.NET 6: Modify JSON in Memory with the System.Text.Json.Nodes Namespace

This blog post contains some notes to help me remember how to use the System.Text.Json.Nodes namespace in .NET 6 and beyond to modify JSON in memory.

The C# code that I previously migrated from Newtonsoft.Json to System.Text.Json serialized and deserialized between objects and JSON, but never explicitly updated the JSON. Recently, I had a need to update JSON before deserialization and realized that, until .NET 6, System.Text.Json is read-only, and therefore useful only for serialization and deserialization, not for modifying the JSON node tree in memory.

• Migrating C# from Newtonsoft.Json to System.Text.Json for .NET 5 – Deliverystack.net

.NET six introduces the System.Text.Json.Nodes namespace for mutable JSON documents. Visual Studio community edition does not support the .NET 6 SDK, so I downloaded a prerelease version of JetBrains Rider that does.

• System.Text.Json.Nodes Namespace | Microsoft Docs
  • Provides types for handling an in-memory writeable document object model (DOM) for random access of the JSON elements within a structured view of the data.
• JsonArray Class (System.Text.Json.Nodes) | Microsoft Docs
  • Represents a mutable JSON array.
  • Object > JsonNode > JsonArray
• JsonNode Class (System.Text.Json.Nodes) | Microsoft Docs
  • The base class that represents a single node within a mutable JSON document.
  • Object > JsonNode
• JsonObject Class (System.Text.Json.Nodes) | Microsoft Docs
  • Represents a mutable JSON object.
  • Object > JsonNode > JsonObject
• JsonValue Class (System.Text.Json.Nodes) | Microsoft Docs
  • Represents a mutable JSON value.
  • Object > JsonNode > JsonValue

The System.Text.Json.Nodes namespace contains classes for in-memory writable structured JSON manipulation. I think of JsonNode as read-only, use JsonObject to update, and I haven’t found a need for JsonArray or JsonValue.

I don’t know if you can convert a JsonElement (read-only in the System.Text.Json namespace) to a JsonNode, but you can parse its raw JSON.

JsonNode jsonNode = JsonNode.Parse(jsonElement.GetRawText());

For demonstration purposes, assume that I have a key named container and I want to move its child keys to the root. So, this:

{
  "container": {
    "move": "this is or was in the container",
    "this": {
      "also": "need to be moved"
    },
    "too": "."
  }
}

Would become:

{
  "move": "this is or was in the container",
  "this": {
    "also": "need to be moved"
  },
  "too": "."
}

This test code seems to work correctly.

using System.Text.Json.Nodes;

// source JSON to process
string jsonString = File.ReadAllText("test.json");
Console.WriteLine(jsonString);

// root node (opening curly brace)
JsonObject? root = JsonNode.Parse(jsonString)?.AsObject();

// if the root contains no key named "container" 
JsonNode? containerNode = root?["container"];

if (containerNode == null)
{
    return;
}

// get the names of the keys under the container key
List<string> keys = containerNode.AsObject().Select(
    child => child.Key).ToList();

// convert read-only JsonNode to writable JsonObject 
JsonObject container = containerNode.AsObject();

// iterate and move keys from container to root
foreach (string key in keys)
{
    JsonNode? move = containerNode[key];
    container.Remove(key);
    root?.Add(key, move);
}

root?.Remove("container");

Console.WriteLine(root);

Note that at least with Rider using .NET 6 with C# 10, the code for a command line tool apparently does not need a static class declaration and main() method (entry point), so this is actually the complete program (minus the test.json file). And I could still access something named args that I couldn’t see declared, and I think use types defined in the System namespace.