Custom JSON converter to de-serialise Enum description value to Enum value

It’s common to have description attributes on Enum types. This allows adding a longer description string for each Enum value. In one of my recent projects, I have a Web API that needs to accept this description value from the JSON request body and map to the correct Enum value. In this post, I’m going to add a custom JSON converter that will allow de-serialising the description value back to the Enum value and serialising an Enum value to its description value and.

Let’s say we have an Enum like this

public enum MonthOfYear
{
    [Description("First month of the year")]
    January,

    [Description("Second month of the year")]
    February,

    [Description("Third month of the year")]
    March

    // ... and so on
}

And a sample request model that looks like this

    public class SampleRequest
    {
        public MonthOfYear MonthOfYear { get; set; }
    }

If we try to de-serialise the next JSON string to SampleRequest, it would work

{
  "MonthOfYear": 2
}

{
  "MonthOfYear": "March"
}

However, if we use this JSON string, it will throw an exception because it cannot parse to the Enum

{
  "MonthOfYear": "Third month of the year"
}

Next, we’re going to add a custom JSON converter to support this. In this post, I showed how to add extension method to get the description attribute value for an Enum. We’re going to be reusing this in the custom JSON converter.

    public class EnumConverter : JsonConverter
    {
        public override bool CanConvert(Type objectType)
        {
            var enumType = Nullable.GetUnderlyingType(objectType) ?? objectType;
            return enumType.IsEnum;
        }

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            if (reader.Value is long)
                return Enum.ToObject(objectType, reader.Value);

            string description = (string)reader.Value;

            if (description is null) return null;

            foreach (var field in objectType.GetFields())
            {
                if (Attribute.GetCustomAttribute(field,
                typeof(DescriptionAttribute)) is DescriptionAttribute attribute)
                {
                    if (attribute.Description == description)
                        return field.GetValue(null);
                }
                else
                {
                    if (field.Name == description)
                        return field.GetValue(null);
                }
            }

            throw new ArgumentException("Not found.", nameof(description));
        }

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

        }
    }

In the CanConvert, we only allow if type is an Enum. ReadJson will be called when de-serialising JSON string and WriteJson will be called when serialising JSON string. We should now be able to de-serialise the previous JSON string using this converter.

And it also works when serialising the request model.