Question

So I have an object the inherits a list.

public class Foo: List<Bar>

And I want to reorder it by BarDate which is a property on Bar

For example

Foo testFoo = GetFoo();

testFoo = testFoo.OrderBy(b => b.BarDate).ToList();

but testFoo is of type Foo not List so how could I do the cast?

Thanks

Answer 1

You can’t do that. You are setting a reference to a derived class an instance of its base which won’t work.

The reference testFoo can only be a Foo or an object derived from Foo (List<Bar> is a base class of Foo)

You can, however, do something like this:

public class Foo : List<Bar>
{
    public Foo()
    {}

    public Foo(IEnumerable<Bar> collection)
        : base(collection)
    {}
}

And in your code later on:

Foo testFoo = new Foo();
testFoo = new Foo(testFoo.OrderBy(b => b.BarDate));

The LINQ expression will return an IEnumerable<Bar>, so is compatible with one of the constructors of List<T>

Incidentally, the reason the compiler error message says “An explicit conversion exists (are you missing a cast?)” is because if you have an object that really is a Foo that happens to be referenced as a List<Bar> then you can cast it. e.g.

Foo myFoo = new Foo();
List<Bar> myList = myFoo;
Foo sameFoo = (Foo)myList;

All three references (myFoo, myList, and sameFoo) all reference the exact same Foo instance and casting will work. However, if this is the scenario:

List<Bar> actualList = new List<Bar>();
Foo listedFoo = (Foo)actualList;

Then the cast will fail because actualList isn’t a Foo or derivative of Foo.