Operator '?' cannot be applied to operand

问题:

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.

回答1:

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.

回答2:

T must be a reference type or a nullable type

public abstract class Feature<T> where T : class
{
    // ...
}

回答3:

As far as I know the ?. operator is hardcoded to work with null, that is, it works for reference types or nullable value types, but not normal value types. The problem is likely that the operator returns null if the expression was null instead of default(T).

You might be able to fix it by restricting T to class here.