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

Question

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.

Anyone know a workaround?

Answer 1

try this

public static class EnumExtension
{    
    public static bool IsSet( this Enum input, Enum matchTo )   
    {            
        return (input & matchTo) != 0;    
    }
}

if i remember right, that works. doesn't let you enforce the same enumeration though, but you could do that easily enough in the logic and throw an exception if they aren't the same type.

Answer 2

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;
}

Answer 3

Using your original code, inside the method you can also use reflection to test that T is an enum:

public static class EnumExtension
{
    public static bool IsSet<T>( this T input, T matchTo )
    {
        if (!typeof(T).IsEnum)
        {
            throw new ArgumentException("Must be an enum", "input");
        }
        return (input & matchTo) != 0;
    }
}

Answer 4

The way I do it is put a struct constraint, then check that T is an enum at runtime. This doesn't eliminate the problem completely, but it does reduce it somewhat

Answer 5

Jon Skeet actually blogged about this yesterday:

http://msmvps.com/blogs/jon_skeet/archive/2009/09/10/generic-constraints-for-enums-and-delegates.aspx

Answer 6

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...

Answer 7

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.

Answer 8

Here's some code that I just did up that seems to work like you want without having to do anything too crazy. It's not restricted to only enums set as Flags, but there could always be a check put in if need be.

public static class EnumExtensions
{
    public static bool ContainsFlag(this Enum source, Enum flag)
    {
        var sourceValue = ToUInt64(source);
        var flagValue = ToUInt64(flag);

        return (sourceValue & flagValue) == flagValue;
    }

    public static bool ContainsAnyFlag(this Enum source, params Enum[] flags)
    {
        var sourceValue = ToUInt64(source);

        foreach (var flag in flags)
        {
            var flagValue = ToUInt64(flag);

            if ((sourceValue & flagValue) == flagValue)
            {
                return true;
            }
        }

        return false;
    }

    // found in the Enum class as an internal method
    private static ulong ToUInt64(object value)
    {
        switch (Convert.GetTypeCode(value))
        {
            case TypeCode.SByte:
            case TypeCode.Int16:
            case TypeCode.Int32:
            case TypeCode.Int64:
                return (ulong)Convert.ToInt64(value, CultureInfo.InvariantCulture);

            case TypeCode.Byte:
            case TypeCode.UInt16:
            case TypeCode.UInt32:
            case TypeCode.UInt64:
                return Convert.ToUInt64(value, CultureInfo.InvariantCulture);
        }

        throw new InvalidOperationException("Unknown enum type.");
    }
}

Answer 9

This doesn't answer the original question, but there is now a method in .NET 4 called Enum.HasFlag which does what you are trying to do in your example