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Question

I have a piece of code where I get enum flags from an API. When this API changes and updates, I might get values that are not defined in my enum structure.

Example:

[Flags]
public enum TestEnum
{
   Enum1 = 1,
   Enum2 = 2,
   Enum3 = 4
}

When I say (TestEnum)7 , I get, as a string representation when outputting said data, "Enum1|Enum2|Enum3". However, if now the API changes, and gives me an additional flag "8" that is set with (TestEnum)15 , the String representation, especially in Visual Studio, will not give me the flags, it will only Display "0xF"

Is there a way to either get all the bits that are mapped properly to be displayed, or like in my example, to "forget" Enum4, the forth bit. Is there a way to do this?

Answer 1

What you have to do is mask the value with the combination of all defined values and do "ToString()" on that, and then mask the value with the compliment of all defined values and do ToString() on the remainder:

    TestEnum value = (TestEnum)(64 - 1);
    var all = ((TestEnum[])Enum.GetValues(typeof(TestEnum))).Aggregate((TestEnum)0, (a, c) => a | c);
    var str = (value & all).ToString() + ((value & ~all) == 0 ? "" : ", 0x" + (value & ~all).ToString("X"));
    Debug.WriteLine(str);

And the output is

    Enum1, Enum2, Enum3, 0x00000038

Doing this as a generic extension method is weirdly hard, because there's no generic enum constraint in c#, and thus no way to do bitmasking directly in any generic method targetted at enums. Here's one attempt:

public static class EnumHelper
{
    // Generic singleton remembering basic properties about specified enums, cached for performance.
    sealed class DataSingleton<TEnum> where TEnum : struct, IConvertible, IComparable, IFormattable
    {
        static readonly DataSingleton<TEnum> instance = new DataSingleton<TEnum>();

        readonly bool isSigned;
        readonly TEnum allValues;
        readonly bool hasFlags;
        readonly bool isEnum;

        // Explicit static constructor to tell C# compiler
        // not to mark type as beforefieldinit
        static DataSingleton()
        {
        }

        DataSingleton()
        {
            var type = typeof(TEnum);
            isEnum = type.IsEnum;
            if (isEnum)
            {
                isSigned = GetIsSigned();
                allValues = GetAll();
                hasFlags = GetHasFlags();
            }
            else
            {
                isSigned = false;
                allValues = default(TEnum);
                hasFlags = false;
            }
        }

        static bool GetHasFlags()
        {
            var attributes = typeof(TEnum).GetCustomAttributes(typeof(FlagsAttribute), false);
            return attributes != null && attributes.Length > 0;
        }

        static bool GetIsSigned()
        {
            var underlyingType = Enum.GetUnderlyingType(typeof(TEnum));
            bool isSigned = (underlyingType == typeof(long) || underlyingType == typeof(int) || underlyingType == typeof(short) || underlyingType == typeof(sbyte));
            bool isUnsigned = (underlyingType == typeof(ulong) || underlyingType == typeof(uint) || underlyingType == typeof(ushort) || underlyingType == typeof(byte));
            if (!isSigned && !isUnsigned)
                throw new InvalidOperationException();
            return isSigned;
        }

        static TEnum GetAll()
        {
            if (GetIsSigned())
            {
                long value = 0;
                foreach (var v in Enum.GetValues(typeof(TEnum)))
                    // Not sure I need the culture but Microsoft passes it in Enum.ToUInt64(Object value) - http://referencesource.microsoft.com/#mscorlib/system/enum.cs
                    value |= Convert.ToInt64(v, CultureInfo.InvariantCulture);
                return (TEnum)Enum.ToObject(typeof(TEnum), value);
            }
            else
            {
                ulong value = 0;
                foreach (var v in Enum.GetValues(typeof(TEnum)))
                    // Not sure I need the culture but Microsoft passes it in Enum.ToUInt64(Object value) - http://referencesource.microsoft.com/#mscorlib/system/enum.cs
                    value |= Convert.ToUInt64(v, CultureInfo.InvariantCulture);
                return (TEnum)Enum.ToObject(typeof(TEnum), value);
            }
        }

        public bool HasFlags { get { return hasFlags; } }

        public bool IsSigned { get { return isSigned; } }

        public TEnum AllValues { get { return allValues; } }

        public bool IsEnum { get { return isEnum; } }

        public static DataSingleton<TEnum> Instance { get { return instance; } }
    }

    private static void ThrowOnNonEnum<TEnum>(DataSingleton<TEnum> data) where TEnum : struct, IConvertible, IComparable, IFormattable
    {
        if (!data.IsEnum)
        {
            throw (new ArgumentException("The generic argument [<TEnum>] must be an enumeration.", "TEnum: " + typeof(TEnum).FullName));
        }
    }

    private static void ThrowOnEnumWithFlags<TEnum>(DataSingleton<TEnum> data) where TEnum : struct, IConvertible, IComparable, IFormattable
    {
        if (!data.IsEnum || data.HasFlags)
        {
            throw (new ArgumentException("The generic argument [<TEnum>] must be an enumeration without the [FlagsAttribute] applied.", "TEnum: " + typeof(TEnum).FullName));
        }
    }

    private static void ThrowOnEnumWithoutFlags<TEnum>(DataSingleton<TEnum> data) where TEnum : struct, IConvertible, IComparable, IFormattable
    {
        if (!data.IsEnum || !data.HasFlags)
        {
            throw (new ArgumentException("The generic argument [<TEnum>] must be an enumeration with the [FlagsAttribute] applied.", "TEnum: " + typeof(TEnum).FullName));
        }
    }

    public static TEnum GetAll<TEnum>() where TEnum : struct, IConvertible, IComparable, IFormattable
    {
        var data = DataSingleton<TEnum>.Instance;
        ThrowOnEnumWithoutFlags<TEnum>(data);
        return data.AllValues;
    }

    static ulong ToUInt64<TEnum>(TEnum value) where TEnum : struct, IConvertible, IComparable, IFormattable
    {
        // Silently convert the value to UInt64 from the other base 
        // types for enum without throwing an exception.
        // This is need since the Convert functions do overflow checks.
        TypeCode typeCode = value.GetTypeCode();
        ulong result;

        switch (typeCode)
        {
            case TypeCode.SByte:
            case TypeCode.Int16:
            case TypeCode.Int32:
            case TypeCode.Int64:
                unchecked
                {
                    result = (UInt64)value.ToInt64(CultureInfo.InvariantCulture);
                }
                break;

            case TypeCode.Byte:
            case TypeCode.UInt16:
            case TypeCode.UInt32:
            case TypeCode.UInt64:
            case TypeCode.Boolean:
            case TypeCode.Char:
                unchecked
                {
                    result = value.ToUInt64(CultureInfo.InvariantCulture);
                }
                break;

            default:
                throw new InvalidOperationException();
        }
        return result;
    }

    public static string ToDebugString<TEnum>(this TEnum anEnum) where TEnum : struct, IConvertible, IComparable, IFormattable
    {
        var data = DataSingleton<TEnum>.Instance;
        if (!data.IsEnum || !data.HasFlags)
            return anEnum.ToString();
        var allLong = ToUInt64(data.AllValues);
        var enumLong = ToUInt64(anEnum);
        string str1 = ((TEnum)Enum.ToObject(typeof(TEnum), enumLong & allLong)).ToString(CultureInfo.InvariantCulture);
        var compliment = enumLong & ~(allLong);
        if (compliment == 0)
            return str1;
        return str1 + ", 0x" + ((TEnum)Enum.ToObject(typeof(TEnum), compliment)).ToString("X", CultureInfo.InvariantCulture);
    }
}