C＃检查Flags枚举是否只有一个值集

Question

I have a Flags enum and want to assert that a given instance of it, is one of the primitive values, ie it has exactly one '1' in its binary representation, ie it's a power of two.

What's the best way to check this?

(I suppose "best" isn't necessarily well-defined, so ...)

• What's the fastest way to check this?
• What's the most semantically clear way to check this?
• How do I check this, whilst writing the least code?

Answer 1

Using Framework functionality and be more flexible concerning the number of set flags one could use something like:

[Flags] 
public enum FlagsEnum {
   None = 0,
   One = 1,
   Two = 2,
   Three = 4,
}

void Main()
{
    var flags = FlagsEnum.Two;
    var hasOneElement = Enum.GetValues(typeof(FlagsEnum)).OfType<FlagsEnum>().Where(i => i != FlagsEnum.None && flags.HasFlag(i)).Count() == 1;
}

Answer 2

bit operation will yield the fastest result

((anInstanceOfTheFlaggedEnum & (anInstanceOfTheFlaggedEnum -1)) != 0)

over the more readable built in function

Enum.IsDefined(typeof(yourFlaggedEnumType), anInstanceOfTheFlaggedEnum)

I just ran a test and it was ~175 times faster..