C# 7.3 Enum constraint: Why can't I use the nullable enum?

Question

Now that we have enum constraint, why doesn\'t compiler allow me to write this code?

public static TResult? ToEnum(this String value, TResult?         


        

Answer 1

You can, but you have to add another constraint: the struct constraint.

public static void DoSomething<T>(T? defaultValue) where T : struct, Enum
{
}

Answer 2

Because System.Enum is a class, you cannot declare a variable of type Nullable<Enum> (since Nullable<T> is only possible if T is a struct).

Thus:

Enum? bob = null;

won't compile, and neither will your code.

This is definitely strange (since Enum itself is a class, but a specific Enum that you define in your code is a struct) if you haven't run into it before, but it is clearly a class (not a struct) as per the docs and the source code.