Question

If I have an object which looks like:

class Person : IProxy
{
    // Properties
}

And I have a method which returns an object which is actually a List<Person>:

object GetList()
{
    List<Person> people = new List<Person>();

    person.Add(new Person());
    person.Add(new Person());

    return people;
}

Why does the following code result in null?

var obj = GetList() as List<IProxy>;

But the code below returns a List:

var obj = GetList() as List<Person>;

In the Watch panel of Visual Studio, my type is reported as:

object {System.Collections.Generic.List<Person>}

Answer 1

A List<Person> and List<IProxy> are two different types so converting one to another may yield null.

GetList().OfType<IProxy>()

will do what you want. You can also use

GetList().Cast<IProxy>()

Personally I prefer OfType because it doesn’t throw an exception when the collection contains heterogeneous types

Covariance and Contravariance FAQ may answer more of your questions.