Question

I’m targeting version 4.0 of the .Net framework. I wrote the following simple code:

public class A
{
    public A()
    {
    }
}

public class B
{
    public B()
    {
    }

    public static implicit operator A(B b)
    {
        return new A();
    }
}

Then I created a generic list:

 var mylist = typeof(List<>).MakeGenericType(typeof(A)).GetConstructor(Type.EmptyTypes).Invoke(new object[]{});

When I want add a new instance of B the following code works:

((IList<A>)mylist).Add(new B());

However if I run the below code the following exception is thrown:

The value "B" is not of type "A" and cannot be used in this generic collection.

Parameter name: value

((IList)mylist).Add(new B());

Answer 1

The implicit conversion happens only in the first Add as you are adding an object of type B to a IList<A>. In the second case, you are adding it to the non-generic IList, which is a collection of objects and the implicit conversion doesn’t take place and bombs when the object is actually added to your List<A>

Even this fails:

var mylist = new List<A>();
((IList<A>)mylist).Add(new B());
((IList)mylist).Add(new B())

You can do this, however:

((IList)mylist).Add((A)new B());