How to add property in existing json using System.Text.Json library?

Question

       {
          "TestData":{
              "Year__of__Account":"2019",
              "Tax___x0025_":"0.06",
              "Buildings__1":"1000",
              "Contents__1":"400",
              "Total_Insurable_Value":"100",
              "Buildings__Prem":"2560.8",
              "Contents__Prem":"1707.2",
              "YB__1":"1950",
              "No__Buildings":"55",
              "Location_Sprinklers_YN":"No",
              "test":"test"
           }
        }

In the above sample JSON I want to add a property called "Name" with Value "John" inside property "TestData". How can I achieve this using .net Core 3.0 System.Text.Json library.

I have tried using methods of Utf8JsonWriter but it is creating a new JSON object instead of appending it to the above existing JSON.

        using (MemoryStream memoryStream1 = new MemoryStream())
        {
            using (Utf8JsonWriter utf8JsonWriter1 = new Utf8JsonWriter(memoryStream1))
            {
                using (JsonDocument jsonDocument = JsonDocument.Parse(json))
                {
                    utf8JsonWriter1.WriteStartObject();
                    utf8JsonWriter1.WritePropertyName("Name");
                    utf8JsonWriter1.WriteStringValue("John");
                    utf8JsonWriter1.WriteEndObject();

                    // how can I add above properties to JsonDocument object??
                }
            }
        }

Answer 1

Assuming there may be several properties and you want to add a name only to "TestData" property:

using (MemoryStream memoryStream1 = new MemoryStream())
{
    using (Utf8JsonWriter utf8JsonWriter1 = new Utf8JsonWriter(memoryStream1))
    {
        using (JsonDocument jsonDocument = JsonDocument.Parse(json))
        {
            utf8JsonWriter1.WriteStartObject();

            foreach (var element in jsonDocument.RootElement.EnumerateObject())
            {
                if (element.Name == "TestData")
                {
                    utf8JsonWriter1.WritePropertyName(element.Name);

                    // Staring new object
                    utf8JsonWriter1.WriteStartObject();

                    // Adding "Name" property 
                    utf8JsonWriter1.WritePropertyName("Name");
                    utf8JsonWriter1.WriteStringValue("John");

                    // Copying existing values from "TestData" object
                    foreach (var testDataElement in element.Value.EnumerateObject())
                    {
                        testDataElement.WriteTo(utf8JsonWriter1);
                    }

                    utf8JsonWriter1.WriteEndObject();
                }
                else
                {
                    element.WriteTo(utf8JsonWriter1);
                }
            }

            utf8JsonWriter1.WriteEndObject();
        }
    }

    var resultJson = Encoding.UTF8.GetString(memoryStream1.ToArray());
}

Here for each property (except for "TestData" property) I write the whole value as is (by calling element.WriteTo(utf8JsonWriter1)), and for "TestData" property I start a new object, add "Name" property and then copy each of the "TestData" object's properties.

P.S. This works, but I'm pretty sure a much better solution should exist.