Convert long number as string in the serialization

You could probably create a custom JsonConverter and apply it on your property.

Following is an example (NOTE: I haven't used this api before so it could probably be improved more, but following should give you a rough idea):

public class Person{    [JsonConverter(typeof(IdToStringConverter))]    public long ID { get; set; }    public string Name { get; set; }}public class IdToStringConverter : JsonConverter{    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)    {        JToken jt = JValue.ReadFrom(reader);        return jt.Value<long>();    }    public override bool CanConvert(Type objectType)    {        return typeof(System.Int64).Equals(objectType);    }    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)    {        serializer.Serialize(writer, value.ToString());    }}

Web API Action:

public Person Post([FromBody]Person person){    return person;}

Request:

POST http://asdfasdf/api/values HTTP/1.1  Host: servername:9095  Connection: Keep-Alive  Content-Type: application/json  Content-Length: 42  {"ID":"1306270928525862400","Name":"Mike"}

Response:

HTTP/1.1 200 OK  Content-Length: 42  Content-Type: application/json; charset=utf-8  Server: Microsoft-HTTPAPI/2.0  Date: Fri, 28 Jun 2013 17:02:18 GMT  {"ID":"1306270928525862400","Name":"Mike"}

EDIT:
if you do not want to decorate the property with an attribute, you could instead add it to the Converters collection. Example:

config.Formatters.JsonFormatter.SerializerSettings.Converters.Add(new IdToStringConverter());

How about a view model that only has string properties, like this:

public class CustomerEntityViewModel{    public string ID { get; set; }    public string Name { get; set; }}

Now you are only dealing with strings and JSON serialization problems go away.

I would probably just create an anonymous type when I call the json serializer like;

 JsonConvert.SerializeObject(new { instance.Name, instance.ID.ToString() } );

In the case where your class has 20 or something fields and this becomes a very ugly solution I would add a string to the class called ID, change the long to lID or something and use the serializer settings to ignore the long when serializing so the resulting json will only have the string version.

There are several different attributes that will accomplish this for example;

 public class CustomerEntity {      [ScriptIgnore]      public long _ID { get; set; }      public string ID { get; set; }      public string Name { get; set; } }

Will not serialize _ID