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

Asked 2008-08-10T17:14:10.260

What I want to do is something like this: I have enums with combined flagged values.

public static class EnumExtension
{
    public static bool IsSet<T>( this T input, T matchTo ) 
        where T:enum //the constraint I want that doesn't exist in C#3
    {    
        return (input & matchTo) != 0;
    }
}

So then I could do:

MyEnum tester = MyEnum.FlagA | MyEnum.FlagB

if( tester.IsSet( MyEnum.FlagA ) )
    //act on flag a

Unfortunately, C#'s generic where constraints have no enum restriction, only class and struct. C# doesn't see enums as structs (even though they are value types) so I can't add extension types like this.

Does anyone know a workaround?

c# .net enums flags

Edit Report

2 Answers

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...

answered 2009-09-11T09:12:34.137

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;
}

answered 2008-08-10T22:53:05.613

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

2 Answers

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