How to add an extra property into a serialized JSON string using json.net?

Following is the cleanest way I could implement this

dynamic obj = JsonConvert.DeserializeObject(jsonstring);
obj.NewProperty = "value";
var payload = JsonConvert.SerializeObject(obj);

You have a few options.

The easiest way, as @Manvik suggested, is simply to add another property to your class and set its value prior to serializing.

If you don't want to do that, the next easiest way is to load your object into a JObject, append the new property value, then write out the JSON from there. Here is a simple example:

class Item
{
    public int ID { get; set; }
    public string Name { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        Item item = new Item { ID = 1234, Name = "FooBar" };
        JObject jo = JObject.FromObject(item);
        jo.Add("feeClass", "A");
        string json = jo.ToString();
        Console.WriteLine(json);
    }
}

Here is the output of the above:

{
  "ID": 1234,
  "Name": "FooBar",
  "feeClass": "A"
}

Another possibility is to create a custom JsonConverter for your Item class and use that during serialization. A JsonConverter allows you to have complete control over what gets written during the serialization process for a particular class. You can add properties, suppress properties, or even write out a different structure if you want. For this particular situation, I think it is probably overkill, but it is another option.