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• Casting generics and the generic type

  - by Kragen

  Consider, I have the following 3 classes / interfaces: class MyClass<T> { } interface IMyInterface { } class Derived : IMyInterface { } And I want to be able to cast a MyClass<Derived> into a MyClass<IMyInterface> or visa-versa: MyClass<Derived> a = new MyClass<Derived>(); MyClass<IMyInterface> b = (MyClass<IMyInterface>)a; But I get compiler errors if I try: Cannot convert type 'MyClass<Derived>' to 'MyClass<IMyInterface>' I'm sure there is a very good reason why I cant do this, but I can't think of one. As for why I want to do this - The scenario I'm imagining is one whereby you ideally want to work with an instance of MyClass<Derived> in order to avoid lots of nasty casts, however you need to pass your instance to an interface that accepts MyClass<IMyInterface>. So my question is twofold: Why can I not cast between these two types? Is there any way of keeping the niceness of working with an instance of MyClass<Derived> while still being able to cast this into a MyClass<IMyInterface>?

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• C# Generics and Winform

  - by Nathan

  Is it possible to pass a generic type T into an instance of a winform so T is usable throughout the form?

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• C# .NET 4.0 and Generics

  - by Mr Snuffle

  I was wondering if anyone could tell me if this kind of behaviour is possible in C# 4.0 I have an object hierarchy I'd like to keep strongly typed. Something like this class ItemBase {} class ItemType<T> where T : ItemBase { T Base { get; set; } } class EquipmentBase : ItemBase {} class EquipmentType : ItemType<EquipmentBase> {} What I want to be able to do to have something like this ItemType item = new EquipmentType(); And I want item.Base to return type ItemBase. Basically I want to know if it's smart enough to strongly typed generic to a base class without the strong typing. Benefit of this being I can simply cast an ItemType back to an EquipmentType and get all the strongly typedness again. I may be thinking about this all wrong...

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• Get a class name with generics information

  - by Damien

  Hello, In a java serialization problem, I want to save some classes name and I have some problems with generic classes. For example : - If I have ArrayList<String> listToDump = new ArrayList<String>(); - If I take the name : listToDump.getName(); or listToDump.getCanonicalName(); - I will have java.util.ArrayList or ArrayList - And I want to have java.util.ArrayList<String> or ArrayList<String> Any ideas on how I can do this? Damien.

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• Abstract Factory Using Generics: Is Explicitly Converting a Specified Type to Generic a Bad Practice

  - by Merritt

  The question's title says it all. I like how it fits into the rest of my code, but does it smell? public interface IFoo<T> { T Bar { get; set; } } public class StringFoo : IFoo<string> { public string Bar { get; set; } } public static class FooFactory { public static IFoo<T> CreateFoo<T>() { if (typeof(T) == typeof(string)) { return new StringFoo() as IFoo<T>; } throw new NotImplementedException(); } } UPDATE: this is sort of a duplicate of Is the StaticFactory in codecampserver a well known pattern?

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• Weird generics compile error.

  - by Jouke van der Maas

  I have two classes, a base class and a child class. In the base class i define a generic virtual method: protected virtual Bar Foo<T() where T : Bar {} Then in my child class i try to do this: protected override BarChild Foo() // BarChild inherits Bar { return base.Bar<T as BarChild() } Visual studio gives this weird error: The type 'T' cannot be used as type parameter 'T' in the generic type or method 'Foo()'. There is no boxing conversion or type parameter conversion from 'T' to 'Bar'. Repeating the where clause on the child's override also gives an error: Constraints for override and explicit interface implementation methods are inherited from the base method, so they cannot be specified directly So what am i doing wrong here?

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• Interfaces with structs, by reference using Generics

  - by Fraga

  I can't pass by reference an interface with a struct in it, what am I doing wrong? Here is the example code: class Processor<T> where T : new() { public Processor() { Data = new T(); } public T Data; } class PeriodsProcessor : Processor<Periods> { public PeriodsProcessor() { DataBase DB = new DataBase(); Console.WriteLine(Data.Value); DB.ModifyData<Period>(Data); Console.WriteLine(Data.Value); Console.ReadLine(); } } public class Period { public string Name; } public interface IDataTable<T> { string Value { get; set; } T Filter { get; set; } } [Serializable] public struct Periods : IDataTable<Period> { public string Value { get; set; } public Period Filter { get; set; } } public class DataBase { public void ModifyData<T>(IDataTable<T> data) where T : new() { data.Value = "CHANGE"; } } class Program { static void Main(string[] args) { PeriodsProcessor PeriodsProcessor = new PeriodsProcessor(); } }

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• bounding java generics by 'super' keyword

  - by mohsenof

  Why I can use 'super' just with wildcards and not with type parameters? for example why in Collection interface they've not written toArray method like this interface Collection"<"T{ public "< S super T S[] toArray(S[] a){ } } (sorry, I don't know how to deal with "<")

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• Using generics in Unity ... InvalidCastException

  - by Sunny D

  Hi, My interface definition is: public interface IInterface where T:UserControl My class definition is: public partial class App1Control : UserControl, IInterface The unity section of my app.config looks as below: <unity> <typeAliases> <typeAlias alias="singleton" type="Microsoft.Practices.Unity.ContainerControlledLifetimeManager, Microsoft.Practices.Unity" /> <typeAlias alias="myInterface" type="MyApplication.IInterface`1, MyApplication" /> <typeAlias alias="App1" type="MyApplication.App1Control, MyApplication" /> </typeAliases> <containers> <container> <types> <type type="myInterface" mapTo="App1" name="Application 1"> <lifetime type="singleton"/> </type> </types> </container> </containers> </unity> The app runs fine but, the following code gives a InvalidCastException container.Resolve<IInterface<UserControl>>("Application 1"); The error message is : Unable to cast object of type 'MyApplication.App1Control' to type 'MyApplication.IInterface`1[System.Windows.Forms.UserControl]' I believe there is a minor mistake in my code ... but am not able to figure out what. Any thoughts?

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• Collections of generics

  - by Luis Sep

  According to what I've read, I think this can't be done, but I'd like to be sure. I have a class OpDTO and several other *DTO extends OpDTO. Then, I want to have a method to extract just certain elements from lists of these child DTOs, and return the extracted elements in another list: public List<? extends OpDTO> getLastOp (List<? extends OpDTO> listDTOs) { List<? extends OpDTO> last = new ArrayList<? extends OpDTO>(); //compile error: Cannot instantiate the type ArrayList<? extends OpDTO> //processing return last; } I want ult to be a list of elements of the same kind as elements in listDTOs, and use only OpDTO's methods, but it produces a compile error: Cannot instantiate the type ArrayList<? extends OpDTO> I also tried doing something like: public <T> List<T> getLastOp (List<T> listDTOs) { List<T> last = new ArrayList<T>(); //processing return last; } But then I can't enforce elements in listDTOs to be a subclass of OpDTO, and can't instantiate T. Any idea?

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• Java generics SuppressWarnings("unchecked") mystery

  - by Johannes Ernst

  Why does code alternative(1) compile without warnings, and code alternative(2) produce an "unchecked cast" warning? Common for both: class Foo<T> { Foo( T [] arg ) { } } Alternative (1): class Bar<T> extends Foo<T> { protected static final Object [] EMPTY_ARRAY = {}; @SuppressWarnings("unchecked") Bar() { super( (T []) EMPTY_ARRAY ); } } Alternative (2): class Bar<T> extends Foo<T> { @SuppressWarnings("unchecked") Bar() { super( (T []) EMPTY_ARRAY ); } protected static final Object [] EMPTY_ARRAY = {}; } Alternative (2) produces: javac -Xlint:unchecked Foo.java Bar.java Bar.java:4: warning: [unchecked] unchecked cast super( (T []) EMPTY_ARRAY ); ^ required: T[] found: Object[] where T is a type-variable: T extends Object declared in class Bar 1 warning This is: java version "1.7.0_07" Java(TM) SE Runtime Environment (build 1.7.0_07-b10) Java HotSpot(TM) 64-Bit Server VM (build 23.3-b01, mixed mode)

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• Why does using Collections.emptySet() with generics work in assignment but not as a method parameter

  - by Karl von L

  So, I have a class with a constructor like this: public FilterList(Set<Integer> labels) { ... } and I want to construct a new FilterList object with an empty set. Following Joshua Bloch's advice in his book Effective Java, I don't want to create a new object for the empty set; I'll just use Collections.emptySet() instead: FilterList emptyList = new FilterList(Collections.emptySet()); This gives me an error, complaining that java.util.Set<java.lang.Object> is not a java.util.Set<java.lang.Integer>. OK, how about this: FilterList emptyList = new FilterList((Set<Integer>)Collections.emptySet()); This also gives me an error! Ok, how about this: Set<Integer> empty = Collections.emptySet(); FilterList emptyList = new FilterList(empty); Hey, it works! But why? After all, Java doesn't have type inference, which is why you get an unchecked conversion warning if you do Set<Integer> foo = new TreeSet() instead of Set<Integer> foo = new TreeSet<Integer>(). But Set<Integer> empty = Collections.emptySet(); works without even a warning. Why is that?

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• WPF View/ViewModels using Generics- how?

  - by Investor5555

  New to WPF, so please bear with me... Suppose I have 2 tables in SQL Thing OtherThing Both have the exact same fields: ID (int) Name (string) Description (string) IsActive (bit/bool) DateModified (DateTime) So, I want to create one Model (not two) and do something like this: BaseModel<T>() { public int ID {get;set;} ... } etc. (of course, using the INotifyPropertyChanged, just trying to keep the code simple). I want to be able to create a BaseView and BaseViewModel that would work with whatever model conforms to the Thing/OtherThing. I am really at a loss as to what to do here to make this generic, so I don't have to have a ThingView/ThingViewModel and a OtherThingView/OtherThingViewModel... It seems that this should be simple, but I cannot seem to figure it out. Does anyone have a code example where they could interchange various items from a dropdown list using one view, one ViewModel, and one base datamodel (and switching out the type from a dropdown)? For example, a combobox has 3 identical table structures for Thing OtherThing SomeThing and on selection changed, I want to pull the data from whatever table was selected, to be able to do standard CRUD operations on any of these 3 tables, without having to create concrete classes for each view/viewmodel.

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• Refactoring exercise with generics

  - by Berryl

  I have a variation on a Quantity (Fowler) class that is designed to facilitate conversion between units. The type is declared as: public class QuantityConvertibleUnits<TFactory> where TFactory : ConvertableUnitFactory, new() { ... } In order to do math operations between dissimilar units, I convert the right hand side of the operation to the equivalent Quantity of whatever unit the left hand side is in, and do the math on the amount (which is a double) before creating a new Quantity. Inside the generic Quantity class, I have the following: protected static TQuantity _Add<TQuantity>(TQuantity lhs, TQuantity rhs) where TQuantity : QuantityConvertibleUnits<TFactory>, new() { var toUnit = lhs.ConvertableUnit; var equivalentRhs = _Convert<TQuantity>(rhs.Quantity, toUnit); var newAmount = lhs.Quantity.Amount + equivalentRhs.Quantity.Amount; return _Convert<TQuantity>(new Quantity(newAmount, toUnit.Unit), toUnit); } protected static TQuantity _Subtract<TQuantity>(TQuantity lhs, TQuantity rhs) where TQuantity : QuantityConvertibleUnits<TFactory>, new() { var toUnit = lhs.ConvertableUnit; var equivalentRhs = _Convert<TQuantity>(rhs.Quantity, toUnit); var newAmount = lhs.Quantity.Amount - equivalentRhs.Quantity.Amount; return _Convert<TQuantity>(new Quantity(newAmount, toUnit.Unit), toUnit); } ... same for multiply and also divide I need to get the typing right for a concrete Quantity, so an example of an add op looks like: public static ImperialLengthQuantity operator +(ImperialLengthQuantity lhs, ImperialLengthQuantity rhs) { return _Add(lhs, rhs); } The question is those verbose methods in the Quantity class. The only change between the code is the math operator (+, -, *, etc.) so it seems that there should be a way to refactor them into a common method, but I am just not seeing it. How can I refactor that code? Cheers, Berryl

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• c# Generics problem

  - by UpTheCreek

  Can anyone tell me why this does not work? I would have thought the constraint would make it valid. public class ClassA<T> where T : ICommon { public ClassA() { ClassB b = new b(); IEnumerable<T> alist = new IList<T>; b.items = alist; //Error: cannot convert from IEnumerable<T> to IEnumerable<ICommon>' } } public class ClassB { IEnumerable<ICommon> items { get; set;} .... }

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• C# simpler run time generics

  - by Hellfrost

  Is there a way to invoke a generic function with a type known only at run time? I'm trying to do something like: static void bar() { object b = 6; string c = foo<typeof(b)>(); } static string foo<T>() { return typeof (T).Name; } Basically I want to decide on the type parameter only at run time, but the function I'm calling depends on the type parameter. Also I know this can be done with reflections... but it's not the nicest solution to the problem... I'm sort of looking for dynamic features in C#...

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• C# Fun with Generics - Mutual Dependencies

  - by Kenneth Cochran

  As an experiment I'm trying to write a generic MVP framework. I started with: public interface IPresenter<TView> where TView: IView<IPresenter<... { TView View { get; set;} } public interface IView<TPresenter> where TPresenter:IPresenter<IView<... { TPresenter Presenter { get; set; } } Obviously this can't work because the types of TView and TPresenter can't be resolved. You'd be writing Type<Type<... forever. So my next attempt looked like this: public interface IView<T> where T:IPresenter { ... } public interface IView:IView<IPresenter> { } public interface IPresenter<TView> where TView: IView { ... } public interface IPresenter: IPresenter<IView> { ... } This actually compiles and you can even inherit from these interfaces like so: public class MyView : IView, IView<MyPresenter> { ... } public class MyPresenter : IPresenter, IPresenter<MyView> { ... } The problem is in the class definition you have to define any members declared in the generic type twice. Not ideal but it still compiles. The problem's start creeping up when you actually try to access the members of a Presenter from a View or vice versa. You get an Ambiguous reference when you try to compile. Is there any way to avoid this double implementation of a member when you inherit from both interfaces? Is it even possible to resolve two mutually dependent generic types at compile time?

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• JAVA Inheritance Generics and Casting.

  - by James Moore

  Hello, I have two classes which both extends Example. public class ClassA extends Example { public ClassA() { super("a", "class"); } .... } public class ClassB extends Example { public ClassB() { super("b", "class"); } .... } public class Example () { public String get(String x, String y) { return "Hello"; } } So thats all very well. So suppose we have another class called ExampleManager. With example manager I want to use a generic type and consequently return that generic type. e.g. public class ExampleManager<T extends Example> { public T getExample() { return new T("example","example"); // So what exactly goes here? } } So where I am returning my generic type how do i get this to actually work correctly and cast Example as either classA or classB? Many Thanks

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• With this generics code why am I getting "Argument 1: cannot convert from 'ToplogyLibrary.Relationsh

  - by Greg

  Hi, Any see why I'm getting a "Argument 1: cannot convert from 'ToplogyLibrary.RelationshipBase' to 'TRelationship'" in the code below, in CreateRelationship() ? public class TopologyBase<TKey, TNode, TRelationship> where TNode : NodeBase<TKey>, new() where TRelationship : RelationshipBase<TKey>, new() { // Properties public Dictionary<TKey, TNode> Nodes { get; private set; } public List<TRelationship> Relationships { get; private set; } // Constructors protected TopologyBase() { Nodes = new Dictionary<TKey, TNode>(); Relationships = new List<TRelationship>(); } // Methods public TNode CreateNode(TKey key) { var node = new TNode {Key = key}; Nodes.Add(node.Key, node); return node; } public void CreateRelationship(TNode parent, TNode child) { // Validation if (!Nodes.ContainsKey(parent.Key) || !Nodes.ContainsKey(child.Key)) { throw new ApplicationException("Can not create relationship as either parent or child was not in the graph: Parent:" + parent.Key + ", Child:" + child.Key); } // Add Relationship var r = new RelationshipBase<TNode>(); r.Parent = parent; r.Child = child; Relationships.Add(r); // *** HERE *** "Argument 1: cannot convert from 'ToplogyLibrary.RelationshipBase<TNode>' to 'TRelationship'" } } public class RelationshipBase<TNode> { public TNode Parent { get; set; } public TNode Child { get; set; } } public class NodeBase<T> { public T Key { get; set; } public NodeBase() { } public NodeBase(T key) { Key = key; } }

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• Using generics in F# to create an EnumArray type

  - by Matthew

  I've created an F# class to represent an array that allocates one element for each value of a specific enum. I'm using an explicit constructor that creates a dictionary from enum values to array indices, and an Item property so that you can write expressions like: let my_array = new EnumArray<EnumType, int> my_array.[EnumType.enum_value] <- 5 However, I'm getting the following obscure compilation error at the line marked with '// FS0670' below. error FS0670: This code is not sufficiently generic. The type variable ^e when ^e : enum<int> and ^e : equality and ^e : (static member op_Explicit : ^e -> int) could not be generalized because it would escape its scope. I'm at a loss - can anyone explain this error? type EnumArray< 'e, 'v when 'e : enum<int> and 'e : equality and ^e : (static member op_Explicit : ^e -> int) > = val enum_to_int : Dictionary<'e, int> val a : 'v array new() as this = { enum_to_int = new Dictionary<'e, int>() a = Array.zeroCreate (Enum.GetValues(typeof<'e>).Length) } then for (e : obj) in Enum.GetValues(typeof<'e>) do this.enum_to_int.Add(e :?> 'e, int(e :?> 'e)) member this.Item with get (idx : 'e) : 'v = this.a.[this.enum_to_int.[idx]] // FS0670 and set (idx : 'e) (c : 'v) = this.a.[this.enum_to_int.[idx]] <- c

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• Service locator for generics

  - by vittore

  Hi everyone, I have say a dozen types T which inherit from EntityObject and IDataObject. I have generic the following interface IDataManager<T> where T : EntityObject, IDataObject ... I have also base class for data managers BaseDataManager<T> : IDataManager<T> where T : EntityObject, IDataObject .... And i have particular classes public class Result : EntityObject, IDataObject .... public class ResultDataManager : BaseDataManager<Result> ... I want to implement service locator, which will return instance of IDataManager<T> for T But I stucked how to implement it in a neat way without a lot of castings. Any ideas?

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• C# generics method invocation

  - by Firat KÜÇÜK

  Hi, i have some polymorphic methods and i want to call via using an intermediate method. Following class is the simplified version of my program. class Program { public class A { } public class B { } public class C { } public void SomeMethod(A value) { Console.WriteLine("A value"); } public void SomeMethod(B value) { Console.WriteLine("B value"); } public void SomeMethod(C value) { Console.WriteLine("C value"); } static void Main(string[] args) { Program p = new Program(); // code block p.IntermediateMethod<A>(new A()); p.IntermediateMethod<B>(new B()); p.IntermediateMethod<C>(new C()); } public void IntermediateMethod<T>(T value) { // code block SomeMethod(value); // code block } }

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• Java generics: Illegal forward reference

  - by Arian

  Given a generic interface interface Foo<A, B> { } I want to write an implementation that requires A to be a subclass of B. So I want to do class Bar<A, B super A> implements Foo<A, B> { } // --> Syntax error or class Bar<A extends B, B> implements Foo<A, B> { } // --> illegal forward reference But the only solution that seems to work is this: class Bar<B, A extends B> implements Foo<A, B> { } which is kind of ugly, because it reverses the order of the generic parameters. Are there any solutions or workarounds to this problem?

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• Java Generics Class Parameter Type Inference

  - by Pindatjuh

  Given the interface: public interface BasedOnOther<T, U extends BasedList<T>> { public T getOther(); public void staticStatisfied(final U list); } The BasedOnOther<T, U extends BasedList<T>> looks very ugly in my use-cases. It is because the T type parameter is already defined in the BasedList<T> part, so the "uglyness" comes from that T needs to be typed twice. Problem: is it possible to let the Java compiler infer the generic T type from BasedList<T> in a generic class/interface definition? Ultimately, I'd like to use the interface like: class X implements BasedOnOther<BasedList<SomeType>> { public SomeType getOther() { ... } public void staticStatisfied(final BasedList<SomeType> list) { ... } } // Does not compile, due to invalid parameter count. Instead: class X implements BasedOnOther<SomeType, BasedList<SomeType>> { public SomeType getOther() { ... } public void staticStatisfied(final BasedList<SomeType> list) { ... } }

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• C# Pass Generics At Runtime

  - by TheCloudlessSky

  I have a method like the following: public IEnumerable<T> GetControls<T>() : where T : ControlBase { // removed. } I then created a class: public class HandleBase<TOwner> : ControlBase : TOwner { // Removed } I'd like to be able to call GetControls<HandleBase<this.GetType()>>; where it would use the type of THIS class to pass to the HandleBase. This would in essentially get all HandleBase that have an owner of THIS type. How can I achieve this?

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