How to check if any flags of a flag combination are set?

Question

Let's say I have this enum:

[Flags]
enum Letters
{
     A = 1,
     B = 2,
     C = 4,
     AB = A | B,
     All = A | B | C,
}

To check if for example AB is set I can do this:

if((letter & Letters.AB) == Letters.AB)

Is there a simpler way to check if any of the flags of a combined flag constant are set than the following?

if((letter & Letters.A) == Letters.A || (letter & Letters.B) == Letters.B)

Could one for example swap the & with something?

Answer 1

In .NET 4 you can use the Enum.HasFlag method :

using System;

[Flags] public enum Pet {
   None = 0,
   Dog = 1,
   Cat = 2,
   Bird = 4,
   Rabbit = 8,
   Other = 16
}

public class Example
{
   public static void Main()
   {
      // Define three families: one without pets, one with dog + cat and one with a dog only
      Pet[] petsInFamilies = { Pet.None, Pet.Dog | Pet.Cat, Pet.Dog };
      int familiesWithoutPets = 0;
      int familiesWithDog = 0;

      foreach (Pet petsInFamily in petsInFamilies)
      {
         // Count families that have no pets. 
         if (petsInFamily.Equals(Pet.None))
            familiesWithoutPets++;
         // Of families with pets, count families that have a dog. 
         else if (petsInFamily.HasFlag(Pet.Dog))
            familiesWithDog++;
      }
      Console.WriteLine("{0} of {1} families in the sample have no pets.", 
                        familiesWithoutPets, petsInFamilies.Length);   
      Console.WriteLine("{0} of {1} families in the sample have a dog.", 
                        familiesWithDog, petsInFamilies.Length);   
   }
}

The example displays the following output:

//       1 of 3 families in the sample have no pets. 
//       2 of 3 families in the sample have a dog.

Answer 2

If you want to know if letter has any of the letters in AB you must use the AND & operator. Something like:

if ((letter & Letters.AB) != 0)
{
    // Some flag (A,B or both) is enabled
}
else
{
    // None of them are enabled
}

Answer 3

I use extension methods to write things like that :

if (letter.IsFlagSet(Letter.AB))
    ...

Here's the code :

public static class EnumExtensions
{
    private static void CheckIsEnum<T>(bool withFlags)
    {
        if (!typeof(T).IsEnum)
            throw new ArgumentException(string.Format("Type '{0}' is not an enum", typeof(T).FullName));
        if (withFlags && !Attribute.IsDefined(typeof(T), typeof(FlagsAttribute)))
            throw new ArgumentException(string.Format("Type '{0}' doesn't have the 'Flags' attribute", typeof(T).FullName));
    }

    public static bool IsFlagSet<T>(this T value, T flag) where T : struct
    {
        CheckIsEnum<T>(true);
        long lValue = Convert.ToInt64(value);
        long lFlag = Convert.ToInt64(flag);
        return (lValue & lFlag) != 0;
    }

    public static IEnumerable<T> GetFlags<T>(this T value) where T : struct
    {
        CheckIsEnum<T>(true);
        foreach (T flag in Enum.GetValues(typeof(T)).Cast<T>())
        {
            if (value.IsFlagSet(flag))
                yield return flag;
        }
    }

    public static T SetFlags<T>(this T value, T flags, bool on) where T : struct
    {
        CheckIsEnum<T>(true);
        long lValue = Convert.ToInt64(value);
        long lFlag = Convert.ToInt64(flags);
        if (on)
        {
            lValue |= lFlag;
        }
        else
        {
            lValue &= (~lFlag);
        }
        return (T)Enum.ToObject(typeof(T), lValue);
    }

    public static T SetFlags<T>(this T value, T flags) where T : struct
    {
        return value.SetFlags(flags, true);
    }

    public static T ClearFlags<T>(this T value, T flags) where T : struct
    {
        return value.SetFlags(flags, false);
    }

    public static T CombineFlags<T>(this IEnumerable<T> flags) where T : struct
    {
        CheckIsEnum<T>(true);
        long lValue = 0;
        foreach (T flag in flags)
        {
            long lFlag = Convert.ToInt64(flag);
            lValue |= lFlag;
        }
        return (T)Enum.ToObject(typeof(T), lValue);
    }

    public static string GetDescription<T>(this T value) where T : struct
    {
        CheckIsEnum<T>(false);
        string name = Enum.GetName(typeof(T), value);
        if (name != null)
        {
            FieldInfo field = typeof(T).GetField(name);
            if (field != null)
            {
                DescriptionAttribute attr = Attribute.GetCustomAttribute(field, typeof(DescriptionAttribute)) as DescriptionAttribute;
                if (attr != null)
                {
                    return attr.Description;
                }
            }
        }
        return null;
    }
}

Answer 4

There is HasFlag method in .NET 4 or higher.

if(letter.HasFlag(Letters.AB))
{
}

Answer 5

If you can use .NET 4 or higher than use HasFlag() method

examples

letter.HasFlag(Letters.A | Letters.B) // both A and B must be set

same as

letter.HasFlag(Letters.AB)

Answer 6

If it really annoys you, you can write a function like that:

public bool IsSet(Letters value, Letters flag)
{
    return (value & flag) == flag;
}

if (IsSet(letter, Letters.A))
{
   // ...
}

// If you want to check if BOTH Letters.A and Letters.B are set:
if (IsSet(letter, Letters.A & Letters.B))
{
   // ...
}

// If you want an OR, I'm afraid you will have to be more verbose:
if (IsSet(letter, Letters.A) || IsSet(letter, Letters.B))
{
   // ...
}

Answer 7

To check if for example AB is set I can do this:

if((letter & Letters.AB) == Letters.AB)

Is there a simpler way to check if any of the flags of a combined flag constant are set than the following?

This checks that both A and B are set, and ignores whether any other flags are set.

 if((letter & Letters.A) == Letters.A || (letter & Letters.B) == Letters.B)

This checks that either A or B is set, and ignores whether any other flags are set or not.

This can be simplified to:

if(letter & Letters.AB)

Here's the C for binary operations; it should be straightforward to apply this to C#:

enum {
     A = 1,
     B = 2,
     C = 4,
     AB = A | B,
     All = AB | C,
};

int flags = A|C;

bool anything_and_a = flags & A;

bool only_a = (flags == A);

bool a_and_or_c_and_anything_else = flags & (A|C);

bool both_ac_and_anything_else = (flags & (A|C)) == (A|C);

bool only_a_and_c = (flags == (A|C));

Incidentally, the naming of the variable in the question's example is the singular 'letter', which might imply that it represents only a single letter; the example code makes it clear that its a set of possible letters and that multiple values are allowed, so consider renaming the variable 'letters'.

Answer 8

I created a simple extension method that does not need a check on Enum types:

public static bool HasAnyFlag(this Enum value, Enum flags)
{
    return
        value != null && ((Convert.ToInt32(value) & Convert.ToInt32(flags)) != 0);
}

It also works on nullable enums. The standard HasFlag method does not, so I created an extension to cover that too.

public static bool HasFlag(this Enum value, Enum flags)
{
    int f = Convert.ToInt32(flags);

    return
        value != null && ((Convert.ToInt32(value) & f) == f);
}

A simple test:

[Flags]
enum Option
{
    None = 0x00,
    One = 0x01,
    Two = 0x02,
    Three = One | Two,
    Four = 0x04
}

[TestMethod]
public void HasAnyFlag()
{
    Option o1 = Option.One;
    Assert.AreEqual(true, o1.HasAnyFlag(Option.Three));
    Assert.AreEqual(false, o1.HasFlag(Option.Three));

    o1 |= Option.Two;
    Assert.AreEqual(true, o1.HasAnyFlag(Option.Three));
    Assert.AreEqual(true, o1.HasFlag(Option.Three));
}

[TestMethod]
public void HasAnyFlag_NullableEnum()
{
    Option? o1 = Option.One;
    Assert.AreEqual(true, o1.HasAnyFlag(Option.Three));
    Assert.AreEqual(false, o1.HasFlag(Option.Three));

    o1 |= Option.Two;
    Assert.AreEqual(true, o1.HasAnyFlag(Option.Three));
    Assert.AreEqual(true, o1.HasFlag(Option.Three));
}

Enjoy!

Answer 9

There are a lot of answers on here but I think the most idiomatic way to do this with Flags would be Letters.AB.HasFlag(letter) or (Letters.A | Letters.B).HasFlag(letter) if you didn't already have Letters.AB. letter.HasFlag(Letters.AB) only works if it has both.

Answer 10

How about

if ((letter & Letters.AB) > 0)

?

Answer 11

这对你有用吗？

if ((letter & (Letters.A | Letters.B)) != 0)

Answer 12

You can use this extension method on enum, for any type of enums:

public static bool IsSingle(this Enum value)
{
    var items = Enum.GetValues(value.GetType());
    var counter = 0;
    foreach (var item in items)
    {
        if (value.HasFlag((Enum)item))
        {
            counter++;
        }
        if (counter > 1)
        {
            return false;
        }
    }
    return true;
}

Answer 13

if((int)letter != 0) { }

Answer 14

You could just check if the value is not zero.

if ((Int32)(letter & Letters.AB) != 0) { }

But I would consider it a better solution to introduce a new enumeration value with value zero and compare agains this enumeration value (if possible because you must be able to modify the enumeration).

[Flags]
enum Letters
{
    None = 0,
    A    = 1,
    B    = 2,
    C    = 4,
    AB   =  A | B,
    All  = AB | C
}

if (letter != Letters.None) { }

UPDATE

Missread the question - fixed the first suggestion and just ignore the second suggestion.

Answer 15

There are two aproaches that I can see that would work for checking for any bit being set.

Aproach A

if (letter != 0)
{
}

This works as long as you don't mind checking for all bits, including non-defined ones too!

Aproach B

if ((letter & Letters.All) != 0)
{
}

This only checks the defined bits, as long as Letters.All represents all of the possible bits.

For specific bits (one or more set), use Aproach B replacing Letters.All with the bits that you want to check for (see below).

if ((letter & Letters.AB) != 0)
{
}

Answer 16

从 .Net 4 开始，您可以使用速记版本而无需明确指定 &：

if(Letters.AB.HasFlag(Letters.C))

Answer 17

Can we find out easily and efficiently whether at least one flag is set ?

Well, if you are satisfied with checking whether at least one flag bit is set , then yes!

Usage:

if (EnumHelper.HasAnyFlagBitsSet(letter))

Implementation:

public static class EnumHelper
{
    static EnumHelper()
    {
        // Required to get correct behavior in GetNumericValue
        // Because we will overlap the enum type with a ulong, left-aligned
        if (!BitConverter.IsLittleEndian)
            throw new NotSupportedException("This type is only supported on little-endian architectures.");
    }

    /// <summary>
    /// <para>
    /// Returns whether the given enum value has any bits set that occurs in a defined flag for <typeparamref name="T"/>.
    /// </para>
    /// <para>
    /// Throws if the type parameter is not an enum type with the <see cref="FlagsAttribute"/>.
    /// </para>
    /// </summary>
    public static bool HasAnyFlagBitsSet<T>(T enumValue)
        where T : unmanaged, Enum
    {
        var numericValue = GetNumericValue(enumValue);

        // Take the value that has all the permitted bits set
        // Use & to keep only the corresponding bits from the input value
        // Check that the input value provided at least one such bit
        return (numericValue & FlagValueCache<T>.AllFlagsSetValue) != 0;
    }

    /// <summary>
    /// <para>
    /// Returns whether the given enum value has any bits set that are set in <paramref name="flags"/>.
    /// </para>
    /// <para>
    /// Throws if the type parameter is not an enum type with the <see cref="FlagsAttribute"/>.
    /// </para>
    /// </summary>
    public static bool HasAnyFlagBitsSet<T>(T enumValue, T flags)
        where T : unmanaged, Enum
    {
        var numericValue = GetNumericValue(enumValue);
        var numericFlags = GetNumericValue(flags);

        // Use & to keep only the bits present in flags
        // Check that the input value provided at least one such bit
        return (numericValue & flags) != 0;
    }

    // Actually, have a bonus method as well, since this is a common operation:

    /// <summary>
    /// <para>
    /// Returns whether the given enum value consists exclusively of defined flags for <typeparamref name="T"/>.
    /// The result is false if a bit is set that is not part of any value defined by <typeparamref name="T"/>.
    /// </para>
    /// <para>
    /// Throws if the type parameter is not an enum type with the <see cref="FlagsAttribute"/>.
    /// </para>
    /// </summary>
    public static bool HasDefinedFlags<T>(T enumValue)
        where T : unmanaged, Enum
    {
        var numericValue = GetNumericValue(enumValue);

        // Take the value that has all the permitted bits set
        // Use ~ to get a value with all the forbidden bits set
        // Use & to keep only the corresponding bits from the input value
        // Check that the input value provided no such forbidden bits
        return (numericValue & ~FlagValueCache<T>.AllFlagsSetValue) == 0;
    }

    /// <summary>
    /// <para>
    /// Returns the numeric value of the given <paramref name="enumValue"/>.
    /// </para>
    /// <para>
    /// The resulting <see cref="ulong"/> can be cast to the intended integral type, even if it is a signed type.
    /// </para>
    /// </summary>
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ulong GetNumericValue<T>(T enumValue)
        where T : unmanaged, Enum
    {
        Span<ulong> ulongSpan = stackalloc ulong[] { 0UL };
        Span<T> span = MemoryMarshal.Cast<ulong, T>(ulongSpan);

        span[0] = enumValue;

        return ulongSpan[0];
    }

    /// <summary>
    /// Statically caches a "full" flags value each enum type for which this class is accessed.
    /// </summary>
    internal static class FlagValueCache<T>
        where T : unmanaged, Enum
    {
        /// <summary>
        /// Each bit that is set in any of the type's defined values is also set in this value.
        /// </summary>
        public static ulong AllFlagsSetValue { get; }

        static FlagValueCache()
        {
            if (typeof(T).BaseType != typeof(Enum)) throw new Exception("The type parameter must be an enum type.");

            foreach (var value in (T[])Enum.GetValues(typeof(T)))
                AllFlagsSetValue |= GetNumericValue(value);
        }
    }
}

---

What does it mean that we are checking if at least one flag bit is set?

Well, this solution may fail to answer correctly for nonsensical enums like the following:

[Flags]
public enum Nonsense
{
    One = 1,

    // Eh, why does this value need TWO bits when those bits are NOT defined as individual values?
    TwoAndFour = 2 | 4,
}

Here, EnumHelper.HasAnyFlagBitSet((Nonsense)2) would return true , which is technically incorrect, since 2 is not a defined flag.

However, it works perfectly fine for all sensible flags enums, including ones with multi-flags:

[Flags]
public enum Fine
{
    One = 1,
    Two = 2,
    Four = 4,

    // Fine, and sensible, since these flags exist individually
    TwoAndFour = 2 | 4,
}

Answer 18

Sorry, but i will show it in VB :)

   <Flags()> Public Enum Cnt As Integer
        None = 0
        One = 1
        Two = 2
        Three = 4
        Four = 8    
    End Enum

    Sub Test()
    Dim CntValue As New Cnt
    CntValue += Cnt.One
    CntValue += Cnt.Three
    Console.WriteLine(CntValue)
    End Sub

CntValue = 5 So the enum contains 1 + 4