NorthwindDataContext()) { // Constructs the query. IQueryable<Product> query = database.Products.Where(product => product.ProductID > 0) .OrderBy(product => product.ProductName) .Take(2); // database.Provider dynamic dynamicDatabase = database; dynamic results = dynamicDatabase.Provider.Execute(query).ReturnValue; } This throws a RuntimeBinderException at runtime: 'System.Data.Linq.DataContext.Provider' is inaccessible due to its protection level. Here dynamic is able find the specified member. So the next thing is just writing some custom code to access the found member. .NET 4.0 DynamicObject, and DynamicWrapper<T> Where to put the custom code for dynamic? The answer is DynamicObject’s derived class. I first heard of DynamicObject from Anders Hejlsberg's video in PDC2008. It is very powerful, providing useful virtual methods to be overridden, like: TryGetMember() TrySetMember() TryInvokeMember() etc.  (In 2008 they are called GetMember, SetMember, etc., with different signature.) For example, if dynamicDatabase is a DynamicObject, then the following code: dynamicDatabase.Provider will invoke dynamicDatabase.TryGetMember() to do the actual work, where custom code can be put into. Now create a type to inherit DynamicObject: public class DynamicWrapper<T> : DynamicObject { private readonly bool _isValueType; private readonly Type _type; private T _value; // Not readonly, for value type scenarios. public DynamicWrapper(ref T value) // Uses ref in case of value type. { if (value == null) { throw new ArgumentNullException("value"); } this._value = value; this._type = value.GetType(); this._isValueType = this._type.IsValueType; } public override bool TryGetMember(GetMemberBinder binder, out object result) { // Searches in current type's public and non-public properties. PropertyInfo property = this._type.GetTypeProperty(binder.Name); if (property != null) { result = property.GetValue(this._value, null).ToDynamic(); return true; } // Searches in explicitly implemented properties for interface. MethodInfo method = this._type.GetInterfaceMethod(string.Concat("get_", binder.Name), null); if (method != null) { result = method.Invoke(this._value, null).ToDynamic(); return true; } // Searches in current type's public and non-public fields. FieldInfo field = this._type.GetTypeField(binder.Name); if (field != null) { result = field.GetValue(this._value).ToDynamic(); return true; } // Searches in base type's public and non-public properties. property = this._type.GetBaseProperty(binder.Name); if (property != null) { result = property.GetValue(this._value, null).ToDynamic(); return true; } // Searches in base type's public and non-public fields. field = this._type.GetBaseField(binder.Name); if (field != null) { result = field.GetValue(this._value).ToDynamic(); return true; } // The specified member is not found. result = null; return false; } // Other overridden methods are not listed. } In the above code, GetTypeProperty(), GetInterfaceMethod(), GetTypeField(), GetBaseProperty(), and GetBaseField() are extension methods for Type class. For example: internal static class TypeExtensions { internal static FieldInfo GetBaseField(this Type type, string name) { Type @base = type.BaseType; if (@base == null) { return null; } return @base.GetTypeField(name) ?? @base.GetBaseField(name); } internal static PropertyInfo GetBaseProperty(this Type type, string name) { Type @base = type.BaseType; if (@base == null) { return null; } return @base.GetTypeProperty(name) ?? @base.GetBaseProperty(name); } internal static MethodInfo GetInterfaceMethod(this Type type, string name, params object[] args) { return type.GetInterfaces().Select(type.GetInterfaceMap).SelectMany(mapping => mapping.TargetMethods) .FirstOrDefault( method => method.Name.Split('.').Last().Equals(name, StringComparison.Ordinal) && method.GetParameters().Count() == args.Length && method.GetParameters().Select( (parameter, index) => parameter.ParameterType.IsAssignableFrom(args[index].GetType())).Aggregate( true, (a, b) => a && b)); } internal static FieldInfo GetTypeField(this Type type, string name) { return type.GetFields( BindingFlags.GetField | BindingFlags.Instance | BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic).FirstOrDefault( field => field.Name.Equals(name, StringComparison.Ordinal)); } internal static PropertyInfo GetTypeProperty(this Type type, string name) { return type.GetProperties( BindingFlags.GetProperty | BindingFlags.Instance | BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic).FirstOrDefault( property => property.Name.Equals(name, StringComparison.Ordinal)); } // Other extension methods are not listed. } So now, when invoked, TryGetMember() searches the specified member and invoke it. The code can be written like this: dynamic dynamicDatabase = new DynamicWrapper<NorthwindDataContext>(ref database); dynamic dynamicReturnValue = dynamicDatabase.Provider.Execute(query.Expression).ReturnValue; This greatly simplified reflection. ToDynamic() and fluent reflection To make it even more straight forward, A ToDynamic() method is provided: public static class DynamicWrapperExtensions { public static dynamic ToDynamic<T>(this T value) { return new DynamicWrapper<T>(ref value); } } and a ToStatic() method is provided to unwrap the value: public class DynamicWrapper<T> : DynamicObject { public T ToStatic() { return this._value; } } In the above TryGetMember() method, please notice it does not output the member’s value, but output a wrapped member value (that is, memberValue.ToDynamic()). This is very important to make the reflection fluent. Now the code becomes: IEnumerable<Product> results = database.ToDynamic() // Here starts fluent reflection. .Provider.Execute(query.Expression).ReturnValue .ToStatic(); // Unwraps to get the static value. With the help of TryConvert(): public class DynamicWrapper<T> : DynamicObject { public override bool TryConvert(ConvertBinder binder, out object result) { result = this._value; return true; } } ToStatic() can be omitted: IEnumerable<Product> results = database.ToDynamic() .Provider.Execute(query.Expression).ReturnValue; // Automatically converts to expected static value. Take a look at the reflection code at the beginning of this post again. Now it is much much simplified! Special scenarios In 90% of the scenarios ToDynamic() is enough. But there are some special scenarios. Access static members Using extension method ToDynamic() for accessing static members does not make sense. Instead, DynamicWrapper<T> has a parameterless constructor to handle these scenarios: public class DynamicWrapper<T> : DynamicObject { public DynamicWrapper() // For static. { this._type = typeof(T); this._isValueType = this._type.IsValueType; } } The reflection code should be like this: dynamic wrapper = new DynamicWrapper<StaticClass>(); int value = wrapper._value; int result = wrapper.PrivateMethod(); So accessing static member is also simple, and fluent of course. Change instances of value types Value type is much more complex. The main problem is, value type is copied when passing to a method as a parameter. This is why ref keyword is used for the constructor. That is, if a value type instance is passed to DynamicWrapper<T>, the instance itself will be stored in this._value of DynamicWrapper<T>. Without the ref keyword, when this._value is changed, the value type instance itself does not change. Consider FieldInfo.SetValue(). In the value type scenarios, invoking FieldInfo.SetValue(this._value, value) does not change this._value, because it changes the copy of this._value. I searched the Web and found a solution for setting the value of field: internal static class FieldInfoExtensions { internal static void SetValue<T>(this FieldInfo field, ref T obj, object value) { if (typeof(T).IsValueType) { field.SetValueDirect(__makeref(obj), value); // For value type. } else { field.SetValue(obj, value); // For reference type. } } } Here __makeref is a undocumented keyword of C#. But method invocation has problem. This is the source code of TryInvokeMember(): public override bool TryInvokeMember(InvokeMemberBinder binder, object[] args, out object result) { if (binder == null) { throw new ArgumentNullException("binder"); } MethodInfo method = this._type.GetTypeMethod(binder.Name, args) ?? this._type.GetInterfaceMethod(binder.Name, args) ?? this._type.GetBaseMethod(binder.Name, args); if (method != null) { // Oops! // If the returnValue is a struct, it is copied to heap. object resultValue = method.Invoke(this._value, args); // And result is a wrapper of that copied struct. result = new DynamicWrapper<object>(ref resultValue); return true; } result = null; return false; } If the returned value is of value type, it will definitely copied, because MethodInfo.Invoke() does return object. If changing the value of the result, the copied struct is changed instead of the original struct. And so is the property and index accessing. They are both actually method invocation. For less confusion, setting property and index are not allowed on struct. Conclusions The DynamicWrapper<T> provides a simplified solution for reflection programming. It works for normal classes (reference types), accessing both instance and static members. In most of the scenarios, just remember to invoke ToDynamic() method, and access whatever you want: StaticType result = someValue.ToDynamic()._field.Method().Property[index]; In some special scenarios which requires changing the value of a struct (value type), this DynamicWrapper<T> does not work perfectly. Only changing struct’s field value is supported. The source code can be downloaded from here, including a few unit test code.

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• "The system time has changed" events after waking from sleep

  - by Damir Arh

  Sometimes when my computer running Windows 7 wakes up from sleep, it has to adjust the time. When this happens the following system event is logged: <Event xmlns='http://schemas.microsoft.com/win/2004/08/events/event'> <System> <Provider Name='Microsoft-Windows-Kernel-General' Guid='{A68CA8B7-004F-D7B6-A698-07E2DE0F1F5D}'/> <EventID>1</EventID> <Version>0</Version> <Level>4</Level> <Task>0</Task> <Opcode>0</Opcode> <Keywords>0x8000000000000010</Keywords> <TimeCreated SystemTime='2010-03-06T19:09:57.500000000Z'/> <EventRecordID>10672</EventRecordID> <Correlation/> <Execution ProcessID='4' ThreadID='56'/> <Channel>System</Channel> <Computer>GAME</Computer> <Security/> </System> <EventData> <Data Name='NewTime'>2010-03-06T19:09:57.500000000Z</Data> <Data Name='OldTime'>2010-03-06T17:34:32.870117200Z</Data> </EventData> <RenderingInfo Culture='sl-SI'> <Message>The system time has changed to ?2010?-?03?-?06T19:09:57.500000000Z from ?2010?-?03?-?06T17:34:32.870117200Z.</Message> <Level>Information</Level> <Task></Task> <Opcode>Info</Opcode> <Channel>System</Channel> <Provider>Microsoft-Windows-Kernel-General</Provider> <Keywords> <Keyword>Time</Keyword> </Keywords> </RenderingInfo> </Event> When this happens (I noticed it twice until now) the old time always corresponds to the time when computer entered sleep. The problem is that if Windows Media Center is scheduled for recording during this time, it just skips it as if the computer was turned off. I never had this problem running Windows Vista on the same machine. Any ideas what could be causing this problem and how to solve it are welcome.

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• Deleting entire lines in a text file based on a partial string match with Windows PowerShell

  - by Charles

  So I have several large text files I need to sort through, and remove all occurrences of lines which contain a given keyword. So basically, if I have these lines: This is not a test This is a test Maybe a test Definitely not a test And I run the script with 'not', I need to entirely delete lines 1 and 4. I've been trying with: PS C:\Users\Admin (Get-Content "D:\Logs\co2.txt") | Foreach-Object {$_ -replace "3*Program*", ""} | Set-Content "D:\Logs\co2.txt" but it only replaces the 'Program' and not the entire line.

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• "The system time has changed" avents after waking from sleep

  - by Damir Arh

  Sometimes when my computer running Windows 7 wakes up from sleep, it has to adjust the time. When this happens the following system event is logged: <Event xmlns='http://schemas.microsoft.com/win/2004/08/events/event'> <System> <Provider Name='Microsoft-Windows-Kernel-General' Guid='{A68CA8B7-004F-D7B6-A698-07E2DE0F1F5D}'/> <EventID>1</EventID> <Version>0</Version> <Level>4</Level> <Task>0</Task> <Opcode>0</Opcode> <Keywords>0x8000000000000010</Keywords> <TimeCreated SystemTime='2010-03-06T19:09:57.500000000Z'/> <EventRecordID>10672</EventRecordID> <Correlation/> <Execution ProcessID='4' ThreadID='56'/> <Channel>System</Channel> <Computer>GAME</Computer> <Security/> </System> <EventData> <Data Name='NewTime'>2010-03-06T19:09:57.500000000Z</Data> <Data Name='OldTime'>2010-03-06T17:34:32.870117200Z</Data> </EventData> <RenderingInfo Culture='sl-SI'> <Message>The system time has changed to ?2010?-?03?-?06T19:09:57.500000000Z from ?2010?-?03?-?06T17:34:32.870117200Z.</Message> <Level>Information</Level> <Task></Task> <Opcode>Info</Opcode> <Channel>System</Channel> <Provider>Microsoft-Windows-Kernel-General</Provider> <Keywords> <Keyword>Time</Keyword> </Keywords> </RenderingInfo> </Event> When this happens (I noticed it twice until now) the old time always corresponds to the time when computer entered sleep. The problem is that if Windows Media Center is scheduled for recording during this time, it just skips it as if the computer was turned off. I never had this problem running Windows Vista on the same machine. Any ideas what could be causing this problem and how to solve it are welcome.

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• Fedora distribution update pop-up after fresh installation

  - by Sayan Ghosh

  Hi, We do a kickstart installation of FC-10 at our place. I am quite intimidated by the distribution update pop up that comes up after the O/S installation. I want a keyword to put into the kickstart file that would stop Fedora from intimating with an update pop-up. Is it possible to include such a switch in the kickstart OR a script that could be added to post.bash? Thanks, Sayan

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• How to search inside files in Windows 7?

  - by Revolter

  In Windows XP we can search for files witch contains a defined keyword (inside all files types) Windows 7 can look inside files for a keywords, okay, but only for text files. (*.doc,*.txt, *.inf, ...), not (*.conf, *.dat, *.*, ...) Microsoft search filters don't contain any filter I can use for this. Any idea?

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• Fedora distribution update pop-up after fresh installation

  - by Sayan Ghosh

  Hi, We do a kickstart installation of FC-10 at our place. I am quite intimidated by the distribution update pop up that comes up after the O/S installation. I want a keyword to put into the kickstart file that would stop Fedora from intimating with an update pop-up. Is it possible to include such a switch in the kickstart OR a script that could be added to post.bash? Thanks, Sayan

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• is there a ctrl+tab equivalent on OSX

  - by olle

  I want to cycle trough the windows of the currently active application on OSX. Some applications respond to ctrl+tab but they all seem to do something else. I want to switch between entourage mainscreen and the message I am writing for instance. Is there a such a keyword shortcut?

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• Index a set of files to search their text quickly?

  - by Ricket

  I have a unique need: I am frequently searching a large set of text files for a keyword. Right now, I open up Notepad++ and use the "Find in files" feature. It works just fine, but with the amount of files, each search takes several minutes to complete. Is there a good program more suited for this purpose, perhaps that indexes a set of files and then lets you search the set repeatedly and very quickly? It would greatly speed up my workflow.

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• How to leave the open file in eclipse tab after search?

  - by Foolish

  when we open a file from "Package Explorer" in eclipse, after we use it, it will leave in the tabs. If we open a file from "Search" result, then after we search another keyword, it will disappear, we need to see the search result to find out that. Is there any solution that I can leave the opened tab in the tabs after I opened from Search panel ?

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• Portable hard disk with write-protect switch

  - by Wadih M.

  Hi, I'm looking for portable hard disks that have a physical switch (with or without key) to make it "read only" to prevent modification. Is there a particular keyword I should look for that designate those types of drives, or models I can look into, anything that can guide me will help. Thanks

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• Did I find a bug in PHP's `crypt()`?

  - by Nathan Long

  I think I may have found a bug in PHP's crypt() function under Windows. However: I recognize that it's probably my fault. PHP is used by millions and worked on by thousands; my code is used by tens and worked on by me. (This argument is best explained on Coding Horror.) So I'm asking for help: show me my fault. I've been trying to find it for a few days now, with no luck. The setup I'm using a Windows server installation with Apache 2.2.14 (Win32) and PHP 5.3.2. My development box runs Windows XP Professional; the 'production' server (this is an intranet setup) runs Windows Storage Server 2003. The problem happens on both. I don't see anything in php.ini related to crypt(), but will happily answer questions about my config. The problem Several scripts in my PHP app occasionally hang: the page sits there on 'waiting for localhost' and never finishes. Each of these scripts uses crypt to hash a user's password before storing it in the database, or, in the case of the login page, to hash the entered password before comparing it to the version stored in the database. Since the login page is the simplest, I focused on it for testing. I repeatedly logged in, and found that it would hang maybe 4 out of 10 times. As an experiment, I changed the login page to use the plain text password and changed my password in the database to its plain text version. The page stopped hanging. I saw that PHP's latest version lists this bugfix: Fixed bug #51059 (crypt crashes when invalid salt are [sic] given). So I created a very simple test script, as follows, using the same salt given in an official example: $foo = crypt('rasmuslerdorf','r1'); echo $foo; This page, too, will hang, if I reload it like crazy. I only see it hanging in Chrome, but regardless of browser, the effect on Apache is the same. Effect on Apache When these pages hang, Apache's server-status page (which I explained here, regarding a different problem) increments the number of requests being processed and decrements the number of idle workers. The requests being processed almost all have a status of 'Sending Reply,' though sometimes for a moment they will show either 'Reading request' or 'keepalive (read).' Eventually, Apache may crash. When it does, the Windows crash report looks like this: szAppName: httpd.exe szAppVer: 2.2.14.0 szModName: php5ts.dll szModVer: 5.3.1.0 // OK, this report was before I upgraded to PHP 5.3.2, // but that didn't fix it offset: 00a2615 Is it my fault? I'm tempted to file a bug report to PHP on this. The argument against it is, as stated above, that bugs are nearly always my fault. However, my argument in favor of 'it's PHP's fault' is: I'm using Windows, whereas most servers use Linux (I don't get to choose this), so the chances are greater that I've found an edge case There was recently a bug with crypt(), so maybe it still has issues I have made the simplest test case I can, and I still have the problem Can anyone duplicate this? Can you suggest where I've gone wrong? Should I file the bug after all? Thanks in advance for any help you may give.

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• What is the best way to move site to new domain without losing google page rank

  - by T1000

  Hello I just bought new domain for my web page. Is it good to use php redirection to the new domain and what are the exact headers that I must send to google bot to just renew the address and keep the page rank without loosing position in search results for the specific keyword ? Regards

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• Search PDF files from command line or using scripts

  - by arunkumar.cr

  Is there a way to search for some text or keyword in PDF files? I need to do it from the command line or by using some scripts.

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• How to find an alternative proxy server

  - by Stan

  It appears the default ISP proxy has some problem and got high latency. How to find a good alternative proxy at my location? Say google's proxy? I tried to search online (keyword: proxy mylocation), but couldn't tell which is trustworthy. Thanks.

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• How to find an alternative proxy server

  - by Stan

  It appears the default ISP proxy has some problem and got high latency. How to find a good alternative proxy at my location? Say google's proxy? I tried to search online (keyword: proxy mylocation), but couldn't tell which is trustworthy. Thanks.

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• Online modelling tool for server desing / architecture

  - by 2ge

  I am looking for some online (the best is free) tool for designing our servers. We use almost 10 servers now, and it becoming mess, to remember, where, what service is running. So, I'd like to have some online modeling tool, where I can set up things like: - server host - server hw parameters - server os - server services with running programs I am looking for server designing tool like online SQL modeling on http://ondras.zarovi.cz/sql/demo/?keyword=default (WWW SQL designer) Any ideas ?

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• Make Chrome browser prefer DNS over searching

  - by dronus

  Is it possible to let the Chrome browser prefer all DNS resolvable URLs over a search? Everytime I use a local name not matching a usual domain scheme, I got search results for it. I first thought that no nonsense DNS lookup is made if the URL seems to be a search keyword, however Chrome always detect this condition and asks me if I like to go to my domain instead. So the DNS lookup is made anyway.

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• Where does the windows file sharing account info save to?

  - by Stan

  OS: windows server 2003 When open explore and enter \192.168.1.xxx\c$ and prompt to ask login id and password, where does this login info save to? And even if I choose not to save, seems the session will still remain until reboot? Can the community please suggestion some keyword to this and explain how it works a bit? Thanks.

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• Is there a ctrl+tab equivalent on Mac OS X?

  - by olle

  I want to cycle trough the windows of the currently active application on Mac OS X. Some applications respond to ctrl+tab but they all seem to do something else. I want to switch between the Entourage mainscreen and the message I am writing for instance. Is there a such a keyword shortcut?

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• Multi level or progressive or incremental file search

  - by iraSenthil

  I am looking for a GUI tool in Windows, where I can do search and pass the result to next search and continue. I know I can do this in command line by piping one search result to another, but I am looking for a GUI tool. Here is a sample search, I would like to find all files that has extension ".java". From the result, find all files that has a specific word. From the result, select few files, and search only those files with another keyword.

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• Parallelism in .NET – Part 4, Imperative Data Parallelism: Aggregation

  - by Reed

  In the article on simple data parallelism, I described how to perform an operation on an entire collection of elements in parallel.  Often, this is not adequate, as the parallel operation is going to be performing some form of aggregation. Simple examples of this might include taking the sum of the results of processing a function on each element in the collection, or finding the minimum of the collection given some criteria.  This can be done using the techniques described in simple data parallelism, however, special care needs to be taken into account to synchronize the shared data appropriately.  The Task Parallel Library has tools to assist in this synchronization. The main issue with aggregation when parallelizing a routine is that you need to handle synchronization of data.  Since multiple threads will need to write to a shared portion of data.  Suppose, for example, that we wanted to parallelize a simple loop that looked for the minimum value within a dataset: double min = double.MaxValue; foreach(var item in collection) { double value = item.PerformComputation(); min = System.Math.Min(min, value); } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } This seems like a good candidate for parallelization, but there is a problem here.  If we just wrap this into a call to Parallel.ForEach, we’ll introduce a critical race condition, and get the wrong answer.  Let’s look at what happens here: // Buggy code! Do not use! double min = double.MaxValue; Parallel.ForEach(collection, item => { double value = item.PerformComputation(); min = System.Math.Min(min, value); }); This code has a fatal flaw: min will be checked, then set, by multiple threads simultaneously.  Two threads may perform the check at the same time, and set the wrong value for min.  Say we get a value of 1 in thread 1, and a value of 2 in thread 2, and these two elements are the first two to run.  If both hit the min check line at the same time, both will determine that min should change, to 1 and 2 respectively.  If element 1 happens to set the variable first, then element 2 sets the min variable, we’ll detect a min value of 2 instead of 1.  This can lead to wrong answers. Unfortunately, fixing this, with the Parallel.ForEach call we’re using, would require adding locking.  We would need to rewrite this like: // Safe, but slow double min = double.MaxValue; // Make a "lock" object object syncObject = new object(); Parallel.ForEach(collection, item => { double value = item.PerformComputation(); lock(syncObject) min = System.Math.Min(min, value); }); This will potentially add a huge amount of overhead to our calculation.  Since we can potentially block while waiting on the lock for every single iteration, we will most likely slow this down to where it is actually quite a bit slower than our serial implementation.  The problem is the lock statement – any time you use lock(object), you’re almost assuring reduced performance in a parallel situation.  This leads to two observations I’ll make: When parallelizing a routine, try to avoid locks. That being said: Always add any and all required synchronization to avoid race conditions. These two observations tend to be opposing forces – we often need to synchronize our algorithms, but we also want to avoid the synchronization when possible.  Looking at our routine, there is no way to directly avoid this lock, since each element is potentially being run on a separate thread, and this lock is necessary in order for our routine to function correctly every time. However, this isn’t the only way to design this routine to implement this algorithm.  Realize that, although our collection may have thousands or even millions of elements, we have a limited number of Processing Elements (PE).  Processing Element is the standard term for a hardware element which can process and execute instructions.  This typically is a core in your processor, but many modern systems have multiple hardware execution threads per core.  The Task Parallel Library will not execute the work for each item in the collection as a separate work item. Instead, when Parallel.ForEach executes, it will partition the collection into larger “chunks” which get processed on different threads via the ThreadPool.  This helps reduce the threading overhead, and help the overall speed.  In general, the Parallel class will only use one thread per PE in the system. Given the fact that there are typically fewer threads than work items, we can rethink our algorithm design.  We can parallelize our algorithm more effectively by approaching it differently.  Because the basic aggregation we are doing here (Min) is communitive, we do not need to perform this in a given order.  We knew this to be true already – otherwise, we wouldn’t have been able to parallelize this routine in the first place.  With this in mind, we can treat each thread’s work independently, allowing each thread to serially process many elements with no locking, then, after all the threads are complete, “merge” together the results. This can be accomplished via a different set of overloads in the Parallel class: Parallel.ForEach<TSource,TLocal>.  The idea behind these overloads is to allow each thread to begin by initializing some local state (TLocal).  The thread will then process an entire set of items in the source collection, providing that state to the delegate which processes an individual item.  Finally, at the end, a separate delegate is run which allows you to handle merging that local state into your final results. To rewriting our routine using Parallel.ForEach<TSource,TLocal>, we need to provide three delegates instead of one.  The most basic version of this function is declared as: public static ParallelLoopResult ForEach<TSource, TLocal>( IEnumerable<TSource> source, Func<TLocal> localInit, Func<TSource, ParallelLoopState, TLocal, TLocal> body, Action<TLocal> localFinally ) The first delegate (the localInit argument) is defined as Func<TLocal>.  This delegate initializes our local state.  It should return some object we can use to track the results of a single thread’s operations. The second delegate (the body argument) is where our main processing occurs, although now, instead of being an Action<T>, we actually provide a Func<TSource, ParallelLoopState, TLocal, TLocal> delegate.  This delegate will receive three arguments: our original element from the collection (TSource), a ParallelLoopState which we can use for early termination, and the instance of our local state we created (TLocal).  It should do whatever processing you wish to occur per element, then return the value of the local state after processing is completed. The third delegate (the localFinally argument) is defined as Action<TLocal>.  This delegate is passed our local state after it’s been processed by all of the elements this thread will handle.  This is where you can merge your final results together.  This may require synchronization, but now, instead of synchronizing once per element (potentially millions of times), you’ll only have to synchronize once per thread, which is an ideal situation. Now that I’ve explained how this works, lets look at the code: // Safe, and fast! double min = double.MaxValue; // Make a "lock" object object syncObject = new object(); Parallel.ForEach( collection, // First, we provide a local state initialization delegate. () => double.MaxValue, // Next, we supply the body, which takes the original item, loop state, // and local state, and returns a new local state (item, loopState, localState) => { double value = item.PerformComputation(); return System.Math.Min(localState, value); }, // Finally, we provide an Action<TLocal>, to "merge" results together localState => { // This requires locking, but it's only once per used thread lock(syncObj) min = System.Math.Min(min, localState); } ); Although this is a bit more complicated than the previous version, it is now both thread-safe, and has minimal locking.  This same approach can be used by Parallel.For, although now, it’s Parallel.For<TLocal>.  When working with Parallel.For<TLocal>, you use the same triplet of delegates, with the same purpose and results. Also, many times, you can completely avoid locking by using a method of the Interlocked class to perform the final aggregation in an atomic operation.  The MSDN example demonstrating this same technique using Parallel.For uses the Interlocked class instead of a lock, since they are doing a sum operation on a long variable, which is possible via Interlocked.Add. By taking advantage of local state, we can use the Parallel class methods to parallelize algorithms such as aggregation, which, at first, may seem like poor candidates for parallelization.  Doing so requires careful consideration, and often requires a slight redesign of the algorithm, but the performance gains can be significant if handled in a way to avoid excessive synchronization.

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• C#/.NET Little Wonders: The EventHandler and EventHandler&lt;TEventArgs&gt; delegates

  - by James Michael Hare

  Once again, in this series of posts I look at the parts of the .NET Framework that may seem trivial, but can help improve your code by making it easier to write and maintain. The index of all my past little wonders posts can be found here. In the last two weeks, we examined the Action family of delegates (and delegates in general), and the Func family of delegates and how they can be used to support generic, reusable algorithms and classes. So this week, we are going to look at a handy pair of delegates that can be used to eliminate the need for defining custom delegates when creating events: the EventHandler and EventHandler<TEventArgs> delegates. Events and delegates Before we begin, let’s quickly consider events in .NET.  According to the MSDN: An event in C# is a way for a class to provide notifications to clients of that class when some interesting thing happens to an object. So, basically, you can create an event in a type so that users of that type can subscribe to notifications of things of interest.  How is this different than some of the delegate programming that we talked about in the last two weeks?  Well, you can think of an event as a special access modifier on a delegate.  Some differences between the two are: Events are a special access case of delegates They behave much like delegates instances inside the type they are declared in, but outside of that type they can only be (un)subscribed to. Events can specify add/remove behavior explicitly If you want to do additional work when someone subscribes or unsubscribes to an event, you can specify the add and remove actions explicitly. Events have access modifiers, but these only specify the access level of those who can (un)subscribe A public event, for example, means anyone can (un)subscribe, but it does not mean that anyone can raise (invoke) the event directly.  Events can only be raised by the type that contains them In contrast, if a delegate is visible, it can be invoked outside of the object (not even in a sub-class!). Events tend to be for notifications only, and should be treated as optional Semantically speaking, events typically don’t perform work on the the class directly, but tend to just notify subscribers when something of note occurs. My basic rule-of-thumb is that if you are just wanting to notify any listeners (who may or may not care) that something has happened, use an event.  However, if you want the caller to provide some function to perform to direct the class about how it should perform work, make it a delegate. Declaring events using custom delegates To declare an event in a type, we simply use the event keyword and specify its delegate type.  For example, let’s say you wanted to create a new TimeOfDayTimer that triggers at a given time of the day (as opposed to on an interval).  We could write something like this: 1: public delegate void TimeOfDayHandler(object source, ElapsedEventArgs e); 2:  3: // A timer that will fire at time of day each day. 4: public class TimeOfDayTimer : IDisposable 5: { 6: // Event that is triggered at time of day. 7: public event TimeOfDayHandler Elapsed; 8:  9: // ... 10: } The first thing to note is that the event is a delegate type, which tells us what types of methods may subscribe to it.  The second thing to note is the signature of the event handler delegate, according to the MSDN: The standard signature of an event handler delegate defines a method that does not return a value, whose first parameter is of type Object and refers to the instance that raises the event, and whose second parameter is derived from type EventArgs and holds the event data. If the event does not generate event data, the second parameter is simply an instance of EventArgs. Otherwise, the second parameter is a custom type derived from EventArgs and supplies any fields or properties needed to hold the event data. So, in a nutshell, the event handler delegates should return void and take two parameters: An object reference to the object that raised the event. An EventArgs (or a subclass of EventArgs) reference to event specific information. Even if your event has no additional information to provide, you are still expected to provide an EventArgs instance.  In this case, feel free to pass the EventArgs.Empty singleton instead of creating new instances of EventArgs (to avoid generating unneeded memory garbage). The EventHandler delegate Because many events have no additional information to pass, and thus do not require custom EventArgs, the signature of the delegates for subscribing to these events is typically: 1: // always takes an object and an EventArgs reference 2: public delegate void EventHandler(object sender, EventArgs e) It would be insane to recreate this delegate for every class that had a basic event with no additional event data, so there already exists a delegate for you called EventHandler that has this very definition!  Feel free to use it to define any events which supply no additional event information: 1: public class Cache 2: { 3: // event that is raised whenever the cache performs a cleanup 4: public event EventHandler OnCleanup; 5:  6: // ... 7: } This will handle any event with the standard EventArgs (no additional information).  But what of events that do need to supply additional information?  Does that mean we’re out of luck for subclasses of EventArgs?  That’s where the generic for of EventHandler comes into play… The generic EventHandler<TEventArgs> delegate Starting with the introduction of generics in .NET 2.0, we have a generic delegate called EventHandler<TEventArgs>.  Its signature is as follows: 1: public delegate void EventHandler<TEventArgs>(object sender, TEventArgs e) 2: where TEventArgs : EventArgs This is similar to EventHandler except it has been made generic to support the more general case.  Thus, it will work for any delegate where the first argument is an object (the sender) and the second argument is a class derived from EventArgs (the event data). For example, let’s say we wanted to create a message receiver, and we wanted it to have a few events such as OnConnected that will tell us when a connection is established (probably with no additional information) and OnMessageReceived that will tell us when a new message arrives (probably with a string for the new message text). So for OnMessageReceived, our MessageReceivedEventArgs might look like this: 1: public sealed class MessageReceivedEventArgs : EventArgs 2: { 3: public string Message { get; set; } 4: } And since OnConnected needs no event argument type defined, our class might look something like this: 1: public class MessageReceiver 2: { 3: // event that is called when the receiver connects with sender 4: public event EventHandler OnConnected; 5:  6: // event that is called when a new message is received. 7: public event EventHandler<MessageReceivedEventArgs> OnMessageReceived; 8:  9: // ... 10: } Notice, nowhere did we have to define a delegate to fit our event definition, the EventHandler and generic EventHandler<TEventArgs> delegates fit almost anything we’d need to do with events. Sidebar: Thread-safety and raising an event When the time comes to raise an event, we should always check to make sure there are subscribers, and then only raise the event if anyone is subscribed.  This is important because if no one is subscribed to the event, then the instance will be null and we will get a NullReferenceException if we attempt to raise the event. 1: // This protects against NullReferenceException... or does it? 2: if (OnMessageReceived != null) 3: { 4: OnMessageReceived(this, new MessageReceivedEventArgs(aMessage)); 5: } The above code seems to handle the null reference if no one is subscribed, but there’s a problem if this is being used in multi-threaded environments.  For example, assume we have thread A which is about to raise the event, and it checks and clears the null check and is about to raise the event.  However, before it can do that thread B unsubscribes to the event, which sets the delegate to null.  Now, when thread A attempts to raise the event, this causes the NullReferenceException that we were hoping to avoid! To counter this, the simplest best-practice method is to copy the event (just a multicast delegate) to a temporary local variable just before we raise it.  Since we are inside the class where this event is being raised, we can copy it to a local variable like this, and it will protect us from multi-threading since multicast delegates are immutable and assignments are atomic: 1: // always make copy of the event multi-cast delegate before checking 2: // for null to avoid race-condition between the null-check and raising it. 3: var handler = OnMessageReceived; 4: 5: if (handler != null) 6: { 7: handler(this, new MessageReceivedEventArgs(aMessage)); 8: } The very slight trade-off is that it’s possible a class may get an event after it unsubscribes in a multi-threaded environment, but this is a small risk and classes should be prepared for this possibility anyway.  For a more detailed discussion on this, check out this excellent Eric Lippert blog post on Events and Races. Summary Generic delegates give us a lot of power to make generic algorithms and classes, and the EventHandler delegate family gives us the flexibility to create events easily, without needing to redefine delegates over and over.  Use them whenever you need to define events with or without specialized EventArgs.   Tweet Technorati Tags: .NET, C#, CSharp, Little Wonders, Generics, Delegates, EventHandler

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