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C# Enum as bit field

Bit field enum

Whenever you wish to express combinations of properties of an object, bit fields are a good way to accomplish this. As a simple example, consider a file in the file system. It can be Readable, Writable, Hidden or a combination these.

The different attributes can be defined as an enum:

[Flags]
public enum FileAttribute
{
  None      = 0b0000;
  Readable  = 0b0001;
  Writeable = 0b0010;
  Hidden    = 0b0100;
}

To indicate that this enum is expected to be used as a bit field I have defined it with the FlagsAttribute. It is important to understand that the FlagsAttribute does nothing more than making some changes to how the ToString method of the enum works, making it possible to print out all flags. It does not introduce any validation or special treatment of the enum in any other way.

I have defined the values of the different fields of the enum using binary representation, this should make it even more clear that this is a bit field and which bit that maps to which property.

Finally, it is very important that the value 0 (zero) represents None. This has to do with how the checking if different bits are set or not is performed. Always include a value of zero that represents None or Nothing.

Setting, checking, and clearing bits

The bit field enum is designed to work with bitwise OR, AND, and XOR operations. To use the code above, let's create a class File that has a FileAttribute property.

public class File
{
  public FileAttribute Attributes { get; set; }
  ...
}

Now, to set the Readable bit, simply use the bitwise OR operator, |

var f = new File();
f.Attributes |= FileAttribute.Readable;

And to later on check if the Readable bit is set, either use bitwise AND, or the HasFlag extension method.

var isReadable = (f.Attributes & FileAttribute.Readable) == FileAttribute.Readable;
// Alternatively
var isReadable = f.Attributes.HasFlag(FileAttribute.Readable));

To clear the Readable bit you should use the bitwise complement operator in combination with bitwise AND.

f.Attributes &= ~FileAttribute.Readable;

NOTE: When looking through the C# programming guide on Enumeration types I noticed that Microsoft uses bitwise XOR to clear the bit in their example. This is bad practice since bitwise XOR will invert the bit, not clear it.

Now, with the examples above it might just look difficult using a bit field. The beauty of it however shows when you start combining the attributes. For example, to set a File to both Readable and Writeable, just include both flags in the set operation.

 f.Attributes |= FileAttribute.Readable | FileAttribute.Writeable;

More information on how to use enumerations as bit flags can be found in additional resources listed below.

More resources

C# Programming Guide on Enumeration Types
The Bitwise complement operator
What does the [Flags] enum attribute mean in C#?

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