C# generic constraints for IEnumerable

c#, generics

Solution

The problem is that generic covariance only applies for reference types. For example, a `List<int>` is not* an `IEnumerable<Comparable>`, but a `List<string>` is.

So if you constrain `T` to be a reference type, it compiles:

public void Foo<T, I>() where T : class, I 
{
    IEnumerable<T> t = new List<T>();
    new List<I>().AddRange(t);
}

Problem

``` public interface I { } public class A : I { } ``` The compiler takes explicit `IEnumerable<A>` as `IEnumerable<I>` : ``` public void Test() { IEnumerable<A> a = new List<A>(); new List<I>().AddRange(a); } ``` But with generic constraints, we get: ``` public void Test<T>() where T : I { IEnumerable<T> t = new List<T>(); new List<I>().AddRange(t); } ^^^ Argument 1: cannot convert from 'IEnumerable<T>' to 'IEnumerable<I>' ``` However, this compiles fine. ``` public void Test<T>(T t) where T : I { new List<I>().Add(t); } ``` Hence the question: is this a correct behavior, or is it a bug?

Original source