Add JObject to JObject

Answer 1

You are getting this error because you are trying to construct a JObject with a string (which gets converted into a JValue). A JObject cannot directly contain a JValue, nor another JObject, for that matter; it can only contain JProperties (which can, in turn, contain other JObjects, JArrays or JValues).

To make it work, change your second line to this:

json.Add(new JProperty(fm.Name, new JObject()));

Working demo: https://dotnetfiddle.net/cjtoJn

Answer 2

One more example

var jArray = new JArray {
    new JObject
    {
        new JProperty("Property1",
            new JObject
            {
                new JProperty("Property1_1", "SomeValue"),
                new JProperty("Property1_2", "SomeValue"),
            }
        ),
        new JProperty("Property2", "SomeValue"),
    }
};

Answer 3

json["report"] = new JObject
    {
        { "name", fm.Name }
    };

Newtonsoft is using more direct-like approach, where You can access any property via square brackets []. You just need to set the JObject, which have to be created based on Newtonsoft specifics.

Full code:

var json = JObject.Parse(@"
{
    ""report"": {},
    ""expense"": {},
    ""invoices"": {},
    ""settings"": {
        ""users"" : {}
    },
}");

Console.WriteLine(json.ToString());

json["report"] = new JObject
    {
        { "name", fm.Name }
    };

Console.WriteLine(json.ToString());

Output:

{
  "report": {},
  "expense": {},
  "invoices": {},
  "settings": {
    "users": {}
  }
}

{
  "report": {
    "name": "SomeValue"
  },
  "expense": {},
  "invoices": {},
  "settings": {
    "users": {}
  }
}

As a reference, You can look at this link: https://www.newtonsoft.com/json/help/html/ModifyJson.htm