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• Degrafa color changer

  - by proyb2

  <degrafa:LinearGradientFill id="bluedream"> <degrafa:GradientStop color="#6ab5d0"/> <degrafa:GradientStop color="#388aae"/> </degrafa:LinearGradientFill> <degrafa:GeometryComposition graphicsTarget="{[bgCanvas]}"> <degrafa:RoundedRectangle id="color_preset" fill="{bluedream}"/> </degrafa:GeometryComposition> I have issue with degrafa code which I have defined a set of different gradients for color_preset.fill to be dynamic change when user select different color in the combobox. I replaced fill="{bluedream}" with fill="using_variable" and lead to error compiled message: Initializer for 'fill': values of type com.degrafa.core.IGraphicsFill cannot be represented in text. Is there a solution to use this code as a color changer?

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• Inheritance - initialization problem

  - by dumbquestion

  I have a c++ class derived from a base class in a framework. The derived class doesn't have any data members because I need it to be freely convertible into a base class and back - the framework is responsible for loading and saving the objects and I can't change it. My derived class just has functions for accessing the data. But there are a couple of places where I need to store some temporary local variables to speed up access to data in the base class. mydata* MyClass::getData() { if ( !m_mydata ) { // set to NULL in the constructor m_mydata = some_long_and complex_operation_to_get_the_data_in_the_base() } return m_mydata; } The problem is if I just access the object by casting the base class pointer returned from the framework to MyClass* the ctor for MyClass is never called and m_mydata is junk. Is there a way of only initializing the m_mydata pointer once?

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• c++ Initializing a struct with an array as a member

  - by Drew Shafer

  I've got the following reduced testcase: typedef struct TestStruct { int length; int values[]; }; TestStruct t = {3, {0, 1, 2}}; This works with Visual C++, but doesn't compile with g++ under linux. Can anyone help me make this specific kind of initializer portable? Additional details: the actual structure I'm working with has several other int values, and the array can range in length from a single entry to over 1800 entries. Any help much appreciated. Thanks!

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• Constant template parameter class manages to link externally

  - by the_drow

  I have a class foo with an enum template parameter and for some reason it links to two versions of the ctor in the cpp file. enum Enum { bar, baz }; template <Enum version = bar> class foo { public: foo(); }; // CPP File #include "foo.hpp" foo<bar>::foo() { cout << "bar"; } foo<baz>::foo() { cout << "baz"; } I'm using msvc 2008, is this the standard behavior? Are only type template parameters cannot be linked to cpp files?

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• an error "variable of field declared void"

  - by lego69

  I have this code: header - test.h Inside header I have some class Z and definitions of two functions test and test2 I call function test2 from test void test2(Z z, Z const *za); this is implementation of the function: void test2(Z z, Z const *za){ int i = z; //this row works cout << i << endl; } I call it from test: test2(z1, za1); // za1 is pinter to object and z1 is some object but in my header I receive an 3 errors: Multiple markers at this line - initializer expression list treated as compound expression - `A' was not declared in this scope - variable or field `quiz2' declared void can somebody please explain why? thanks in advance

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• How to default-initialize local variables of built-in types in C++?

  - by sharptooth

  How do I default-initialize a local variable of primitive type in C++? For example if a have a typedef: typedef unsigned char boolean;//that's Microsoft RPC runtime typedef I'd like to change the following line: boolean variable = 0; //initialize to some value to ensure reproduceable behavior retrieveValue( &variable ); // do actual job into something that would automagically default-initialize the variable - I don't need to assign a specific value to it, but instead I only need it to be intialized to the same value each time the program runs - the same stuff as with a constructor initializer list where I can have: struct Struct { int Value; Struct() : Value() {} }; and the Struct::Value will be default-initialized to the same value every time an instance is cinstructed, but I never write the actual value in the code. How can I get the same behavior for local variables?

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• C++0x optimizing compiler quality

  - by aaa

  hello. I do some heavy numbercrunching and for me floating-point performance is very important. I like performance of Intel compiler very much and quite content with quality of assembly it produces. I am thinking at some point to try C++0x mainly for sugar parts, like auto, initializer list, etc, but also lambdas. at this point I use those features in regular C++ by the means of boost. How good of assembly code do compilers C++0x generate? specifically Intel and gcc compilers. Do they produce SSE code? is performance comparable to C++? are there any benchmarks? My Google search did not reveal much. Thank you.

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• Statically Init a derived class

  - by AC

  With c++, Is there a way to get a derived class to inherit its own static initializer? I am trying to do something like the following: class Base { public: class StaticInit { public: virtual StaticInit() =0; }; }; class Derived: public Base { public: virtual StaticInit::StaticInit() { //do something with the derived class } static StaticInit init; } static Derived::StaticInit init; it would also be nice if I didn't have to define the init var in each derived class. I am currently redefining the StaticInit internal class in each derived class but it seems redundant. Each derived class is a singleton, and I need the instance to be stored in a lookup table at program startup.

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• PrivateFontCollection fails on appharbor

  - by grennis

  I am trying to load a custom font (ttf file) and draw into an image with the Graphics object. This code runs fine locally: PrivateFontCollection fonts = new PrivateFontCollection(); string path = context.Server.MapPath("~/App_Data/Futura LT Bold.ttf"); if (!System.IO.File.Exists(path)) { throw new InvalidOperationException("Font file is not deployed: " + path); } fonts.AddFontFile(path); However when run on appharbor, the call to AddFontFile fails with the exception: System.ArgumentException: Font 'Futura LT Book' does not support style 'Regular'. at System.Drawing.Font.CreateNativeFont() at System.Drawing.Font.Initialize(FontFamily family, Single emSize, FontStyle style, GraphicsUnit unit, Byte gdiCharSet, Boolean gdiVerticalFont) at System.Drawing.Font..ctor(FontFamily family, Single emSize) at LumenboxWeb.Controllers.GalleryController.FontTest() in d:\temp\h5oqslma.udd\input\src\LumenboxWeb\Controllers\GalleryController.cs:line 59 at lambda_method(Closure , ControllerBase , Object[] ) at System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters) at System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) I have tried different fonts, and they all work locally, but none work on appharbor. Is it possible to load fonts dynamically on appharbor?

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• Using an observer within an Engine

  - by Tim

  I've created an Engine which is basically used for all of our projects. Now what I want to do is add a before_create callback to all of the models in this Engine. After some searching I found out that an observer is the way to go. So, I've created this observer: class AuthObserver < ActiveRecord::Observer def before_create( record ) p record end end And now I need to add it to the application, but of course in my Engine there is no such file as application.rb. What I tried is adding it to an initializer located in /config/initializers/observers.rb Like so: Rails.application.config.active_record.observers = :auth_observer But this doesn't work, and it throws no errors. Anybody out here has experience using an observer inside an engine? Thanks a lot!

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• Rails: Creating subfolders in model?

  - by keruilin

  I'm going to have a ton of subclasses, so want to organize them under a subfolder called stream. I added the following line to the environment.rb so that all classes in the subfolder would be loaded: Rails::Initializer.run do |config| ... config.load_paths += Dir["#{RAILS_ROOT}/app/models/*"].find_all { |f| File.stat(f).directory? } ... end I thought this would solve the issue in which by convention the model class is namespaced into an according module. However, when I try to call one of the classes called stream in the stream folder, I get the following error: NoMethodError: undefined method `new' for Stream:Module from (irb):28 from /usr/local/bin/irb:12:in `<main>' Here's the model for the parent and one child: class Stream end class EventStream < Stream end Any idea what the issue is?

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• Is it a bad idea to create tests that rely on each other within a test fixture?

  - by nbolton

  For example: // NUnit-like pseudo code (within a TestFixture) Ctor() { m_globalVar = getFoo(); } [Test] Create() { a(m_globalVar) } [Test] Delete() { // depends on Create being run b(m_globalVar) } … or… // NUnit-like pseudo code (within a TestFixture) [Test] CreateAndDelete() { Foo foo = getFoo(); a(foo); // depends on Create being run b(foo); } … I’m going with the later, and assuming that the answer to my question is: No, at least not with NUnit, because according to the NUnit manual: The constructor should not have any side effects, since NUnit may construct the class multiple times in the course of a session. ... also, can I assume it's bad practice in general? Since tests can usually be run separately. So the result of Create may never be cleaned up by Delete.

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• CoreData Model Objects for API

  - by theiOSguy

  I am using CoreData in my application. I want to abstract out all the CoreData related stuff as an API so that the consume can use the API instead of directly using CoreData and its generated model objects. CoreData generates the managed objects model as following @interface Person : NSManagedObject @end I want to define my API for example MyAPI and it has a function called as createPerson:(Person*)p; So the consumer of this createPerson API needs to create a Person data object (like POJO in java world) and invoke this API. But I cannot create Person object using Person *p = [Person alloc] init] because the designated initializer for this Person model created by CoreData does not allow this type of creation. So should I define corresponding user facing data object may be PersonDO and this API should take that instead to carry the data into the API implementation? Is my approach right? Any expert advise if design the API this way is a good design pattern?

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• Log your SQL in Rails application inside unit test

  - by Phuong Nguy?n

  I want to install a logger so that I can dump all executed SQL of my rails application. Problem is, such logger is associated with AbstractAdapter which initialized very soon under test mode, and thus cannot be set by my initializer code. I try to put ActiveRecord::Base.logger = MyCustomLogger.new(STDOUT) in the end of environment.rb like someone advised but it only works when being run in console environment (kicked by script/console), not when run under test mode. I wonder if there is any way to config such logger so that I will sure to be invoked under any environment (test, development, production, console)

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• subscribing an event to a class object before initializing it.

  - by GN

  I have two classes class A and B.I have a delegate n event published in class B.The class B object is declared in class A.All he functionality dependes on the parameterised constru ctor of class B. Before initializing the object of class B i need to subscibe the event for it.how to do it? e.g public class B { public delegate void myDel(string); public event myDel myEvent; B(object obj) { ----------------- ------------------ } } class A { A objA; class XYZ objXYZ; void func() { objA.myEvent+=new myDel(); objA=new A(objXYZ); // hw to attain this? } }

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• Creating Pages with unique PageMaps

  - by Michael Krauklis

  I have a wicket Application that has long lived Pages. I would like each instance of this Page to be created with a unique PageMap for reasons I won't get into here, but when I try to specify a PageMap on the constructor of my Page I end up getting OOM and StackOverflow errors. Here is the code: public <CTOR>() { super(Session.get().newPageMap("" + System.currentTimeMillis())); ... I'm using a custom BookmarkablePageLink (custom in that it does not pass on the current PageMap name to the new Page) to effectively abandon the old Page/PageMap and create a new one with a new PageMap. The errors seem to be coming from page serialization after only two or three pages have been created. Can anyone find issue with this approach?

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• Microsoft Access DB Connection

  - by sikas

  I have a Microsoft Access DB (2003) that I want to connect to it using C# .. The problem I'm facing is I don't have Access installed within the office package .. So I was wondering if it is possible to connect to it as a database to retrieve and update the tables .. Thanks. UPDATE I have received the error below: Error Detected: System.InvalidOperationException: The 'Microsoft.Jet.OLEDB.4.0' provider is not registered on the local machine. at System.Data.OleDb.OleDbServicesWrapper.GetDataSource(OleDbConnectionString constr, DataSourceWrapper& datasrcWrapp er) at System.Data.OleDb.OleDbConnectionInternal..ctor(OleDbConnectionString constr, OleDbConnection connection) at System.Data.OleDb.OleDbConnectionFactory.CreateConnection(DbConnectionOptions options, Object poolGroupProviderInf o, DbConnectionPool pool, DbConnection owningObject) at System.Data.ProviderBase.DbConnectionFactory.CreateNonPooledConnection(DbConnection owningConnection, DbConnection PoolGroup poolGroup) at System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection) at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory conne ctionFactory) at System.Data.OleDb.OleDbConnection.Open() at SampleNamespace.SampleClass.Main()

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• How do I use a graphics method to pass parameters? Example is below.

  - by sonny5

  private static void getCorners(out float Wx, out float Wy, out float Vx, out float Vy) { // note-- I don't know how to flip the Y axis using this method... this is not a "graphics" method // In other words, I should use something like: // flipY(object sender, System.EventArgs e); // but don't know the syntax or whatever // I tried to do this: //Graphics g = this.CreateGraphic //Matrix myMatrix2 = new Matrix(1, 0, 0, -1, 0, 0); // flip Y axis //g.Transform = myMatrix2; //g.TranslateTransform(0, 480, MatrixOrder.Append); // ...but I get the error: // error CS0026: Keyword 'this' is not valid in a static property, static method, or // static field initializer Wx = 1.00F; Wy = 1.00F; // make this 1.00 (not 3.00F) down from the TOP since cannot get Y flipped Vx = ((Wx- WXmin)*((VXmax-VXmin)+VXmin)/(WXmax-WXmin)); Vy = ((Wy-WYmin)*(VYmax-VYmin)/(WYmax-WYmin)+VYmin); } thanks, Sonny5

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• An array of structures in C...

  - by 00010000

  For the life of me I can't figure out the proper syntax for creating an array of structures in C. I tried this: struct foo { int x; int y; } foo[][] = { { { 1, 2 }, { 4, 5 }, { -1, -1 } }, { { 55, 44 } { 100, 200 }, } }; So for example foo[1][0].x == 100, foo[0][1].y == 5, etc. But GCC spits out a lot of errors. If anyone could provide the proper syntax that'd be great. EDIT: Okay, I tried this: struct foo { const char *x; int y; }; struct foo bar[2][] = { { { "A", 1 }, { "B", 2 }, { NULL, -1 }, }, { { "AA", 11 }, { "BB", 22 }, { NULL, -1 }, }, { { "ZZ", 11 }, { "YY", 22 }, { NULL, -1 }, }, { { "XX", 11 }, { "UU", 22 }, { NULL, -1 }, }, }; But GCC gives me "elements of array bar have incomplete type" and "excess elements in array initializer".

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• Django GenericRelation doesn't save related object's id - is this a bug or am I doing it wrong?

  - by pinkeen

  I have a model with a generic relation (call it A), when creating an instance of this object I pass an instance of another model (call it B) as the initializer of the content_object field (via kwargs of the constructor). If I don't save B before creating A then when saving A the content_object_id is saved to the db as NULL. If I save B before passing it to the constructor of A then everything's allright. It's not logical. I assumed that the ID of the related object (B) is fetched when doing A.save() and it should throw some kind of an exception if B isn't saved yet but it just fails silently. I don't like the current solution (saving B beforhand) because we don't know yet if I will be always willing to keep the object, not just scrap it, and there are performance considerations - what if I will add some another data and save it once more shortly after. class BaseNodeData(models.Model): ... extnodedata_content_type = models.ForeignKey(ContentType, null=True) extnodedata_object_id = models.PositiveIntegerField(null=True) extnodedata = generic.GenericForeignKey(ct_field='extnodedata_content_type', fk_field='extnodedata_object_id') class MarkupNodeData(models.Model): raw_content = models.TextField() Suppose we do: markup = MarkupNodeData(raw_content='...') base = BaseNodeData(..., extnodedata=markup) markup.save() base.save() # both records are inserted to the DB but base is stored with extnodedata_object_id=NULL markup = MarkupNodeData(raw_content='...') base = BaseNodeData(..., extnodedata=markup) base.save() markup.save() # no exception is thrown and everything is the same as above markup = MarkupNodeData(raw_content='...') markup.save() base = BaseNodeData(..., extnodedata=markup) base.save() # this works as expected Of course I can do it this way, but it doesn't change anything: base = BaseNodeData(...) base.extnodedata = markup My question is - is this a bug in django which I should report or maybe I'm doing something wrong. Docs on GenericRelations aren't exactly verbose.

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• Is read-only auto-imlemented property possible?

  - by abatishchev

  Hello. I found a topic on MSDN that talks that yes, this is possible. I did a test that seems to break this statement: using System; namespace Test { class Program { static void Main(string[] args) { Foo f = new Foo("1"); Console.WriteLine(f.Bar); // prints 1 f.Test("2"); Console.WriteLine(f.Bar);// successfully prints 2 } } class Foo { public Foo(string b) { this.Bar = b; } public string Bar { get; private set; } public void Test(string b) { // this would be impossible for readonly field! // next error would be occur: CS0191 or CS0191 // A readonly field cannot be assigned to (except in a constructor or a variable initializer) this.Bar = b; } } } Where am I wrong?

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• Illegal characters for SharePoint 2010 Content Type name

  - by Kelly Jones

  Quick tip: you can’t include a backslash in the name of the SharePoint 2010 Content Type.  In fact, there are several illegal characters:  \  / : * ? " # % < > { } | ~ & , two consecutive periods (..), or special characters such as a tab. What, you didn’t know that after entering one of these characters in the name?  Is it because you saw this screen: Oh, that’s right….you need to turn off custom errors in the layouts folder…See this blog post for details and you’ll also need to turn off for the web application. Once you do that, you’ll see this: I wonder why the SharePoint team just doesn’t let the user know that the content type name contains illegal characters before the user hits the create button. Here’s a copy of the complete error (for the search engines): Server Error in '/' Application. -------------------------------------------------------------------------------- The content type name 'asdfadsf\asdfasf' cannot contain: \  / : * ? " # % < > { } | ~ & , two consecutive periods (..), or special characters such as a tab. 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: Microsoft.SharePoint.SPInvalidContentTypeNameException: The content type name 'asdfadsf\asdfasf' cannot contain: \  / : * ? " # % < > { } | ~ & , two consecutive periods (..), or special characters such as a tab. 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: [SPInvalidContentTypeNameException: The content type name 'asdfadsf\asdfasf' cannot contain: \  / : * ? " # % < > { } | ~ & , two consecutive periods (..), or special characters such as a tab.]    Microsoft.SharePoint.SPContentType.ValidateName(String name) +27419522    Microsoft.SharePoint.SPContentType.ValidateNameWithResource(String strVal, String& strLocalized) +423    Microsoft.SharePoint.SPContentType.set_Name(String value) +151    Microsoft.SharePoint.SPContentType.Initialize(SPContentType parentContentType, SPContentTypeCollection collection, String name) +112    Microsoft.SharePoint.SPContentType..ctor(SPContentType parentContentType, SPContentTypeCollection collection, String name) +132    Microsoft.SharePoint.ApplicationPages.ContentTypeCreatePage.BtnOK_Click(Object sender, EventArgs e) +497    System.Web.UI.WebControls.Button.OnClick(EventArgs e) +115    System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +140    System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +29    System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2981   -------------------------------------------------------------------------------- Version Information: Microsoft .NET Framework Version:2.0.50727.4927; ASP.NET Version:2.0.50727.4927

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• Built-in card-reader doesn't work. HP Compaq nx6325 notebook

  - by user10940

  I have a HP-Compaq nx6325 notebook with an built-in card-reader (SD, MS/Pro, MMC, SM, XD) and the ubuntu (10.10.) don't see it. I've tried to install it manually, with this steps (and with this tifmxx driver), but doesn't work. The compile log: $ echo /home/tvera/downloads/cr_install /home/tvera/downloads/cr_install $ make -C /lib/modules/2.6.35-25-generic/build M=/home/tvera/downloads/cr_install make[1]: Entering directory `/usr/src/linux-headers-2.6.35-25-generic' CC [M] /home/tvera/downloads/cr_install/tifm_core.o In file included from /home/tvera/downloads/cr_install/tifm_core.c:12: /home/tvera/downloads/cr_install/linux/tifm.h:128: error: field ‘cdev’ has incomplete type /home/tvera/downloads/cr_install/tifm_core.c: In function ‘tifm_uevent’: /home/tvera/downloads/cr_install/tifm_core.c:69: warning: passing argument 1 of ‘add_uevent_var’ from incompatible pointer type include/linux/kobject.h:244: note: expected ‘struct kobj_uevent_env *’ but argument is of type ‘char **’ /home/tvera/downloads/cr_install/tifm_core.c:69: warning: passing argument 2 of ‘add_uevent_var’ makes pointer from integer without a cast include/linux/kobject.h:244: note: expected ‘const char *’ but argument is of type ‘int’ /home/tvera/downloads/cr_install/tifm_core.c: At top level: /home/tvera/downloads/cr_install/tifm_core.c:161: warning: initialization from incompatible pointer type /home/tvera/downloads/cr_install/tifm_core.c: In function ‘tifm_free’: /home/tvera/downloads/cr_install/tifm_core.c:170: warning: type defaults to ‘int’ in declaration of ‘__mptr’ /home/tvera/downloads/cr_install/tifm_core.c:170: warning: initialization from incompatible pointer type /home/tvera/downloads/cr_install/tifm_core.c: At top level: /home/tvera/downloads/cr_install/tifm_core.c:177: error: unknown field ‘release’ specified in initializer /home/tvera/downloads/cr_install/tifm_core.c:178: warning: initialization from incompatible pointer type /home/tvera/downloads/cr_install/tifm_core.c: In function ‘tifm_alloc_adapter’: /home/tvera/downloads/cr_install/tifm_core.c:190: error: implicit declaration of function ‘class_device_initialize’ /home/tvera/downloads/cr_install/tifm_core.c: In function ‘tifm_add_adapter’: /home/tvera/downloads/cr_install/tifm_core.c:211: error: ‘BUS_ID_SIZE’ undeclared (first use in this function) /home/tvera/downloads/cr_install/tifm_core.c:211: error: (Each undeclared identifier is reported only once /home/tvera/downloads/cr_install/tifm_core.c:211: error: for each function it appears in.) /home/tvera/downloads/cr_install/tifm_core.c:212: error: implicit declaration of function ‘class_device_add’ /home/tvera/downloads/cr_install/tifm_core.c: In function ‘tifm_remove_adapter’: /home/tvera/downloads/cr_install/tifm_core.c:237: error: implicit declaration of function ‘class_device_del’ /home/tvera/downloads/cr_install/tifm_core.c: In function ‘tifm_free_adapter’: /home/tvera/downloads/cr_install/tifm_core.c:243: error: implicit declaration of function ‘class_device_put’ /home/tvera/downloads/cr_install/tifm_core.c: In function ‘tifm_alloc_device’: /home/tvera/downloads/cr_install/tifm_core.c:275: error: ‘struct device’ has no member named ‘bus_id’ /home/tvera/downloads/cr_install/tifm_core.c:275: error: ‘BUS_ID_SIZE’ undeclared (first use in this function) make[2]: *** [/home/tvera/downloads/cr_install/tifm_core.o] Error 1 make[1]: *** [_module_/home/tvera/downloads/cr_install] Error 2 make[1]: Leaving directory `/usr/src/linux-headers-2.6.35-25-generic' make: *** [all] Error 2 The output of lsusb: Bus 001 Device 005: ID 05e3:0702 Genesys Logic, Inc. USB 2.0 IDE Adapter Bus 003 Device 003: ID 0458:003a KYE Systems Corp. (Mouse Systems) NetScroll+ Mini Traveler Bus 003 Device 002: ID 08ff:2580 AuthenTec, Inc. AES2501 Fingerprint Sensor Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

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• Subterranean IL: Exception handler semantics

  - by Simon Cooper

  In my blog posts on fault and filter exception handlers, I said that the same behaviour could be replicated using normal catch blocks. Well, that isn't entirely true... Changing the handler semantics Consider the following: .try { .try { .try { newobj instance void [mscorlib]System.Exception::.ctor() // IL for: // e.Data.Add("DictKey", true) throw } fault { ldstr "1: Fault handler" call void [mscorlib]System.Console::WriteLine(string) endfault } } filter { ldstr "2a: Filter logic" call void [mscorlib]System.Console::WriteLine(string) // IL for: // (bool)((Exception)e).Data["DictKey"] endfilter }{ ldstr "2b: Filter handler" call void [mscorlib]System.Console::WriteLine(string) leave.s Return } } catch object { ldstr "3: Catch handler" call void [mscorlib]System.Console::WriteLine(string) leave.s Return } Return: // rest of method If the filter handler is engaged (true is inserted into the exception dictionary) then the filter handler gets engaged, and the following gets printed to the console: 2a: Filter logic 1: Fault handler 2b: Filter handler and if the filter handler isn't engaged, then the following is printed: 2a:Filter logic 1: Fault handler 3: Catch handler Filter handler execution The filter handler is executed first. Hmm, ok. Well, what happens if we replaced the fault block with the C# equivalent (with the exception dictionary value set to false)? .try { // throw exception } catch object { ldstr "1: Fault handler" call void [mscorlib]System.Console::WriteLine(string) rethrow } we get this: 1: Fault handler 2a: Filter logic 3: Catch handler The fault handler is executed first, instead of the filter block. Eh? This change in behaviour is due to the way the CLR searches for exception handlers. When an exception is thrown, the CLR stops execution of the thread, and searches up the stack for an exception handler that can handle the exception and stop it propagating further - catch or filter handlers. It checks the type clause of catch clauses, and executes the code in filter blocks to see if the filter can handle the exception. When the CLR finds a valid handler, it saves the handler's location, then goes back to where the exception was thrown and executes fault and finally blocks between there and the handler location, discarding stack frames in the process, until it reaches the handler. So? By replacing a fault with a catch, we have changed the semantics of when the filter code is executed; by using a rethrow instruction, we've split up the exception handler search into two - one search to find the first catch, then a second when the rethrow instruction is encountered. This is only really obvious when mixing C# exception handlers with fault or filter handlers, so this doesn't affect code written only in C#. However it could cause some subtle and hard-to-debug effects with object initialization and ordering when using and calling code written in a language that can compile fault and filter handlers.

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• Subterranean IL: Fault exception handlers

  - by Simon Cooper

  Fault event handlers are one of the two handler types that aren't available in C#. It behaves exactly like a finally, except it is only run if control flow exits the block due to an exception being thrown. As an example, take the following method: .method public static void FaultExample(bool throwException) { .try { ldstr "Entering try block" call void [mscorlib]System.Console::WriteLine(string) ldarg.0 brfalse.s NormalReturn ThrowException: ldstr "Throwing exception" call void [mscorlib]System.Console::WriteLine(string) newobj void [mscorlib]System.Exception::.ctor() throw NormalReturn: ldstr "Leaving try block" call void [mscorlib]System.Console::WriteLine(string) leave.s Return } fault { ldstr "Fault handler" call void [mscorlib]System.Console::WriteLine(string) endfault } Return: ldstr "Returning from method" call void [mscorlib]System.Console::WriteLine(string) ret } If we pass true to this method the following gets printed: Entering try block Throwing exception Fault handler and the exception gets passed up the call stack. So, the exception gets thrown, the fault handler gets run, and the exception propagates up the stack afterwards in the normal way. If we pass false, we get the following: Entering try block Leaving try block Returning from method Because we are leaving the .try using a leave.s instruction, and not throwing an exception, the fault handler does not get called. Fault handlers and C# So why were these not included in C#? It seems a pretty simple feature; one extra keyword that compiles in exactly the same way, and with the same semantics, as a finally handler. If you think about it, the same behaviour can be replicated using a normal catch block: try { throw new Exception(); } catch { // fault code goes here throw; } The catch block only gets run if an exception is thrown, and the exception gets rethrown and propagates up the call stack afterwards; exactly like a fault block. The only complications that occur is when you want to add a fault handler to a try block with existing catch handlers. Then, you either have to wrap the try in another try: try { try { // ... } catch (DirectoryNotFoundException) { // ... // leave.s as normal... } catch (IOException) { // ... throw; } } catch { // fault logic throw; } or separate out the fault logic into another method and call that from the appropriate handlers: try { // ... } catch (DirectoryNotFoundException ) { // ... } catch (IOException ioe) { // ... HandleFaultLogic(); throw; } catch (Exception e) { HandleFaultLogic(); throw; } To be fair, the number of times that I would have found a fault handler useful is minimal. Still, it's quite annoying knowing such functionality exists, but you're not able to access it from C#. Fortunately, there are some easy workarounds one can use instead. Next time: filter handlers.

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