Question

Was looking at another question, and was curious if there’s any difference at all (in operation or performance) between these two.

Given:

[Flags]
enum TransportModes
{
   None = 0,
   Bus = 1,
   Train = 2,
   Plane = 4
}

And a variable

var trip = TransportModes.Bus | TransportModes.Train;

1. if((trip & TransportModes.Bus) == TransportModes.Bus) ...

2. if((trip & TransportModes.Bus)) != 0) ...

I know what they do bit wise, and I know that HasFlag replaces them. But Jon Skeet recommends one, and the MSDN docs recommend another.

Answer 1

Your second option will return true, if the values you gave your enum values are not powers of two. The first option doesn’t have this problem.

Example:

[Flags]
enum TransportModes
{
   None = 0,
   Bus = 1,
   Train = 2,
   Plane = 5
}

var trip = TransportModes.Bus | TransportModes.Train;

if((trip & TransportModes.Plane) != 0)
    // will be executed
if((trip & TransportModes.Plane) == TransportModes.Plane)
    // won't be executed

Explanation:
trip & TransportModes.Plane is 1 which is apparently != 0, but not equal to TransportModes.Plane which has a value of 5.

However, if you don’t use powers of two for the values of a flag enum, you most likely have bigger issues. Think about what happens, if Plane would have the value 3: You couldn’t tell Bus | Train and Plane apart…