Check if a bitmask consists of multiple flags

Question

Note, this is not a dupe of this question:

Testing if a bitmask has one and only one flag

I need to validate whether or not a bitmask consists of multiple flags. I've come up with this method, but I don't like it a whole lot because of the enumeration and casting.

[Flags]
enum MyFlags { a = 1, b = 2, c = 4, d = 8 }

var flags = Enum.GetValues(typeof(MyFlags)).Cast<MyFlags>();

Console.WriteLine(flags.Any(f => f == (MyFlags.a | MyFlags.c))); //false
Console.WriteLine(flags.Any(f => f == MyFlags.b)); //true

Might be clearer with your example if you made clear what the input/value under test is. I assume it's where you've currently got literal expressions, but it's presumably a `MyFlags` parameter/variable that has already been set? Also, not entirely sure what you're expecting if you're working with an enum which has named combinations (e.g. [FileShare.ReadWrite](https://msdn.microsoft.com/en-us/library/system.io.fileshare(v=vs.110).aspx)) — Damien_The_Unbeliever, Dec 08 '16 at 10:51
If it's literally just "this value has more than a single bit set" then what you're looking for would seem to be a `population count`. — Damien_The_Unbeliever, Dec 08 '16 at 10:52

score 0 · Answer 1 · edited May 23 '17 at 12:30

If what you want is to test if a bitmask consist of flags, you can do it like this

MyFlags flag = MyFlags.b | MyFlags.d | MyFlags.a;

Console.WriteLine(flag.HasFlag(MyFlags.b));             // true
Console.WriteLine(flag.HasFlag(MyFlags.a | MyFlags.b)); // true
Console.WriteLine(flag.HasFlag(MyFlags.c | MyFlags.b)); // false

Edit
You can also have a look at the BitArray class
https://msdn.microsoft.com/en-us/library/system.collections.bitarray(v=vs.110).aspx

Basically you just have to count the number of bit set to one in the BitArray

You can use this fast GetCardinality method to count them (alternatively, you can also do it with a for loop): Counting bits set in a .Net BitArray Class. Then:

public static bool HasExactlyOneBitSet(Enum e)
{
    return GetCardinality(new BitArray(new[] { (int)(object)e })) == 1;
}

And you will use it like this:

HasExactlyOneBitSet(MyFlags.A);             // true
HasExactlyOneBitSet(MyFlags.A | MyFlags.C); // false

This is not what I want to do. This checks if a bitmask consists of specific set flags. I want to ensure that the bitmask passed in consists of a single flag in the available set of flags. I also won't know the enum type, so it has to be dynamic like in my OP example. Lastly, HasFlags is evil and you should not use it. — oscilatingcretin, Dec 08 '16 at 11:12

Check if a bitmask consists of multiple flags

1 Answers1