How do I check if more than one enum flag is set?

Question

I just want to know if exactly one enum flag is set, not which ones. My current thinking is to check if it is a power of 2. Is there a better way built into enum types?

[Flags]
enum Foo
{
Flag1 = 0x01,
Flag2 = 0x02,
Flag3 = 0x04,
Flag4 = 0x08,
Flag5 = 0x10,
Flag6 = 0x20,
Flag7 = 0x40,
Flag8 = 0x80
}

private bool ExactlynOneFlagSet(Foo myFoo)
{
  var x = (byte) myFoo;
  return (x != 0) && ((x & (x - 1)) == 0); //Check if a power of 2
}

if(!ExactlynOneFlagSet(Foo myFoo))
{
   //Do something
}

Answer 1

Its a Bit operation!

if ((myFoo & (myFoo -1)) != 0) //has more than 1 flag

The statement checks if the value of myFoo is not power of two. Or, vice versa, the statement (myFoo & (myFoo -1)) == 0 checks for power of two. The idea is that only single flag values will be power of two. Setting more than one flag will result in a non power of two value of myFoo.

More information can be found in this answer to a similar question: https://stackoverflow.com/a/1662162/2404788.

For more information about bit operations go to http://en.wikipedia.org/wiki/Bitwise_operation

Answer 2

If the enum doesn't define explicit combinations of flags, you can just check if the value is defined in the enum:

private bool ExactlynOneFlagSet(Foo myFoo)
{
    return Enum.IsDefined(typeof(Foo), myFoo);
}

Answer 3

private bool ExatlyOneFlagSet(Foo myFoo)
{
  return !myFoo.ToString().Contains(',');
}