Why does AddAfterSelf return 'JProperty cannot have multiple values' when used with SelectToken?

Question

I want to add a new JProperty to a JSON object using a string path. I'm retrieving an existing path and then adding a new value proximal to it. It seems no matter how I select a token, or no matter what Add method I call (most relevant is AddAfterSelf) or what I supply as a new value, I receive the exception:

Run-time exception (line 9): Newtonsoft.Json.Linq.JProperty cannot have multiple values.

You can see this failing here: https://dotnetfiddle.net/mnvmOI

Why can't I add a JProperty in this situation?

using System;
using Newtonsoft.Json.Linq;

public class Program
{
    public static void Main()
    {
        JObject test = JObject.Parse("{\"test\":123,\"deeper\":{\"another\":\"value\"}}");
        test.SelectToken("deeper.another").AddAfterSelf(new JProperty("new name","new value"));
    }
}

Answer 1

The reason that the exception is thrown is that SelectToken() returns the property's JValue not the JProperty itself. Specifically, it returns a JValue owned by the JProperty with name "another". You can see this if you do:

Console.WriteLine("Result type: {0}; result parent type: {1}", result.GetType(), result.Parent.GetType());

Which results in

Result type: Newtonsoft.Json.Linq.JValue; result parent type: Newtonsoft.Json.Linq.JProperty

And if you further print the object types from the top of the JToken hierarchy to the value returned by SelectToken(), you will see the JValue tokens contained inside the JProperty tokens:

Depth: 0, Type: JObject
Depth: 1, Type: JProperty: deeper
Depth: 2, Type: JObject
Depth: 3, Type: JProperty: another
Depth: 4, Type: JValue: value

The Json.NET documentation also indicates that SelectToken() returns the selected property's value:

string name = (string)o.SelectToken("Manufacturers[0].Name");

Console.WriteLine(name);
// Acme Co

Since a JProperty cannot have more than one value, when you try to add a JProperty immediately after the value in the hierarchy, you are trying to add it as a child of its parent JProperty, which throws the exception.

Instead, add it to parent's parent:

test.SelectToken("deeper.another").Parent.AddAfterSelf(new JProperty("new name","new value"));

Sample fiddle showing all of the above.