Operator '?' cannot be applied to operand of type 'T'

c#, c#-6.0, delegates, generics, null-propagation-operator

Solution

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.

Problem

Trying to make `Feature` generic and then suddenly compiler said Operator '?' cannot be applied to operand of type 'T' Here is the code ``` public abstract class Feature<T> { public T Value { get { return GetValue?.Invoke(); } // here is error set { SetValue?.Invoke(value); } } public Func<T> GetValue { get; set; } public Action<T> SetValue { get; set; } } ``` It is possible to use this code instead ``` get { if (GetValue != null) return GetValue(); return default(T); } ``` But I am wondering how to fix that nice C# 6.0 one-liner.

Original source