Operator ‘?’ cannot be applied to operand of type ‘T’

Since not everything can be null, you have to narrow down T to be something nullable (aka an object). Structs can’t be null, and neither can enums.

Adding a where on class does fix the issue:

public abstract class Feature<T> where T : class

So why doesn’t it just work?

Invoke() yields T. If GetValue is null, the ? operator sets the return value of type T to null, which it can’t. If T is int for example, it can’t make it nullable (int?) since the actual type required (T = int) isn’t.

If you change T to be int in your code, you will see the problem very clearly. The end result of what you ask is this:

get
{
    int? x = GetValue?.Invoke();
    return x.GetValueOrDefault(0);
}

This is not something the null-propagation operator will do for you. If you revert to the use of default(T) it does know exactly what to do and you avoid the ‘problematic’ null-propagation.