How can I ignore unknown enum values during json deserialization?

Question

How can I get Json.net not to throw up when my enum doesn't match string value provided in the json property?

This happens when I create enum based on current documentation, but the third party API adds more enum values later.

I would be happy with either marking special value as Unknown or using a nullable enum and unmatched value would return null.

Answer 1

You can solve this problem with a custom JsonConverter. Here is one I put together using a few pieces from the StringEnumConverter class that comes from Json.Net. It should give you the flexibility to handle things whatever way you decide. Here's how it works:

• If the value found in the JSON matches the enum (either as a string or an integer), that value is used. (If the value is integer and there are multiple possible matches, the first of those is used.)
• Otherwise if the enum type is nullable, then the value is set to null.
• Otherwise if the enum has a value called "Unknown", then that value is used.
• Otherwise the first value of the enum is used.

Here is the code. Feel free to change it to meet your needs.

class TolerantEnumConverter : JsonConverter
{
    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;

        string[] names = Enum.GetNames(enumType);

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

            if (!string.IsNullOrEmpty(enumText))
            {
                string match = names
                    .Where(n => string.Equals(n, enumText, StringComparison.OrdinalIgnoreCase))
                    .FirstOrDefault();

                if (match != null)
                {
                    return Enum.Parse(enumType, match);
                }
            }
        }
        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 defaultName = names
                .Where(n => string.Equals(n, "Unknown", StringComparison.OrdinalIgnoreCase))
                .FirstOrDefault();

            if (defaultName == null)
            {
                defaultName = names.First();
            }

            return Enum.Parse(enumType, defaultName);
        }

        return null;
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        writer.WriteValue(value.ToString());
    }

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

Here is a demo which puts it the converter through its paces using a couple of different enums (one has an "Unknown" value, and the other does not):

[JsonConverter(typeof(TolerantEnumConverter))]
enum Status
{
    Ready = 1,
    Set = 2,
    Go = 3
}

[JsonConverter(typeof(TolerantEnumConverter))]
enum Color
{
    Red = 1,
    Yellow = 2,
    Green = 3,
    Unknown = 99
}

class Foo
{
    public Status NonNullableStatusWithValidStringValue { get; set; }
    public Status NonNullableStatusWithValidIntValue { get; set; }
    public Status NonNullableStatusWithInvalidStringValue { get; set; }
    public Status NonNullableStatusWithInvalidIntValue { get; set; }
    public Status NonNullableStatusWithNullValue { get; set; }

    public Status? NullableStatusWithValidStringValue { get; set; }
    public Status? NullableStatusWithValidIntValue { get; set; }
    public Status? NullableStatusWithInvalidStringValue { get; set; }
    public Status? NullableStatusWithInvalidIntValue { get; set; }
    public Status? NullableStatusWithNullValue { get; set; }

    public Color NonNullableColorWithValidStringValue { get; set; }
    public Color NonNullableColorWithValidIntValue { get; set; }
    public Color NonNullableColorWithInvalidStringValue { get; set; }
    public Color NonNullableColorWithInvalidIntValue { get; set; }
    public Color NonNullableColorWithNullValue { get; set; }

    public Color? NullableColorWithValidStringValue { get; set; }
    public Color? NullableColorWithValidIntValue { get; set; }
    public Color? NullableColorWithInvalidStringValue { get; set; }
    public Color? NullableColorWithInvalidIntValue { get; set; }
    public Color? NullableColorWithNullValue { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        string json = @"
        {
            ""NonNullableStatusWithValidStringValue"" : ""Set"",
            ""NonNullableStatusWithValidIntValue"" : 2,
            ""NonNullableStatusWithInvalidStringValue"" : ""Blah"",
            ""NonNullableStatusWithInvalidIntValue"" : 9,
            ""NonNullableStatusWithNullValue"" : null,
            ""NullableStatusWithValidStringValue"" : ""Go"",
            ""NullableStatusWithValidIntValue"" : 3,
            ""NullableStatusWithNullValue"" : null,
            ""NullableStatusWithInvalidStringValue"" : ""Blah"",
            ""NullableStatusWithInvalidIntValue"" : 9,
            ""NonNullableColorWithValidStringValue"" : ""Green"",
            ""NonNullableColorWithValidIntValue"" : 3,
            ""NonNullableColorWithInvalidStringValue"" : ""Blah"",
            ""NonNullableColorWithInvalidIntValue"" : 0,
            ""NonNullableColorWithNullValue"" : null,
            ""NullableColorWithValidStringValue"" : ""Yellow"",
            ""NullableColorWithValidIntValue"" : 2,
            ""NullableColorWithNullValue"" : null,
            ""NullableColorWithInvalidStringValue"" : ""Blah"",
            ""NullableColorWithInvalidIntValue"" : 0,
        }";

        Foo foo = JsonConvert.DeserializeObject<Foo>(json);
        foreach (PropertyInfo prop in typeof(Foo).GetProperties())
        {
            object val = prop.GetValue(foo, null);
            Console.WriteLine(prop.Name + ": " + 
                             (val == null ? "(null)" : val.ToString()));
        }
    }
}

Output:

NonNullableStatusWithValidStringValue: Set
NonNullableStatusWithValidIntValue: Set
NonNullableStatusWithInvalidStringValue: Ready
NonNullableStatusWithInvalidIntValue: Ready
NonNullableStatusWithNullValue: Ready
NullableStatusWithValidStringValue: Go
NullableStatusWithValidIntValue: Go
NullableStatusWithInvalidStringValue: (null)
NullableStatusWithInvalidIntValue: (null)
NullableStatusWithNullValue: (null)
NonNullableColorWithValidStringValue: Green
NonNullableColorWithValidIntValue: Green
NonNullableColorWithInvalidStringValue: Unknown
NonNullableColorWithInvalidIntValue: Unknown
NonNullableColorWithNullValue: Unknown
NullableColorWithValidStringValue: Yellow
NullableColorWithValidIntValue: Yellow
NullableColorWithInvalidStringValue: (null)
NullableColorWithInvalidIntValue: (null)
NullableColorWithNullValue: (null)

Answer 2

Looking through the handful of suggestions that exist for this problem, all of them use StringEnumConverter as a backbone, but no suggestions use it through inheritance. If your scenario was like mine, I was taking a 3rd party API response, which has ton of possible enum values, that may change over time. I only care about maybe 10 of those values, so all the other values I want to fallback on a default value(like Unknown). Here's my enum converter to do this:

/// <inheritdoc />
/// <summary>
/// Defaults enum values to the base value if 
/// </summary>
public class DefaultUnknownEnumConverter : StringEnumConverter
{
    /// <summary>
    /// The default value used to fallback on when a enum is not convertable.
    /// </summary>
    private readonly int defaultValue;

    /// <inheritdoc />
    /// <summary>
    /// Default constructor. Defaults the default value to 0.
    /// </summary>
    public DefaultUnknownEnumConverter() 
    {}

    /// <inheritdoc />
    /// <summary>
    /// Sets the default value for the enum value.
    /// </summary>
    /// <param name="defaultValue">The default value to use.</param>
    public DefaultUnknownEnumConverter(int defaultValue)
    {
        this.defaultValue = defaultValue;
    }

    /// <inheritdoc />
    /// <summary>
    /// Reads the provided JSON and attempts to convert using StringEnumConverter. If that fails set the value to the default value.
    /// </summary>
    /// <param name="reader">Reads the JSON value.</param>
    /// <param name="objectType">Current type that is being converted.</param>
    /// <param name="existingValue">The existing value being read.</param>
    /// <param name="serializer">Instance of the JSON Serializer.</param>
    /// <returns>The deserialized value of the enum if it exists or the default value if it does not.</returns>
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        try
        {
            return base.ReadJson(reader, objectType, existingValue, serializer);
        }
        catch
        {
            return Enum.Parse(objectType, $"{defaultValue}");
        }
    }

    /// <inheritdoc />
    /// <summary>
    /// Validates that this converter can handle the type that is being provided.
    /// </summary>
    /// <param name="objectType">The type of the object being converted.</param>
    /// <returns>True if the base class says so, and if the value is an enum and has a default value to fall on.</returns>
    public override bool CanConvert(Type objectType)
    {
        return base.CanConvert(objectType) && objectType.GetTypeInfo().IsEnum && Enum.IsDefined(objectType, defaultValue);
    }
}

Usage is the same as other examples:

[JsonConverter(typeof(DefaultUnknownEnumConverter))]
public enum Colors
{
    Unknown,
    Red,
    Blue,
    Green,
}

[JsonConverter(typeof(DefaultUnknownEnumConverter), (int) NotFound)]
public enum Colors
{        
    Red = 0,
    Blue,
    Green,
    NotFound
}