Question

I figured I could pass an IList<ChildClass> as an IEnumerable<ParentClass>, since obviously each object in a ChildType list is also an instance of ParentType. But I’m gettin no love from the compiler. What am I missing?

EDIT: Added function Foo3 which does what I want. Thanks!

namespace StackOverflow {     public class ParentClass     {     }      public class ChildClass : ParentClass     {     }      public class Test     {         // works         static void Foo(ParentClass bar2)         {         }          // fails         static void Foo2(IEnumerable<ParentClass> bar)         {         }          // EDIT: here's the right answer, obtained from the          // Charlie Calvert blog post          static void Foo3<T>(IEnumerable<T> bar) where T : ParentClass         {         }          public static void Main()         {             var childClassList = new List<ChildClass>();              // this works as expected             foreach (var obj in childClassList)                 Foo(obj);              // this won't compile             // Argument '1': cannot convert from              // 'System.Collections.Generic.List<ChildClass>'              // to 'System.Collections.Generic.IEnumerable<ParentClass>'              Foo2(childClassList);              // EDIT: this works and is what I wanted             Foo3(childClassList);         }     } } 

Answer 1

Because generics aren’t co/contra variant:

Eric Lippert’s blog has a great post on this.

Another article from Charlie Calvert is here.