Using Bitwise operators on flags

Question

I have four flags

Current = 0x1   Past = 0x2   Future = 0x4   All = 0x7 

Say I receive the two flags Past and Future (setFlags(PAST | FUTURE)). How can I tell if Past is in it? Likewise how can I tell that Current is not in it? That way I don't have to test for every possible combination.

Answer 1

If you want all bits in the test mask to match:

if((value & mask) == mask) {...} 

If you want any single bit in the test mask to match:

if((value & mask) != 0) {...} 

The difference is most apparent when you are testing a value for multiple things.

To test for exclusion:

if ((value & mask) == 0) { } 

Answer 2

First of all - use enums with FlagAttribute. That's what it's for.

[Flags] public enum Time {     None = 0     Current = 1,     Past = 2,     Future = 4     All = 7 } 

Testing then is done like this:

if ( (x & Time.Past) != 0 ) 

Or this:

if ( (x & Time.Past) == Time.Past ) 

The latter will work better if "Past" was a combination of flags and you wanted to test them all.

Setting is like this:

x |= Time.Past; 

Unsetting is like this:

x &= ~Time.Past;