Extension method for nullable enum

Question

I'm trying to write an Extension method for nullable Enums.
Like with this example:

// ItemType is an enum
ItemType? item;
...

item.GetDescription();

So I wrote this method which doesn't compile for some reason that I don't understand:

public static string GetDescription(this Enum? theEnum)
{
    if (theEnum == null)
        return string.Empty;

    return GetDescriptionAttribute(theEnum);
}

I'm getting the following error on Enum?:

only non-nullable value type could be underlying of system.nullable

Why? Enum can not have the value null!

Update:

If have lots of enums, ItemType is just an example of one of them.

Answer 1

System.Enum is a class, so just drop the ? and this should work.

(By "this should work", I mean if you pass in a null-valued ItemType?, you'll get a null Enum in the method.)

public static string GetDescription(this Enum theEnum)
{
    if (theEnum == null)
        return string.Empty;
    return GetDescriptionAttribute(theEnum);
}
enum Test { blah }

Test? q = null;
q.GetDescription(); // => theEnum parameter is null
q = Test.blah;
q.GetDescription(); // => theEnum parameter is Test.blah

Answer 2

You can simply make your extension method generic:

public static string GetDescription<T>(this T? theEnum) where T : struct
{ 
    if (!typeof(T).IsEnum)
        throw new Exception("Must be an enum.");

    if (theEnum == null) 
        return string.Empty; 
 
    return GetDescriptionAttribute(theEnum); 
}

Unfortunately, you cannot use System.Enum in a generic constraint, so the extension method will show for all nullable values (hence the extra check).

EDIT: C# 7.3 introduced new generic constraints which now allow restricting a generic argument to an enum, like so:

public static string GetDescription<T>(this T? theEnum) where T : Enum
{ 
    if (theEnum == null) 
        return string.Empty; 
 
    return GetDescriptionAttribute(theEnum); 
}

Thanks @JeppeStigNielsen for pointing that out.