Custom JsonConverter that can convert decimal.MinValue to empty string and back

asp.net-mvc-4, c#, json.net

Solution

The following converter implementation should give you what you want:

public class DecimalJsonConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return (objectType == typeof(decimal));
    }

    public override object ReadJson(JsonReader reader, Type objectType, 
                                    object existingValue, JsonSerializer serializer)
    {
        if (reader.TokenType == JsonToken.String)
        {
            if ((string)reader.Value == string.Empty)
            {
                return decimal.MinValue;
            }
        }
        else if (reader.TokenType == JsonToken.Float || 
                 reader.TokenType == JsonToken.Integer)
        {
            return Convert.ToDecimal(reader.Value);
        }

        throw new JsonSerializationException("Unexpected token type: " +
                                             reader.TokenType.ToString());
    }

    public override void WriteJson(JsonWriter writer, object value, 
                                   JsonSerializer serializer)
    {
        decimal dec = (decimal)value;
        if (dec == decimal.MinValue)
        {
            writer.WriteValue(string.Empty);
        }
        else
        {
            writer.WriteValue(dec);
        }
    }
}

Here is a round-trip demo:

class Program
{
    static void Main(string[] args)
    {
        string json = @"
        {
            ""MinValueAsString"" : """",
            ""ARealDecimal"" : 3.14159,
            ""AnInteger"" : 42
        }";

        JsonSerializerSettings settings = new JsonSerializerSettings();
        settings.Converters.Add(new DecimalJsonConverter());
        settings.Formatting = Formatting.Indented;

        Foo foo = JsonConvert.DeserializeObject<Foo>(json, settings);

        Console.WriteLine("MinValueAsString: " + foo.MinValueAsString);
        Console.WriteLine("ARealDecimal: " + foo.ARealDecimal);
        Console.WriteLine("AnInteger: " + foo.AnInteger);
        Console.WriteLine();

        json = JsonConvert.SerializeObject(foo, settings);
        Console.WriteLine(json);
    }

    class Foo
    {
        public decimal MinValueAsString { get; set; }
        public decimal ARealDecimal { get; set; }
        public decimal AnInteger { get; set; }
    }
}

Output:

MinValueAsString: -79228162514264337593543950335
ARealDecimal: 3.14159
AnInteger: 42

{
  "MinValueAsString": "",
  "ARealDecimal": 3.14159,
  "AnInteger": 42.0
}

Fiddle: https://dotnetfiddle.net/dip85r

Problem

I need to create a custom JSON.NET converter for a legacy system that flags null decimal values with... ``` var nulldec = decimal.MinValue; ``` (This was created before nullable types were introduced, and to change how this works will be a ton of work.) When these min-values are serialized to JSON they need to be serialized as an empty string. When they are deserialized, if the value is empty string, it needs to convert to `decimal.MinValue`. Here is what I have so far (yes, not much). I think this is a simple conversion, but I am not seeing any articles that point me to how to work this for my situation nor any documentation on how to create custom converters. Can someone help please? ``` public class DecimalJsonConverter : JsonConverter { public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) { } public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) { } public override bool CanConvert(Type objectType) { return typeof(System.Decimal).IsAssignableFrom(objectType); } } ```

Original source