Brtdku

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;

}

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

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

    {

    }

}

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

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

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;

    }

}

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());

    }