Anyone know a good workaround for the lack of an enum generic constraint?

Solution 1:

EDIT: This is now live in version 0.0.0.2 of UnconstrainedMelody.

(As requested on my blog post about enum constraints. I've included the basic facts below for the sake of a standalone answer.)

The best solution is to wait for me to include it in UnconstrainedMelody¹. This is a library which takes C# code with "fake" constraints such as

where T : struct, IEnumConstraint

and turns it into

where T : struct, System.Enum

via a postbuild step.

It shouldn't be too hard to write IsSet... although catering for both Int64-based and UInt64-based flags could be the tricky part. (I smell some helper methods coming on, basically allowing me to treat any flags enum as if it had a base type of UInt64.)

What would you want the behaviour to be if you called

tester.IsSet(MyFlags.A | MyFlags.C)

? Should it check that all the specified flags are set? That would be my expectation.

I'll try to do this on the way home tonight... I'm hoping to have a quick blitz on useful enum methods to get the library up to a usable standard quickly, then relax a bit.

EDIT: I'm not sure about IsSet as a name, by the way. Options:

• Includes
• Contains
• HasFlag (or HasFlags)
• IsSet (it's certainly an option)

Thoughts welcome. I'm sure it'll be a while before anything's set in stone anyway...

¹ or submit it as a patch, of course...

Solution 2:

As of C# 7.3, there is now a built-in way to add enum constraints:

public class UsingEnum<T> where T : System.Enum { }

source: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/where-generic-type-constraint

Solution 3:

Darren, that would work if the types were specific enumerations - for general enumerations to work you have to cast them to ints (or more likely uint) to do the boolean math:

public static bool IsSet( this Enum input, Enum matchTo )
{
    return ( Convert.ToUInt32( input ) & Convert.ToUInt32( matchTo ) ) != 0;
}

Solution 4:

Actually, it is possible, with an ugly trick. However, it cannot be used for extension methods.

public abstract class Enums<Temp> where Temp : class {
    public static TEnum Parse<TEnum>(string name) where TEnum : struct, Temp {
        return (TEnum)Enum.Parse(typeof(TEnum), name); 
    }
}
public abstract class Enums : Enums<Enum> { }

Enums.IsSet<DateTimeKind>("Local")

If you want to, you can give Enums<Temp> a private constructor and a public nested abstract inherited class with Temp as Enum, to prevent inherited versions for non-enums.