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

Keith_905 0
Viewed 22720

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?

flags enums .net c#
12 Answers
user_4446340

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;
}
user_1930

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;
}
user_15542080

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

user_794390

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;
    }
}
Scott_Dorman_15590
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