Enums and subtraction operator

Question

Does anyone know (and perhaps: since when) the -= operator is supported on enum values?

I was happily coding away today when I, for some reason, wrote this to exclude a value from a flags enum:

flags -= FlagsEnum.Value1;

After rereading and evaluating my code, I was surprised that it compiled actually worked.

Writing the statement as

flags = flags - FlagsEnum.Value1

however does not compile.

I couldn't find anything in the documentation and on internet so far. Also, other operators (apart from the bit operators, of course) are not supported: += (including a flag) , *= (intersection in Pascal) don't work.

Is this some syntactic sugar built into the C# compiler? If so, any reason why they chose not to include other operators?

A simple code sample:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication4
{
    [Flags]
    enum FlagsEnum
    {
        None = 0,
        Value1 = 1,
        Value2 = 2,
        Value3 = 4
    }

    class Program
    {
        static void Main(string[] args)
        {
            FlagsEnum flags = FlagsEnum.Value1 | FlagsEnum.Value2 | FlagsEnum.Value3;
            Console.WriteLine(flags);
            flags -= FlagsEnum.Value1;
            Console.WriteLine(flags);
            flags -= FlagsEnum.Value3;
            Console.WriteLine(flags);
            flags -= FlagsEnum.Value2;
            Console.WriteLine(flags);
            Console.ReadLine();
        }
    }
}

Answer 1

Iirc, - is still integer subtraction.

To do exclusion you want:

value = value & ~SomeEnum.Flag;

And inclusion:

value = value | SomeEnum.Flag;

Likewise, to test for a partial match (any bit from Flag):

if((value & SomeEnum.Flag) != 0) {...}

Or a full match (all the bits in Flag):

if((value & SomeEnum.Flag) == SomeEnum.Flag) {...}