C# Enum Flags Comparison

Question

Given the following flags,

  [Flags]
    public enum Operations
    {
        add = 1,
        subtract = 2,
        multiply = 4,
        divide = 8,
        ev         


        

Answer 1

Wow, I can't believe all of the wrong answers..

It's important to understand bitwise math if you're working with flags. In your case, you have the following (for the first condition):

1 in binary is  00001
16 in binary is 10000

  00001
& 10000
--------
  00000

So, say we have Subtract (2) as the operation

2 in binary is     00010
previous result is 00000

  00010
& 00000
--------
  00000

Since the previous result is 00000 anything AND'd with it will be zero. So your condition will always evaluate to true since 0 == 0.

If we just switch this to OR, then we have the following:

1 in binary is  00001
16 in binary is 10000

  00001
| 10000
--------
  10001 (17)

Now, say we have Add (1) as the operation

1 in binary is     00001
previous result is 10001 (17)

  00001
& 10001
--------
  00001

So, 1 & 17 => 1 and thus your final condition is (1 & ( 1 | 16 ) ) == ( 1 | 16 ) => 1 & 17 == 17 => 1 == 17 => false (still false!)

So what you actually want is:

((operation | Operations.add | Operations.eval) & (Operations.add | Operations.eval)) == (Operations.add | Operations.eval)

This becomes ((1 | 1 | 16 ) & ( 1 | 16 )) == ( 1 | 16 ) => ( 17 & 17 ) == 17 => 17 == 17 == true

This is obviously not readable, so you should opt for extracting this into a method (as suggested). But it's still important to understand why your condition is incorrect.