你可以检查这个值是否不为零。

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

但是我认为引入一个值为0的新枚举值并与该枚举值进行比较是更好的解决方案(如果可能的话，因为您必须能够修改枚举)。

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

if (letter != Letters.None) { }

更新

看错了问题——修正了第一个建议，忽略第二个建议。

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

在. net 4中，您可以使用Enum。HasFlag方法:

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);   
   }
}

使用实例显示如下信息:

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

如何

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

?

我使用扩展方法来写这样的东西:

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

代码如下:

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;
    }
}

你可以检查这个值是否不为零。

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

但是我认为引入一个值为0的新枚举值并与该枚举值进行比较是更好的解决方案(如果可能的话，因为您必须能够修改枚举)。

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

if (letter != Letters.None) { }

更新

看错了问题——修正了第一个建议，忽略第二个建议。