问题:

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

public static TResult? ToEnum<TResult>(this String value, TResult? defaultValue)
    where TResult : Enum
{
    return String.IsNullOrEmpty(value) ? defaultValue : (TResult?)Enum.Parse(typeof(TResult), value);
}

The compiler says:

Error CS0453 The type 'TResult' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'Nullable'

回答1:

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

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

回答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.