System Text JSON Dom And Streaming

DOM & streaming APIs

Required setup

The DOM types are not in the System.Text.Json namespace. JsonNode/JsonObject/JsonArray live in System.Text.Json.Nodes; forgetting that using is the usual reason this code won't compile (JsonDocument, JsonElement, Utf8JsonReader and Utf8JsonWriter are in System.Text.Json):

using System.Text.Json;         // JsonSerializer, JsonSerializerOptions, JsonDocument, JsonElement, Utf8JsonReader/Writer
using System.Text.Json.Nodes;   // JsonNode, JsonObject, JsonArray, JsonValue

Build one options instance and reuse it — a fresh one is expensive (it caches per-type metadata on first use). options in the examples below is that instance:

static readonly JsonSerializerOptions Options = new(JsonSerializerDefaults.Web);

Mutable DOM — JsonNode (the JObject replacement)

Use when you need to read/modify JSON without a CLR type.

JsonNode root = JsonNode.Parse(json)!;
string name = (string)root["user"]!["name"]!;      // index like a dictionary/array
root["user"]!["active"] = true;                    // mutate
root["tags"]!.AsArray().Add("new");
string outJson = root.ToJsonString(new JsonSerializerOptions { WriteIndented = true });

• JsonNode (mutable) → JsonObject / JsonArray / JsonValue. This is the Newtonsoft JObject/ JArray/JToken analog.
• node.GetValue<T>() or explicit casts pull typed values; missing paths return null (guard with !).

Read-only DOM — JsonDocument / JsonElement

Fastest way to inspect JSON you won't mutate. JsonDocument is IDisposable (it rents pooled buffers) — dispose it, and don't let a JsonElement outlive its document:

using JsonDocument doc = JsonDocument.Parse(json);
JsonElement root = doc.RootElement;
foreach (JsonElement item in root.GetProperty("items").EnumerateArray())
    Console.WriteLine(item.GetProperty("id").GetInt32());

Low-level Utf8JsonReader / Utf8JsonWriter (hottest paths)

Allocation-free forward-only read/write over UTF-8 bytes — reach for these only when profiling demands it (custom converters use them internally):

var writer = new Utf8JsonWriter(bufferWriter);
writer.WriteStartObject();
writer.WriteString("name", "Ada");
writer.WriteNumber("count", 3);
writer.WriteEndObject();
writer.Flush();

var reader = new Utf8JsonReader(utf8Bytes);
while (reader.Read()) { /* switch on reader.TokenType */ }

Bytes & streams — skip the intermediate string

For I/O, prefer the UTF-8 / Stream overloads over Serialize→string→Encoding.UTF8.GetBytes:

byte[] utf8 = JsonSerializer.SerializeToUtf8Bytes(value, options);
await JsonSerializer.SerializeAsync(stream, value, options);
var value = await JsonSerializer.DeserializeAsync<MyType>(stream, options);

Streaming large arrays — DeserializeAsyncEnumerable

Process a huge top-level JSON array without buffering it all in memory:

await foreach (var item in JsonSerializer.DeserializeAsyncEnumerable<Item>(stream, options))
    Handle(item);   // yields items as they are parsed

• Only the current item is materialized — ideal for large feeds/logs.
• All of the above compose with source generation: pass a JsonTypeInfo<T> from a generated context instead of options to stay AOT-safe. Source generation is covered separately.