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  • returning a Void object

    - by Robert
    What is the correct way to return a Void type, when it isn't a primitive? Eg. I currently use null as below. interface B<E>{ E method(); } class A implements B<Void>{ public Void method(){ // do something return null; } }

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  • When using a repository is it possible for a type to return a Func that the repository uses to test for existing entities?

    - by Scott Rickman
    For example given a Factory with a method public static T Save<T>(T item) where T : Base, new() { /* item.Id == Guid.Empty therefore item is new */ if (item.Id == Guid.Empty && repository.GetAll<T>(t => t.Name == item.Name)) { throw new Exception("Name is not unique"); } } how do I create a property of Base (say MustNotAlreadyExist) so that I can change the method above to public static T Save<T>(T item) where T : Base, new() { /* item.Id == Guid.Empty therefore item is new */ if (item.Id == Guid.Empty && repository.GetAll<T>(t.MustNotAlreadyExist)) { throw new Exception("Name is not unique"); } } public class Base { ... public virtual Expression<Func<T, bool>> MustNotAlreadyExist() { return (b => b.Name == name); /* <- this clearly doesn't work */ } } and then how can I override MustNotAlreadyExist in Account : Base public class Account : Base { ... public override Expression<Func<T, bool>> MustNotAlreadyExist() { return (b => b.Name == name && b.AccountCode == accountCode); /* <- this doesn't work */ } ... }

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  • How to use method hiding (new) with generic constrained class

    - by ongle
    I have a container class that has a generic parameter which is constrained to some base class. The type supplied to the generic is a sub of the base class constraint. The sub class uses method hiding (new) to change the behavior of a method from the base class (no, I can't make it virtual as it is not my code). My problem is that the 'new' methods do not get called, the compiler seems to consider the supplied type to be the base class, not the sub, as if I had upcast it to the base. Clearly I am misunderstanding something fundamental here. I thought that the generic where T: xxx was a constraint, not an upcast type. This sample code basically demonstrates what I'm talking about. using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace GenericPartialTest { class ContextBase { public string GetValue() { return "I am Context Base: " + this.GetType().Name; } public string GetOtherValue() { return "I am Context Base: " + this.GetType().Name; } } partial class ContextSub : ContextBase { public new string GetValue() { return "I am Context Sub: " + this.GetType().Name; } } partial class ContextSub { public new string GetOtherValue() { return "I am Context Sub: " + this.GetType().Name; } } class Container<T> where T: ContextBase, new() { private T _context = new T(); public string GetValue() { return this._context.GetValue(); } public string GetOtherValue() { return this._context.GetOtherValue(); } } class Program { static void Main(string[] args) { Console.WriteLine("Simple"); ContextBase myBase = new ContextBase(); ContextSub mySub = new ContextSub(); Console.WriteLine(myBase.GetValue()); Console.WriteLine(myBase.GetOtherValue()); Console.WriteLine(mySub.GetValue()); Console.WriteLine(mySub.GetOtherValue()); Console.WriteLine("Generic Container"); Container<ContextBase> myContainerBase = new Container<ContextBase>(); Container<ContextSub> myContainerSub = new Container<ContextSub>(); Console.WriteLine(myContainerBase.GetValue()); Console.WriteLine(myContainerBase.GetOtherValue()); Console.WriteLine(myContainerSub.GetValue()); Console.WriteLine(myContainerSub.GetOtherValue()); Console.ReadKey(); } } }

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  • Generic delegate instances

    - by Luc C
    I wonder if C# (or the underlying .NET framework) supports some kind of "generic delegate instances": that is a delegate instance that still has an unresolved type parameter, to be resolved at the time the delegate is invoked (not at the time the delegate is created). I suspect this isn't possible, but I'm asking it anyway... Here is an example of what I'd like to do, with some "???" inserted in places where the C# syntax seems to be unavailable for what I want. (Obviously this code doesn't compile) class Foo { public T Factory<T>(string name) { // implementation omitted } } class Test { public void TestMethod() { Foo foo = new Foo(); ??? magic = foo.Factory; // No type argument given here yet to Factory! // What would the '???' be here (other than 'var' :) )? string aString = magic<string>("name 1"); // type provided on call int anInt = magic<int>("name 2"); // another type provided on another call // Note the underlying calls work perfectly fine, these work, but i'd like to expose // the generic method as a delegate. string aString2 = foo.Factory<string>("name 1"); int anInt2 = foo.Factory<int>("name 2"); } } Is there a way to actually do something like this in C#? If not, is that a limitation in the language, or is it in the .NET framework?

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  • Html.Editor() helper in ASP.NET MVC 3 does not work as expected with array in model

    - by SlimShaggy
    In my ASP.NET MVC 3 application I have classes like the following: public class Localization<T> { public int VersionID { get; set; } public T Value { get; set; } ... } public class Localizable<T> { public Localization<T>[] Name { get; set; } ... } Then, I have the following view: @model dynamic ... @for (int i = 0; i < VersionCount; i++) { ... @Html.Editor(string.Format("Name[{0}.Value", i)) ... } Now, when I display this view, passing a subclass of Localizable<string> as the model, the textboxes for the strings are rendered, but they are empty. If I replace @Html.Editor(string.Format("Name[{0}.Value", i)) with @InputExtensions.TextBox(Html, string.Format("Name[{0}].Value", i), Model.Name[i].Value), the textboxes are correctly filled with values from the model. However, using TextBox instead of Editor is not an option for me, because I want to use different editor templates for different types of T. So, what am I doing wrong, or is it a bug in MVC, and is there any workaround?

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  • Is it possible to write a generic +1 method for numeric box types in Java?

    - by polygenelubricants
    This is NOT homework. Part 1 Is it possible to write a generic method, something like this: <T extends Number> T plusOne(T num) { return num + 1; // DOESN'T COMPILE! How to fix??? } Short of using a bunch of instanceof and casts, is this possible? Part 2 The following 3 methods compile: Integer plusOne(Integer num) { return num + 1; } Double plusOne(Double num) { return num + 1; } Long plusOne(Long num) { return num + 1; } Is it possible to write a generic version that bound T to only Integer, Double, or Long?

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  • Overriding inherited generic methods

    - by jess
    I have this code in base class protected virtual bool HasAnyStuff<TObject>(TObject obj) where TObject:class { return false; } In child class I am overriding protected override bool HasAnyStuff<Customer>(Customer obj) { //some stuff if Customer.sth etc return false; } I am getting this error '''Type parameter declaration must be an identifier not a type''' What is it I am doing wrong here?

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  • Why isn't the new() generic constraint satisfied by a class with optional parameters in the construc

    - by Joshua Flanagan
    The following code fails to compile, producing a "Widget must be a non-abstract type with a public parameterless constructor" error. I would think that the compiler has all of the information it needs. Is this a bug? An oversight? Or is there some scenario where this would not be valid? public class Factory<T> where T : new() { public T Build() { return new T(); } } public class Widget { public Widget(string name = "foo") { Name = name; } public string Name { get; set; } } public class Program { public static void Main() { var widget = new Widget(); // this is valid var factory = new Factory<Widget>(); // compiler error } }

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  • unable to pas derived List<>

    - by Tarscher
    Hi all, I have class A {} class B : A {} I also have a method that expects a List parameter void AMethod(List<A> parameter) {} Why can't I List<B> bs = new List<B>(); AMethod(bs); And secondly what is the most elegant way to make this work? regards

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  • What's the most efficient way to combine two List(Of String)?

    - by Jason Towne
    Let's say I've got: Dim los1 as New List(Of String) los1.Add("Some value") Dim los2 as New List(Of String) los2.Add("More values") What would be the most efficient way to combine the two into a single List(Of String)? Edit: While I'm loving the solutions everyone has provided so far, I probably should also mention I'm stuck using the .NET 2.0 framework. Any other suggestions?

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  • Generic Dictionary - Getting Conversion Error

    - by pm_2
    The following code is giving me an error: // GetDirectoryList() returns Dictionary<string, DirectoryInfo> Dictionary<string, DirectoryInfo> myDirectoryList = GetDirectoryList(); // The following line gives a compile error foreach (Dictionary<string, DirectoryInfo> eachItem in myDirectoryList) The error it gives is as follows: Cannot convert type 'System.Collections.Generic.KeyValuePair<string,System.IO.DirectoryInfo>' to 'System.Collections.Generic.Dictionary<string,System.IO.DirectoryInfo>’ My question is: why is it trying to perform this conversion? Can I not use a foreach loop on this type of object?

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  • Common type for generic classes of different types

    - by DinGODzilla
    I have (for example) Dictionary of different generic types (d1, d2, d3, d4) and I want to store them in something var d1 = new Dictionary<int, string>(); var d2 = new Dictionary<int, long>(); var d3 = new Dictionary<DateTime, bool>(); var d4 = new Dictionary<string, object>(); var something = ??? //new List<object> {d1, d2, d3, d4}; Is there any other way how to store that in something with common denominator different than object? Thanks :-)

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  • Finding the specific type held in an ArrayList<Object> (ie. Object = String, etc.)

    - by Christopher Griffith
    Say I have an ArrayList that I have cast to an ArrayList of objects. I know that all the objects that were in the ArrayList I cast were of the same type, but not what the type was. Now, if the ArrayList is not empty, I could take one of the objects in it and use the instanceof operator to learn what the actual type is. But what of the case where the ArrayList is empty? How do I determine what type Object actually is then? Is it possible?

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  • Catch a generic exception in Java?

    - by Alex Baranosky
    We use JUnit 3 at work and there is no ExpectedException annotation. I wanted to add a utility to our code to wrap this: try { someCode(); fail("some error message"); } catch (SomeSpecificExceptionType ex) { } So I tried this: public static class ExpectedExceptionUtility { public static <T extends Exception> void checkForExpectedException(String message, ExpectedExceptionBlock<T> block) { try { block.exceptionThrowingCode(); fail(message); } catch (T ex) { } } } However, Java cannot use generic exception types in a catch block, I think. How can I do something like this, working around the Java limitation? Is there a way to check that the ex variable is of type T?

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  • Why does this static factory method involving implied generic types, work?

    - by Cheeso
    Consider public class Tuple<T1, T2> { public Tuple(T1 v1, T2 v2) { V1 = v1; V2 = v2; } public T1 V1 { get; set; } public T2 V2 { get; set; } } public static class Tuple { // MAGIC!! public static Tuple<T1, T2> New<T1, T2>(T1 v1, T2 v2) { return new Tuple<T1, T2>(v1, v2); } } Why does the part labeled "MAGIC" in the above work? It allows syntax like Tuple.New(1, "2") instead of new Tuple<int, string>(1, "2"), but ... how and why? Why do I not need Tuple.New<int,string>(1, "2") ??

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  • Comparing the values of two generic Numbers

    - by PartlyCloudy
    I want to compare to variables, both of type T extends Number. Now I want to know which of the two variables is greater than the other or equal. Unfortunately I don't know the exact type yet, I only know that it will be a subtype of java.lang.Number. How can I do that? Thanks!

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  • (Action<T>).Name does not return expected values

    - by Tomas Lycken
    I have the following method (used to generate friendly error messages in unit tests): protected string MethodName<TTestedType>(Action<TTestedType> call) { return string.Format("{0}.{1}", typeof(TTestedType).FullName, call.Method.Name); } But when I call it as follows, I don't get the expected results: var nm = MethodName<MyController>(ctrl => ctrl.Create()); After running this code, nm contains "<Create_CreateShowsView>b__8", and not (as expected) "Create". How should I change the code to obtain the expected result?

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  • Why can't these generic type parameters be inferred?

    - by Jon M
    Given the following interfaces/classes: public interface IRequest<TResponse> { } public interface IHandler<TRequest, TResponse> where TRequest : IRequest<TResponse> { TResponse Handle(TRequest request); } public class HandlingService { public TResponse Handle<TRequest, TResponse>(TRequest request) where TRequest : IRequest<TResponse> { var handler = container.GetInstance<IHandler<TRequest, TResponse>>(); return handler.Handle(request); } } public class CustomerResponse { public Customer Customer { get; set; } } public class GetCustomerByIdRequest : IRequest<CustomerResponse> { public int CustomerId { get; set; } } Why can't the compiler infer the correct types, if I try and write something like the following: var service = new HandlingService(); var request = new GetCustomerByIdRequest { CustomerId = 1234 }; var response = service.Handle(request); // Shouldn't this know that response is going to be CustomerResponse? I just get the 'type arguments cannot be inferred' message. Is this a limitation with generic type inference in general, or is there a way to make this work?

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  • is this possible: c# collection of Type with constrains, or collection of generic type?

    - by Jon
    I'm trying to store types in a collection, so that i can later instantiate objects of the types in the collection. But I'm not sure how to do this the best way. What i have so far: List<Type> list = new List<Type>(); list.Add(typeof(MyClass)); var obj = (MyClass)Activator.CreateInstance(list[0]); I would like to have some constrains on the Type, or better yet, just a generic type in the collection instead of an instantiated Type object. Is this possible?

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  • Java generic function for performing calculations on integer, on double?

    - by Daniel
    Is this possible? Surely if you passed in a double, any sort of function implementation code which casts an object to an Integer would not be able to work unless the cast 'Integer' was specifically used? I have a function like: public static void increment(Object o){ Integer one = (Integer)o; system.out.println(one++); } I cant see how this could be made generic for a double? I tried public static <E> void increment(E obj){ E one = (E)o; system.out.println(one++); } but it didn't like it?

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  • How can I define multiple types with the same name and different type parameters using Reflection Em

    - by wawa
    How can I generate types like these using the System.Reflection.Emit libraries: public class Test<T> {} public class Test<T1, T2> {} When I call ModuleBuilder.DefineType(string) with the second type declaration, I get an exception because there is already another type in the module with the same name (I've already defined the type parameter on the first type). Any ideas?

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  • Suggestions on Working with this Inherited Generic Method

    - by blu
    We have inherited a project that is a wrapper around a section of the core business model. There is one method that takes a generic, finds items matching that type from a member and then returns a list of that type. public List<T> GetFoos<T>() { List<IFoo> matches = Foos.FindAll( f => f.GetType() == typeof(T) ); List<T> resultList = new List<T>(); foreach (var match in matches) { resultList.Add((T)obj); } } Foos can hold the same object cast into various classes in inheritance hierarchy to aggregate totals differently for different UI presentations. There are 20+ different types of descendants that can be returned by GetFoos. The existing code basically has a big switch statement copied and pasted throughout the code. The code in each section calls GetFoos with its corresponding type. We are currently refactoring that into one consolidated area, but as we are doing that we are looking at other ways to work with this method. One thought was to use reflection to pass in the type, and that worked great until we realized the Invoke returned an object, and that it needed to be cast somehow to the List <T>. Another was to just use the switch statement until 4.0 and then use the dynamic language options. We welcome any alternate thoughts on how we can work with this method. I have left the code pretty brief, but if you'd like to know any additional details please just ask.

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  • [C#] How do I make hierarchy of objects from two alternating classes?

    - by Millicent
    Here's the scenario: I have two classes ("Page" and "Field"), that are descended from a common class ("Pield"). They represent tags in an XML file and are in the following hierarchy: <page> <field> <page> ... </page> ... </field> ... </page> I.e.: Page and Field objects are in a hierarchy of alternating type (there may be more than one Page or Field to each rung of the hierarchy). Every Field and Page object has a parent property, which points to the respective parent object of the other type. This is not a problem unless the parent-child mechanism is controlled by the base class (Pield), which is shared by the two descended classes (Page and Field). Here is one try, that fails at the line "Pield child = new Pield(pchild, this);": class Pield<T> { private T _pield_parent; ... private void get_children() { ... Pield<Page> child = new Pield<Page>(pchild, this); ... } ... } class Page : Pield<Field> { ... } class Field : Pield<Page> { ... } Any ideas about how to solve this elegantly? Best, Millicent

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  • A strange error in java generic.

    - by ???
    This is ok: Class<? extends String> stringClass = "a".getClass(); But this gets error: <T> void f(T obj) { Class<? extends T> objClass = obj.getClass(); } I know I can cast it like: <T> void f(T obj) { @SuppressWarnings("unchecked") Class<? extends T> objClass = (Class<? extends T>) obj.getClass(); } But why the previous error? Will the next release of Java 7 will support such usage?

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