Question

I would like to know why the generic method below does not compile (C# 4.0). Neither my colleagues nor me can figure it out.

The compiler complains that this which is a SelectableRowBase cannot be converted to a TItem which is constrained to derive from SelectableRowBase. If I put an explicit cast everything works (and that is what I checked in source control), but I would like to know why the compiler is complaining here.

Is that a compiler error? Am I missing something?

public class SelectableRowBase
{
    public void AddToSelectedListWhenIsSelectedChanges<TItem>
            (ObservableCollection<TItem> selectedList)

        where TItem : SelectableRowBase
    {
        // Causes error:
        // The best overloaded method match for 
        //     'System.Collections.ObjectModel.Collection<TItem>.Add(TItem)'
        //      has some invalid arguments
        // Argument 1: cannot convert from SelectableRowBase' to 'TItem'
        Action actionOnSelected = () => selectedList.Add(this);
        Action actionOnDeselected = () => selectedList.Remove(this);

        // Compiles and works fine
        // only difference is the explicit cast
        Action actionOnSelected = () => selectedList.Add((TItem)this);
        Action actionOnDeselected = () => selectedList.Remove((TItem)this);
    }
}

Answer 1

It’s entirely reasonable for it not to work.

You’re trying to add an instance of a base type to a collection of a potential derived type.

That’s like trying to write:

List<string> strings = new List<string>();
strings.Add(new object());

That would violate type safety.

Basically, there’s absolutely no guarantee that this is an instance of TItem. It might be which is why the cast compiles – but equally it might not be. That cast is a really bad idea – it’s leaving type safety to execution time, which is the opposite of the purpose of generics.