C#, using the FlagsAttribute on enumerations

[rajanipro]

Please explain to me the following C# program, which I came across in MSDN help:

// enumFlags.cs
// Using the FlagsAttribute on enumerations.
using System;

[Flags]
public enum CarOptions
{
SunRoof = 0x01,
Spoiler = 0x02,
FogLights = 0x04,
TintedWindows = 0x08,
}

class FlagTest
{
static void Main()
{
CarOptions options = CarOptions.SunR oof | CarOptions.FogL ights;
Console.WriteLi ne(options);
Console.WriteLi ne((int)options );
}
}


Output:
SunRoof, FogLights
5


Comments:
Notice that if you remove FlagsAttribute, the example will output the following:
5
5

[TRScheel]

If you have each value follow a single bit pattern you can use multiple enum values (hence, flags) in a single enum. This is useful when a flag represents multiple things. I see it commonly used in DirectX.

To get this result, declare you enum with the values being incrementally larger versions of 2^n. A sample flagable enum would start at 1, and continue from there, as follows:

1
2
4
8
16
32
64
128
256
512
1024
2048
4096
... etc

Using bytes, this is shown as:

0000 0001 = 1
0000 0010 = 2
0000 0100 = 4
0000 1000 = 8
0001 0000 = 16
0010 0000 = 32
0100 0000 = 64
1000 0000 = 128

How this works:

If you flag any number of enum objects following that pattern, you can easily discern what value they correspond to you. Take for instance:

0000 0001
0001 0000

If you flagged those with the OR symbol, the resulting enum value would be:

0001 0001

Now there are only two possible values that would create that byte. This can be used to any size too. For instance:

1001 0101

Only grouping four of those unique bytes will result in this byte.

If it is still confusing, I will explain more.

[TRScheel]

Some simple bitwise operations include:

The AND operation, only returns 1 if both are true

1 & 0 = 0
0 & 1 = 0
1 & 1 = 1
0 & 0 = 0

The OR operation, returns 1 if either are true

1 | 0 = 1
0 | 1 = 1
1 | 1 = 1
0 | 0 = 0

The NOT operation returns 1 if the bit is 0

! 1 = 0
! 0 = 1

The XOR operation returns 1 if the bits are different

1 ^ 0 = 1
0 ^ 1 = 1
1 ^ 1 = 0
0 ^ 0 = 0

[rajanipro]

Cool! I got it.

But why is the output:
5
5


different when compiled without the FlagsAttribute [Flags]?

[TRScheel]

Ah that flag. Well it informs the framework that your enum is usable as flags, and thus can do more advanced work on it.

I suggest looking at this site:

MSDN - FlagsAttribute

[rajanipro]

Thank you very much indeed!