C# Type Conversion Error Despite Generic Constraint
c#, constraints, covariance, generics, type-conversion
Solution
This is an example of C#'s lack of covariance on generic types (C# does support array covariance). C# 4 will add this feature on interface types and also will update several BCL interface types to support it as well.
Please see C# 4.0: Covariance and Contravariance:
In this article I’ll try to cover one of the C# 4.0 innovations. One of the new features is covariance and contravariance on type parameters that is now supported by generic delegates and generic interfaces. First let’s see what does these words mean :)
Problem
Why, with a generic constraint on type parameter T of class P of "must inherit from A", does the first call succeed but the second call fail with the type conversion error detailed in the comment: ``` abstract class A { } static class S { public static void DoFirst(A argument) { } public static void DoSecond(ICollection<A> argument) { } } static class P<T> where T : A, new() { static void Do() { S.DoFirst(new T()); // this call is OK S.DoSecond(new List<T>()); // this call won't compile with: /* cannot convert from 'System.Collections.Generic.List<T>' to 'System.Collections.Generic.ICollection<A>' */ } } ``` Shouldn't the generic constraint ensure that `List<T>` is indeed `ICollection<A>`?