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c#.netvb.netenumsenum-flags
ElektroStudios
ElektroStudios

Reputation: 20464

Best practice to determine the amount of flags contained in a Enum-flag combination?

In some scenarios, when I pass a Enum to a method, I need to handle whether it is a single Enum value, or otherwise it is a flag combination, for that purpose I wrote this simple extension:

Vb.Net:

<Extension>
Public Function FlagCount(ByVal sender As System.[Enum]) As Integer
    Return sender.ToString().Split(","c).Count()
End Function

C# (online translation):

[Extension()]
public int FlagCount(this System.Enum sender) {
    return sender.ToString().Split(',').Count();
}

Example Usage:

Vb.Net:

Dim flags As FileAttributes = (FileAttributes.Archive Or FileAttributes.Compressed)
Dim count As Integer = flags.FlagCount()
MessageBox.Show(flagCount.ToString())

C# (online translation):

FileAttributes flags = (FileAttributes.Archive | FileAttributes.Compressed);
int count = flags.FlagCount();
MessageBox.Show(flagCount.ToString());

I just would like to ask If exists a more direct and efficient way that what I'm currently doing to avoid represent the flag combination as a String then split it.

Upvotes: 5

Views: 1888

Answers (2)

Shane Paul
Shane Paul

Reputation: 21

Could not let this one go. Best practice when counting bits in an integer is not to convert to a string... Have we lost the ability to work with bits now we all use high level languages? ;)

Since the question was about the most efficient implementation, here is my answer. I have not tried hyper-optimised it as to do so would confuse it I think. I have also used the previous answer as a base to make comparison easier. There are two methods, one which counts the flags and one which exits early if you only want to know if it has a single flag. NB: You cannot remove the flag attribute check because standard non-flag enums can be any number also.

    public static int FlagCount(this System.Enum enumValue){
        var hasFlagAttribute = enumValue.GetType().GetCustomAttributes(typeof(FlagsAttribute), false).Length > 0;
        if (!hasFlagAttribute)
            return 1;
        var count = 0;
        var value = Convert.ToInt32(enumValue);
        while (value != 0){
            if ((value & 1) == 1)
                count++;
            value >>= 1;
        }
        return count;
    }
    public static bool IsSingleFlagCount(this System.Enum enumValue){
        var hasFlagAttribute = enumValue.GetType().GetCustomAttributes(typeof(FlagsAttribute), false).Length > 0;
        if (!hasFlagAttribute)
            return true;
        var isCounted = false;
        var value = Convert.ToInt32(enumValue);
        while (value != 0){
            if ((value & 1) == 1){
                if (isCounted)
                    return false;
                isCounted = true;
            }
            value >>= 1;
        }
        return true;
    }

Upvotes: 1

Avi Turner
Avi Turner

Reputation: 10456

Option A: 

public int FlagCount(System.Enum sender)
{
    bool hasFlagAttribute = sender.GetType().GetCustomAttributes(typeof(FlagsAttribute), false).Length > 0;
    if (!hasFlagAttribute) // No flag attribute. This is a single value.
        return 1;

    var resultString = Convert.ToString(Convert.ToInt32(sender), 2);
    var count = resultString.Count(b=> b == '1');//each "1" represents an enum flag.
    return count;
}

Explanation:

  • If the enum does not have a "Flags attribute", then it is bound to be a single value.
  • If the enum has the "Flags attribute", Convert it to the bit representation and count the "1"s. each "1" represents an enum flag.

Option B:

  1. Get all flaged items.
  2. Count them...

The code: 

public int FlagCount(this System.Enum sender)
{
  return sender.GetFlaggedValues().Count;
}

/// <summary>
/// All of the values of enumeration that are represented by specified value.
/// If it is not a flag, the value will be the only value returned
/// </summary>
/// <param name="value">The value.</param>
/// <returns></returns>
public static List<Enum> GetFlaggedValues(this Enum value)
{
    //checking if this string is a flagged Enum
    Type enumType = value.GetType();
    object[] attributes = enumType.GetCustomAttributes(true);

    bool hasFlags = enumType.GetCustomAttributes(true).Any(attr => attr is System.FlagsAttribute);
    //If it is a flag, add all flagged values
    List<Enum> values = new List<Enum>();
    if (hasFlags)
    {
        Array allValues = Enum.GetValues(enumType);
        foreach (Enum currValue in allValues)
        {
            if (value.HasFlag(currValue))
            {
                values.Add(currValue);
            }
        }
    }
    else//if not just add current value
    {
        values.Add(value);
    }
    return values;
}

Upvotes: 6

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