Most common C# bitwise operations on enums

Question

For the life of me, I can't remember how to set, delete, toggle or test a bit in a bitfield. Either I'm unsure or I mix them up because I rarely need these. So a "bit-cheat-sheet" would be nice to have.

For example:

flags = flags | FlagsEnum.Bit4;  // Set bit 4.

or

if ((flags & FlagsEnum.Bit4)) == FlagsEnum.Bit4) // Is there a less verbose way?

Can you give examples of all the other common operations, preferably in C# syntax using a [Flags] enum?

Answer 1

The idiom is to use the bitwise or-equal operator to set bits:

flags |= 0x04;

To clear a bit, the idiom is to use bitwise and with negation:

flags &= ~0x04;

Sometimes you have an offset that identifies your bit, and then the idiom is to use these combined with left-shift:

flags |= 1 << offset;
flags &= ~(1 << offset);

Answer 2

@Drew

Note that except in the simplest of cases, the Enum.HasFlag carries a heavy performance penalty in comparison to writing out the code manually. Consider the following code:

[Flags]
public enum TestFlags
{
    One = 1,
    Two = 2,
    Three = 4,
    Four = 8,
    Five = 16,
    Six = 32,
    Seven = 64,
    Eight = 128,
    Nine = 256,
    Ten = 512
}


class Program
{
    static void Main(string[] args)
    {
        TestFlags f = TestFlags.Five; /* or any other enum */
        bool result = false;

        Stopwatch s = Stopwatch.StartNew();
        for (int i = 0; i < 10000000; i++)
        {
            result |= f.HasFlag(TestFlags.Three);
        }
        s.Stop();
        Console.WriteLine(s.ElapsedMilliseconds); // *4793 ms*

        s.Restart();
        for (int i = 0; i < 10000000; i++)
        {
            result |= (f & TestFlags.Three) != 0;
        }
        s.Stop();
        Console.WriteLine(s.ElapsedMilliseconds); // *27 ms*        

        Console.ReadLine();
    }
}

Over 10 million iterations, the HasFlags extension method takes a whopping 4793 ms, compared to the 27 ms for the standard bitwise implementation.

Answer 3

To test a bit you would do the following: (assuming flags is a 32 bit number)

Test Bit:

if((flags & 0x08) == 0x08)

(If bit 4 is set then its true) Toggle Back (1 - 0 or 0 - 1):

flags = flags ^ 0x08;

Reset Bit 4 to Zero:

flags = flags & 0xFFFFFF7F;

Answer 4

This was inspired by using Sets as indexers in Delphi, way back when:

/// Example of using a Boolean indexed property
/// to manipulate a [Flags] enum:

public class BindingFlagsIndexer
{
  BindingFlags flags = BindingFlags.Default;

  public BindingFlagsIndexer()
  {
  }

  public BindingFlagsIndexer( BindingFlags value )
  {
     this.flags = value;
  }

  public bool this[BindingFlags index]
  {
    get
    {
      return (this.flags & index) == index;
    }
    set( bool value )
    {
      if( value )
        this.flags |= index;
      else
        this.flags &= ~index;
    }
  }

  public BindingFlags Value 
  {
    get
    { 
      return flags;
    } 
    set( BindingFlags value ) 
    {
      this.flags = value;
    }
  }

  public static implicit operator BindingFlags( BindingFlagsIndexer src )
  {
     return src != null ? src.Value : BindingFlags.Default;
  }

  public static implicit operator BindingFlagsIndexer( BindingFlags src )
  {
     return new BindingFlagsIndexer( src );
  }

}

public static class Class1
{
  public static void Example()
  {
    BindingFlagsIndexer myFlags = new BindingFlagsIndexer();

    // Sets the flag(s) passed as the indexer:

    myFlags[BindingFlags.ExactBinding] = true;

    // Indexer can specify multiple flags at once:

    myFlags[BindingFlags.Instance | BindingFlags.Static] = true;

    // Get boolean indicating if specified flag(s) are set:

    bool flatten = myFlags[BindingFlags.FlattenHierarchy];

    // use | to test if multiple flags are set:

    bool isProtected = ! myFlags[BindingFlags.Public | BindingFlags.NonPublic];

  }
}

Answer 5

In .NET 4 you can now write:

flags.HasFlag(FlagsEnum.Bit4)

Answer 6

.NET's built-in flag enum operations are unfortunately quite limited. Most of the time users are left with figuring out the bitwise operation logic.

In .NET 4, the method HasFlag was added to Enum which helps simplify user's code but unfortunately there are many problems with it.

1. HasFlag is not type-safe as it accepts any type of enum value argument, not just the given enum type.
2. HasFlag is ambiguous as to whether it checks if the value has all or any of the flags provided by the enum value argument. It's all by the way.
3. HasFlag is rather slow as it requires boxing which causes allocations and thus more garbage collections.

Due in part to .NET's limited support for flag enums I wrote the OSS library Enums.NET which addresses each of these issues and makes dealing with flag enums much easier.

Below are some of the operations it provides along with their equivalent implementations using just the .NET framework.

Combine Flags

.NET             flags | otherFlags

Enums.NET flags.CombineFlags(otherFlags)

---

Remove Flags

.NET             flags & ~otherFlags

Enums.NET flags.RemoveFlags(otherFlags)

---

Common Flags

.NET             flags & otherFlags

Enums.NET flags.CommonFlags(otherFlags)

---

Toggle Flags

.NET             flags ^ otherFlags

Enums.NET flags.ToggleFlags(otherFlags)

---

Has All Flags

.NET             (flags & otherFlags) == otherFlags or flags.HasFlag(otherFlags)

Enums.NET flags.HasAllFlags(otherFlags)

---

Has Any Flags

.NET             (flags & otherFlags) != 0

Enums.NET flags.HasAnyFlags(otherFlags)

---

Get Flags

.NET

Enumerable.Range(0, 64)
  .Where(bit => ((flags.GetTypeCode() == TypeCode.UInt64 ? (long)(ulong)flags : Convert.ToInt64(flags)) & (1L << bit)) != 0)
  .Select(bit => Enum.ToObject(flags.GetType(), 1L << bit))`

Enums.NET flags.GetFlags()

---

I'm trying to get these improvements incorporated into .NET Core and maybe eventually the full .NET Framework. You can check out my proposal here.