Question

I have an interface defined as:

public interface MyInterface {
     object foo { get; set; };
}

and a class that implements that interface:

public class MyClass : MyInterface {
     object foo { get; set; }
}

I then create a function that returns a ICollection like so:

public ICollection<MyClass> Classes() {
    List<MyClass> value;

    List<MyInterface> list = new List<MyInterface>(
        new MyInterface[] {
            new MyClass {
                ID = 1
            },
            new MyClass {
                ID = 1
            },
            new MyClass {
                ID = 1
            }
        });

    value = new List<MyClass>((IEnumerable<MyClass>) list);

    return value;
}

It would compile but would throw a

Unable to cast object of type
‘System.Collections.Generic.List1[MyInterface]'
to type
'System.Collections.Generic.IEnumerable1[MyClass]’.

exception. What am I doing wrong?

Answer 1

A List<MyInterface> cannot be converted to a List<MyClass> in general, because the first list might contain objects that implement MyInterface but which aren’t actually objects of type MyClass.

However, since in your case you know how you constructed the list and can be sure that it contains only MyClass objects, you can do this using Linq:

return list.ConvertAll(o => (MyClass)o);