How do I – Use a bitmask enum in C#

Written by Cornelius J. van Dyk on . Posted in How Do I...

In today’s world where processor memory is plentiful, many developers will often times just use a List<bool> for tracking a series of on/off values.  Though the compiler can optimize this code somewhat, the CPU cycles spent managing the List<> structure when super fast bitwise operations could have been done, isn’t really the best way to go.  So when one of my mentees asked about checking values in an enum bitmask, I thought it best to blog the answer for the benefit of others as well.

Let’s assume I’m trying to track the on/off values of say a permissions mask.  I would define my values using the Flags attribute on an enum thus:

public enum Permissions
    All = 4,
    Update = 2,
    Read = 1,
    None = 0

If I now proceed to define a variable of our enum, I can set some of it’s bit values using the the bitwise AND operator thus:

Permissions perm = Permissions.Read & Permissions.Update;

If I now check for a value in the mask that is NOT turned on, it is done with the bitwise OR operator thus:

if ((perm & Permissions.All) != 0)
    //it never gets here

The same applies for values that ARE turned on in the mask thus:

if ((perm & Permissions.Update) != 0)
    //it gets here

In addition, I can turn on more flag altering future check for them thus:

perm = perm & Permissions.All;
if ((perm & Permissions.All) != 0)
    //now it gets here

The major advantage to using the bitmask enum with bitwise operators is speed.  These operations are done by the processor literally just flipping a single bit.

Happy coding!




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Cornelius J. van Dyk

Born and raised in South Africa during the 70's I got my start in computers when a game on my Sinclair ZX Spectrum crashed, revealing it's BASIC source code. The ZX had a whopping 48K of memory which was considered to be a lot in the Commodore Vic20 era, but more importantly, it had BASIC built into the soft touch keyboard. Teaching myself to program, I coded my first commercial program at age 15.

After graduating high school at 17, I joined the South African Air Force, graduating the Academy and becoming a Pilot with the rank of First Lieutenant by age 20. After serving my country for six years, I made my way back into computer software.

Continuing my education, I graduated Suma Cum Laude from the Computer Training Institute before joining First National Bank where my work won the Smithsonian Award for Technological Innovation in the field of Banking and Insurance. Soon I met Will Coleman from Amdahl SA, who introduced me to a little known programming language named Huron/ObjectStar. As fate would have it, this unknown language and Y2K brought me to the USA in 1998.

I got involved with SharePoint after playing around with the Beta for SharePoint Portal Server 2003. Leaving my career at Rexnord to become a consultant in 2004, I was first awarded the Microsoft Most Valuable Professional Award for SharePoint in 2005, becoming only the 9th MVP for WSS at the time. I fulfilled a life long dream by pledging allegiance to the Flag as a US citizen in 2006. I met the love of my life and became a private consultant in 2008. I was honored to receive my ninth MVP award for SharePoint Server in 2013.

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