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.
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