Tolerant JSON.NET enum converter

TolerantEnumConverter.cs

/// <summary>
/// Tolerant Enum converter. Based on code in the StackOverflow post below, but adds EnumMember attribute support.
/// http://stackoverflow.com/questions/22752075/how-can-i-ignore-unknown-enum-values-during-json-deserialization
/// </summary>
public class TolerantEnumConverter : JsonConverter
{
    [ThreadStatic]
    private static Dictionary<Type, Dictionary<string, object>> _fromValueMap; // string representation to Enum value map

    [ThreadStatic]
    private static Dictionary<Type, Dictionary<object, string>> _toValueMap; // Enum value to string map

    public string UnknownValue { get; set; } = "Unknown";

    public override bool CanConvert(Type objectType)
    {
        Type type = IsNullableType(objectType) ? Nullable.GetUnderlyingType(objectType) : objectType;
        return type.IsEnum;
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        bool isNullable = IsNullableType(objectType);
        Type enumType = isNullable ? Nullable.GetUnderlyingType(objectType) : objectType;

        InitMap(enumType);

        if (reader.TokenType == JsonToken.String)
        {
            string enumText = reader.Value.ToString();

            object val = FromValue(enumType, enumText);

            if (val != null)
                return val;
        }
        else if (reader.TokenType == JsonToken.Integer)
        {
            int enumVal = Convert.ToInt32(reader.Value);
            int[] values = (int[])Enum.GetValues(enumType);
            if (values.Contains(enumVal))
            {
                return Enum.Parse(enumType, enumVal.ToString());
            }
        }

        if (!isNullable)
        {
            string[] names = Enum.GetNames(enumType);

            string unknownName = names
                .Where(n => string.Equals(n, UnknownValue, StringComparison.OrdinalIgnoreCase))
                .FirstOrDefault();

            if (unknownName == null)
            {
                throw new JsonSerializationException($"Unable to parse '{reader.Value}' to enum {enumType}. Consider adding Unknown as fail-back value.");
            }

            return Enum.Parse(enumType, unknownName);
        }

        return null;
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        Type enumType = value.GetType();

        InitMap(enumType);

        string val = ToValue(enumType, value);

        writer.WriteValue(val);
    }

    #region Private methods
    private bool IsNullableType(Type t)
    {
        return (t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>));
    }

    private void InitMap(Type enumType)
    {
        if (_fromValueMap == null)
            _fromValueMap = new Dictionary<Type, Dictionary<string, object>>();

        if (_toValueMap == null)
            _toValueMap = new Dictionary<Type, Dictionary<object, string>>();

        if (_fromValueMap.ContainsKey(enumType))
            return; // already initialized

        Dictionary<string, object> fromMap = new Dictionary<string, object>(StringComparer.InvariantCultureIgnoreCase);
        Dictionary<object, string> toMap = new Dictionary<object, string>();

        FieldInfo[] fields = enumType.GetFields(BindingFlags.Static | BindingFlags.Public);

        foreach (FieldInfo field in fields)
        {
            string name = field.Name;
            object enumValue = Enum.Parse(enumType, name);

            // use EnumMember attribute if exists
            EnumMemberAttribute enumMemberAttrbiute = field.GetCustomAttribute<EnumMemberAttribute>();

            if (enumMemberAttrbiute != null)
            {
                string enumMemberValue = enumMemberAttrbiute.Value;

                fromMap[enumMemberValue] = enumValue;
                toMap[enumValue] = enumMemberValue;
            }
            else
            {
                toMap[enumValue] = name;
            }

            fromMap[name] = enumValue;
        }

        _fromValueMap[enumType] = fromMap;
        _toValueMap[enumType] = toMap;
    }

    private string ToValue(Type enumType, object obj)
    {
        Dictionary<object, string> map = _toValueMap[enumType];

        return map[obj];
    }

    private object FromValue(Type enumType, string value)
    {
        Dictionary<string, object> map = _fromValueMap[enumType];

        if (!map.ContainsKey(value))
            return null;

        return map[value];
    } 
    #endregion
}

A little late to the show, and it depends on your use-case, but for enums with flags this won't work. However the default implementation handles that scenario. So you might want to change the CanConvert method to the following to ignore flagged enums:

        public override bool CanConvert(Type objectType)
        {
            Type type = IsNullableType(objectType) ? Nullable.GetUnderlyingType(objectType) : objectType;
            return type.IsEnum && !type.IsDefined(typeof(FlagsAttribute), false);
        }