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

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.

Here is a possible answer

static void Main(string[] args)    {        var jsonString = @"        {            ""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""            }        }        ";        var jsonDoc = JsonSerializer.Deserialize<Dictionary<string, object>>(jsonString);        var testDataDict = JsonSerializer.Deserialize<Dictionary<string, object>>(jsonDoc["TestData"].ToString());        testDataDict.Add("Name", "John");        //replace the test data with the modified test data        jsonDoc["TestData"] = testDataDict;        Console.WriteLine(JsonSerializer.Serialize(jsonDoc));    }

I've just created a NuGet package with some hopefully useful extension methods for JsonElement, which allow properties to be added and/or removed. It's based on using the Utf8JsonWriter to create a new mutated JsonElement based on the original, like the answer above.

GitHub repo | NuGet package

var jsonString = "{ \"Name\": \"Andrew\", \"EmailAddress\": \"a@b.com\" }";var jElement = JsonDocument.Parse(jsonString).RootElement;jElement = jElementAddProperty("Age", 38).AddProperty("Male", true).AddProperty("Female", false).AddNullProperty("Alien").AddProperty("Roles", new string[] { "admin", "user" }).AddProperty("LastUpdated", DateTime.UtcNow).AddProperty("crazyNewObject", new{    Name = "Hobbies",    Value = "bass guitar and writing c# code"});

Hopefully someone will find them useful, but if they don't do quite what you need, please enhance and submit a pull request.