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

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...

• You had to go and mention PostSharp LOL :o postsharp.org/blog/generic-constraints-for-enums-and-delegates - Sam Harwell
• Or actually simpler HasAny() and HasAll() - Keith
• Yes, I agree that's even better. colors.HasAny(Colors.Red | Colors.Blue) looks like very readable code. =) - Blixt
• Yup, I like HasAny and HasAll too. Will go with that. - Jon Skeet
• HasAny and HasAll seem awesome. - IDisposable

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;
}

• And if you have a ridiculous number of flags, you can call GetTypeCode() on the arguments and Convert.ToUint64() - Kit
• Awesome, the combination of 'Enum` and Convert.ToUInt32 I didn't find anywhere else. AFAIK, Its the only decent Pre-Net-4 solution that also works in VB. BTW, if matchTo might have multiple flag bits, then replace != 0 with == Convert.ToUInt32(matchTo). - ToolmakerSteve
• Note that Convert.ToUInt32 used with an enum will use the Convert.ToUInt32(object) overload, meaning that CLR will first box these values before passing then to the ToUInt32 method. In most cases this won't matter, but it's good to know that you'll keep the GC rather busy if you're using something like this to parse millions of enums per second. - Groo

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.

You can achieve this using IL Weaving and ExtraConstraints

Allows you to write this code

public class Sample
{
    public void MethodWithDelegateConstraint<[DelegateConstraint] T> ()
    {        
    }
    public void MethodWithEnumConstraint<[EnumConstraint] T>()
    {
    }
}

What gets compiled

public class Sample
{
    public void MethodWithDelegateConstraint<T>() where T: Delegate
    {
    }

    public void MethodWithEnumConstraint<T>() where T: struct, Enum
    {
    }
}

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

• I did not see your answer before posting mine. +1 - Mik
• Too bad the "accepted" and most widely viewed answer is no longer up-to-date to reflect changes in the language. Someone who does not know and just skimming the the most popular answer will not be getting the most relevant and up-to-date information. - ForeverZer0
• @ForeverZer0 I think most people using SO know that when the answer is rather old, it might exist a better one below it. - Mik
• @Mik Obviously, was just thinking out loud. Thank you for pointing that out. Was just stating because currently it is 4 answers down. - ForeverZer0

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

• Upvoted because at this point, most everyone should be using .NET 4 (or higher) and so they should be using this method instead of trying to hack it together. - CptRobby
• Upvoted. However their solution uses boxing of the argument flag. .NET 4.0 is five years old now. - Jeppe Stig Nielsen

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

• where T : struct, IComparable, IFormattable, IConvertible -- this is the closest you can get to enum :) - Kit

As of C# 7.3, you can use the Enum constraint on generic types:

public static TEnum Parse<TEnum>(string value) where TEnum : Enum
{
    return (TEnum) Enum.Parse(typeof(TEnum), value);
}

If you want to use a Nullable enum, you must leave the orginial struct constraint:

public static TEnum? TryParse<TEnum>(string value) where TEnum : struct, Enum
{
    if( Enum.TryParse(value, out TEnum res) )
        return res;
    else
        return null;
}

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;
    }
}

• Thanks, but that turns a compile time issue (the where constraint) into a runtime one (your exception). Also you'd still need to convert the inputs to ints before you could do anything with them. - Keith

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.");
    }
}

if someone needs generic IsSet (created out of box on fly could be improved on), and or string to Enum onfly conversion (which uses EnumConstraint presented below):

  public class TestClass
  { }

  public struct TestStruct
  { }

  public enum TestEnum
  {
    e1,    
    e2,
    e3
  }

  public static class TestEnumConstraintExtenssion
  {

    public static bool IsSet<TEnum>(this TEnum _this, TEnum flag)
      where TEnum : struct
    {
      return (((uint)Convert.ChangeType(_this, typeof(uint))) & ((uint)Convert.ChangeType(flag, typeof(uint)))) == ((uint)Convert.ChangeType(flag, typeof(uint)));
    }

    //public static TestClass ToTestClass(this string _this)
    //{
    //  // #generates compile error  (so no missuse)
    //  return EnumConstraint.TryParse<TestClass>(_this);
    //}

    //public static TestStruct ToTestStruct(this string _this)
    //{
    //  // #generates compile error  (so no missuse)
    //  return EnumConstraint.TryParse<TestStruct>(_this);
    //}

    public static TestEnum ToTestEnum(this string _this)
    {
      // #enum type works just fine (coding constraint to Enum type)
      return EnumConstraint.TryParse<TestEnum>(_this);
    }

    public static void TestAll()
    {
      TestEnum t1 = "e3".ToTestEnum();
      TestEnum t2 = "e2".ToTestEnum();
      TestEnum t3 = "non existing".ToTestEnum(); // default(TestEnum) for non existing 

      bool b1 = t3.IsSet(TestEnum.e1); // you can ommit type
      bool b2 = t3.IsSet<TestEnum>(TestEnum.e2); // you can specify explicite type

      TestStruct t;
      // #generates compile error (so no missuse)
      //bool b3 = t.IsSet<TestEnum>(TestEnum.e1);

    }

  }

If someone still needs example hot to create Enum coding constraint:

using System;

/// <summary>
/// would be same as EnumConstraint_T&lt;Enum>Parse&lt;EnumType>("Normal"),
/// but writen like this it abuses constrain inheritence on System.Enum.
/// </summary>
public class EnumConstraint : EnumConstraint_T<Enum>
{

}

/// <summary>
/// provides ability to constrain TEnum to System.Enum abusing constrain inheritence
/// </summary>
/// <typeparam name="TClass">should be System.Enum</typeparam>
public abstract class EnumConstraint_T<TClass>
  where TClass : class
{

  public static TEnum Parse<TEnum>(string value)
    where TEnum : TClass
  {
    return (TEnum)Enum.Parse(typeof(TEnum), value);
  }

  public static bool TryParse<TEnum>(string value, out TEnum evalue)
    where TEnum : struct, TClass // struct is required to ignore non nullable type error
  {
    evalue = default(TEnum);
    return Enum.TryParse<TEnum>(value, out evalue);
  }

  public static TEnum TryParse<TEnum>(string value, TEnum defaultValue = default(TEnum))
    where TEnum : struct, TClass // struct is required to ignore non nullable type error
  {    
    Enum.TryParse<TEnum>(value, out defaultValue);
    return defaultValue;
  }

  public static TEnum Parse<TEnum>(string value, TEnum defaultValue = default(TEnum))
    where TEnum : struct, TClass // struct is required to ignore non nullable type error
  {
    TEnum result;
    if (Enum.TryParse<TEnum>(value, out result))
      return result;
    return defaultValue;
  }

  public static TEnum Parse<TEnum>(ushort value)
  {
    return (TEnum)(object)value;
  }

  public static sbyte to_i1<TEnum>(TEnum value)
  {
    return (sbyte)(object)Convert.ChangeType(value, typeof(sbyte));
  }

  public static byte to_u1<TEnum>(TEnum value)
  {
    return (byte)(object)Convert.ChangeType(value, typeof(byte));
  }

  public static short to_i2<TEnum>(TEnum value)
  {
    return (short)(object)Convert.ChangeType(value, typeof(short));
  }

  public static ushort to_u2<TEnum>(TEnum value)
  {
    return (ushort)(object)Convert.ChangeType(value, typeof(ushort));
  }

  public static int to_i4<TEnum>(TEnum value)
  {
    return (int)(object)Convert.ChangeType(value, typeof(int));
  }

  public static uint to_u4<TEnum>(TEnum value)
  {
    return (uint)(object)Convert.ChangeType(value, typeof(uint));
  }

}

hope this helps someone.

I just wanted to add Enum as a generic constraint.

While this is just for a tiny helper method using ExtraConstraints is a bit too much overhead for me.

I decided to just just create a struct constraint and add a runtime check for IsEnum. For converting a variable from T to Enum I cast it to object first.

    public static Converter<T, string> CreateConverter<T>() where T : struct
    {
        if (!typeof(T).IsEnum) throw new ArgumentException("Given Type is not an Enum");
        return new Converter<T, string>(x => ((Enum)(object)x).GetEnumDescription());
    }