Using a bitmask in C

Question

Let s say I have the following  int susan   2    0010 int bob   4    0100 int karen   8    1000   and I pass 10  8   2  as a parameter to a method and I want to decode this to mean susan and karen  I know that 10 is 1010  but how can I do some logic to see if a specific bit is checked as in  if  condition for karen     How to quickly check whether effective karen bit is 1   Right now all i can think of is to check whether the number i passed is   14    1110 12    1100 10    1010 8     1000   When I have a larger number of actual bits in my real world scenario  this seems impractical  what is a better way using a mask to just check whether or not I meet the condition for just karen   I can think of shifting left then back then shifting right then back to clear bits other than the one I m interested in  but this also seems overly complex

User · Answer

To combine bitmasks you want to use bitwise-or. In the trivial case where every value you combine has exactly 1 bit on (like your example), it's equivalent to adding them. If you have overlapping bits however, or'ing them handles the case gracefully.

To decode the bitmasks you and your value with a mask, like so:

if(val & (1<<1)) SusanIsOn();
if(val & (1<<2)) BobIsOn();
if(val & (1<<3)) KarenIsOn();

User · Answer

Easy Way    Flags  public enum MyFlags       None   0      Susan   1      Alice   2      Bob   4      Eve   8     To set the flags use logical  or  operator     MyFlags f   new MyFlags    f   MyFlags Alice   MyFlags Bob    And to check if a flag is included use HasFlag   if f HasFlag MyFlags Alice        true     if f HasFlag MyFlags Eve        false

User · Answer

The traditional way to do this is to use the Flags attribute on an enum    Flags  public enum Names       None   0      Susan   1      Bob   2      Karen   4     Then you d check for a particular name as follows   Names names   Names Susan   Names Bob      evaluates to true bool susanIsIncluded    names  amp  Names Susan     Names None      evaluates to false bool karenIsIncluded    names  amp  Names Karen     Names None    Logical bitwise combinations can be tough to remember  so I make life easier on myself with a FlagsHelper class       The casts to object in the below code are an unfortunate necessity due to    C  s restriction against a where T   Enum constraint   There are ways around    this  but they re outside the scope of this simple illustration   public static class FlagsHelper       public static bool IsSet lt T gt  T flags  T flag  where T   struct               int flagsValue    int  object flags          int flagValue    int  object flag           return  flagsValue  amp  flagValue     0             public static void Set lt T gt  ref T flags  T flag  where T   struct               int flagsValue    int  object flags          int flagValue    int  object flag           flags    T  object  flagsValue   flagValue              public static void Unset lt T gt  ref T flags  T flag  where T   struct               int flagsValue    int  object flags          int flagValue    int  object flag           flags    T  object  flagsValue  amp    flagValue              This would allow me to rewrite the above code as   Names names   Names Susan   Names Bob   bool susanIsIncluded   FlagsHelper IsSet names  Names Susan    bool karenIsIncluded   FlagsHelper IsSet names  Names Karen     Note I could also add Karen to the set by doing this   FlagsHelper Set ref names  Names Karen     And I could remove Susan in a similar way   FlagsHelper Unset ref names  Names Susan      As Porges pointed out  an equivalent of the IsSet method above already exists in  NET 4 0  Enum HasFlag  The Set and Unset methods don t appear to have equivalents  though  so I d still say this class has some merit     Note  Using enums is just the conventional way of tackling this problem  You can totally translate all of the above code to use ints instead and it ll work just as well

User · Answer

if     param  amp  karen      karen          Do stuff     The bitwise  and  will mask out everything except the bit that  represents  Karen  As long as each person is represented by a single bit position  you could check multiple people with a simple   if     param  amp  karen      karen          Do Karen s stuff   if     param  amp  bob      bob        Do Bob s stuff

User · Answer

One other really good reason to use a bitmask vs individual bools is as a web developer  when integrating one website to another  we frequently need to send parameters or flags in the querystring  As long as all of your flags are binary  it makes it much simpler to use a single value as a bitmask than send multiple values as bools  I know there are otherways to send data  GET  POST  etc    but a simple parameter on the querystring is most of the time sufficient for nonsensitive items  Try to send 128 bool values on a querystring to communicate with an external site  This also gives the added ability of not pushing the limit on url querystrings in browsers

User · Answer

I have included an example here which demonstrates how you might store the mask in a database column as an int  and how you would reinstate the mask later on   public enum DaysBitMask   Mon 0  Tues 1  Wed 2  Thu   4  Fri   8  Sat   16  Sun   32     DaysBitMask mask   DaysBitMask Sat   DaysBitMask Thu  bool test  if   mask  amp  DaysBitMask Sat     DaysBitMask Sat      test   true  if   mask  amp  DaysBitMask Thu     DaysBitMask Thu      test   true  if   mask  amp  DaysBitMask Wed     DaysBitMask Wed      test   true      Store the value int storedVal    int mask      Reinstate the mask and re-test DaysBitMask reHydratedMask    DaysBitMask storedVal   if   reHydratedMask  amp  DaysBitMask Sat     DaysBitMask Sat      test   true  if   reHydratedMask  amp  DaysBitMask Thu     DaysBitMask Thu      test   true  if   reHydratedMask  amp  DaysBitMask Wed     DaysBitMask Wed      test   true

[c#] Using a bitmask in C#

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