Question

How can I customize the serialized output of ASP.NET Web API?

Let’s say, I want all values in uppercase.

Answer 1

You have access to the JSON.NET serializer settings. With JSON.NET you can overide conversions using converters e.g. this datetime one.

You can also implement your own from inheritting from the abstract JsonConverter. See here for details.

For your example create the converter:

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

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        return reader.Value.ToString();
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        var outputValue = value as string;
        writer.WriteValue(outputValue == null ? null : outputValue.ToUpper());
    }
}

And then to register this globally add this config:

Registration example from here

JsonMediaTypeFormatter jsonFormatter = GlobalConfiguration.Configuration.Formatters.JsonFormatter;
JsonSerializerSettings jSettings = new Newtonsoft.Json.JsonSerializerSettings();
jSettings.Converters.Add(new UpperCaseStringConverter());
jsonFormatter.SerializerSettings = jSettings;

To add to a single property on a model just add the annotation:

[JsonConverter(typeof(UpperCaseStringConverter))]