Question

I’ve got an object, which I’ll call MyObject. It’s a class that controls a particular data row.

I’ve then got a collection class, called MyObjectCollection:

public class MyObjectCollection : List<MyObject> {}

Why can I not do the following:

List<MyObject> list = this.DoSomethingHere();
MyObjectCollection collection = (MyObjectCollection)list;

Thanks in advance.

Edit: The error is InvalidCastException

Answer 1

My guess is that DoSomethingHere doesn’t return an instance of MyObjectCollection.

Let’s get rid of all the generics etc here, as they’re not relevant. Here’s what I suspect you’re trying to do:

public static object CreateAnObject()
{
    return new object();
}

object o = CreateAnObject();
string s = (string) o;

That will fail (at execution time) and quite rightly so.

To bring it back to your code, unless DoSomethingHere actually returns a MyObjectCollection at execution time, the cast will fail.