Updating JSON Package Readme (#90102)

* Updating Json package readme

* Update src/libraries/System.Text.Json/src/PACKAGE.md

Co-authored-by: Eirik Tsarpalis <[email protected]>

* Apply suggestions from code review

Co-authored-by: Eirik Tsarpalis <[email protected]>

* Remove sharplab comment

* Update PACKAGE.md

* Update PACKAGE.md

* Remove TODO

---------

Co-authored-by: Eirik Tsarpalis <[email protected]>

Author: ericstj

src/libraries/System.Text.Json/src/PACKAGE.md

@@ -1,11 +1,257 @@
 ## About
 
+<!-- A description of the package and where one can find more documentation -->
+
 Provides high-performance and low-allocating types that serialize objects to JavaScript Object Notation (JSON) text and deserialize JSON text to objects, with UTF-8 support built-in. Also provides types to read and write JSON text encoded as UTF-8, and to create an in-memory document object model (DOM), that is read-only, for random access of the JSON elements within a structured view of the data.
 
-The `System.Text.Json` library is built-in as part of the shared framework in .NET Runtime. The package can be installed when you need to use it in other target frameworks.
+## Key Features
+
+<!-- The key features of this package -->
+
+* High-performance reader and writer types for UTF-8 encoded JSON.
+* A fully-featured JSON serializer for .NET types using reflection or source generated contracts.
+* A high-performance read-only JSON DOM (JsonDocument) and a mutable DOM that interoperates with the serializer (JsonNode).
+* Built-in support for async serialization, including IAsyncEnumerable support.
+* Fully customizable contract model for serializable types.
+
+## How to Use
+
+<!-- A compelling example on how to use this package with code, as well as any specific guidelines for when to use the package -->
+
+The System.Text.Json library is built-in as part of the shared framework in .NET Runtime. The package can be installed when you need to use the most recent version in older target frameworks.
+
+Serialization:
+```csharp
+using System;
+using System.Text.Json;
+
+WeatherForecast forecast = new (DateTimeOffset.Now, 26.6f, "Sunny");
+var serialized = JsonSerializer.Serialize(forecast);
+
+Console.WriteLine(serialized);
+// {"Date":"2023-08-02T16:01:20.9025406+00:00","TemperatureCelsius":26.6,"Summary":"Sunny"}
+
+var forecastDeserialized = JsonSerializer.Deserialize<WeatherForecast>(serialized);
+Console.WriteLine(forecast == forecastDeserialized);
+// True
+
+public record WeatherForecast(DateTimeOffset Date, float TemperatureCelsius, string? Summary);
+```
+
+Serialization using the source generator:
+```csharp
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+WeatherForecast forecast = new (DateTimeOffset.Now, 26.6f, "Sunny");
+var serialized = JsonSerializer.Serialize(forecast, SourceGenerationContext.Default.WeatherForecast);
+
+Console.WriteLine(serialized);
+// {"Date":"2023-08-02T16:01:20.9025406+00:00","TemperatureCelsius":26.6,"Summary":"Sunny"}
+
+var forecastDeserialized = JsonSerializer.Deserialize<WeatherForecast>(serialized, SourceGenerationContext.Default.WeatherForecast);
+Console.WriteLine(forecast == forecastDeserialized);
+// True
+
+public record WeatherForecast(DateTimeOffset Date, float TemperatureCelsius, string? Summary);
+
+[JsonSourceGenerationOptions(WriteIndented = true)]
+[JsonSerializable(typeof(WeatherForecast))]
+internal partial class SourceGenerationContext : JsonSerializerContext
+{
+}
+```
+
+Using the JSON DOM:
+```csharp
+
+using System;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+string jsonString =
+@"{
+  ""Date"": ""2019-08-01T00:00:00"",
+  ""Temperature"": 25,
+  ""Summary"": ""Hot"",
+  ""DatesAvailable"": [
+    ""2019-08-01T00:00:00"",
+    ""2019-08-02T00:00:00""
+  ],
+  ""TemperatureRanges"": {
+      ""Cold"": {
+          ""High"": 20,
+          ""Low"": -10
+      },
+      ""Hot"": {
+          ""High"": 60,
+          ""Low"": 20
+      }
+  }
+}
+";
+
+JsonNode forecastNode = JsonNode.Parse(jsonString)!;
+
+
+// Get value from a JsonNode.
+JsonNode temperatureNode = forecastNode["Temperature"]!;
+Console.WriteLine($"Type={temperatureNode.GetType()}");
+Console.WriteLine($"JSON={temperatureNode.ToJsonString()}");
+//output:
+//Type = System.Text.Json.Nodes.JsonValue`1[System.Text.Json.JsonElement]
+//JSON = 25
+
+// Get a typed value from a JsonNode.
+int temperatureInt = (int)forecastNode["Temperature"]!;
+Console.WriteLine($"Value={temperatureInt}");
+//output:
+//Value=25
+
+// Get a typed value from a JsonNode by using GetValue<T>.
+temperatureInt = forecastNode["Temperature"]!.GetValue<int>();
+Console.WriteLine($"TemperatureInt={temperatureInt}");
+//output:
+//Value=25
+
+// Get a JSON object from a JsonNode.
+JsonNode temperatureRanges = forecastNode["TemperatureRanges"]!;
+Console.WriteLine($"Type={temperatureRanges.GetType()}");
+Console.WriteLine($"JSON={temperatureRanges.ToJsonString()}");
+//output:
+//Type = System.Text.Json.Nodes.JsonObject
+//JSON = { "Cold":{ "High":20,"Low":-10},"Hot":{ "High":60,"Low":20} }
+
+// Get a JSON array from a JsonNode.
+JsonNode datesAvailable = forecastNode["DatesAvailable"]!;
+Console.WriteLine($"Type={datesAvailable.GetType()}");
+Console.WriteLine($"JSON={datesAvailable.ToJsonString()}");
+//output:
+//datesAvailable Type = System.Text.Json.Nodes.JsonArray
+//datesAvailable JSON =["2019-08-01T00:00:00", "2019-08-02T00:00:00"]
+
+// Get an array element value from a JsonArray.
+JsonNode firstDateAvailable = datesAvailable[0]!;
+Console.WriteLine($"Type={firstDateAvailable.GetType()}");
+Console.WriteLine($"JSON={firstDateAvailable.ToJsonString()}");
+//output:
+//Type = System.Text.Json.Nodes.JsonValue`1[System.Text.Json.JsonElement]
+//JSON = "2019-08-01T00:00:00"
+
+// Get a typed value by chaining references.
+int coldHighTemperature = (int)forecastNode["TemperatureRanges"]!["Cold"]!["High"]!;
+Console.WriteLine($"TemperatureRanges.Cold.High={coldHighTemperature}");
+//output:
+//TemperatureRanges.Cold.High = 20
+
+// Parse a JSON array
+JsonNode datesNode = JsonNode.Parse(@"[""2019-08-01T00:00:00"",""2019-08-02T00:00:00""]")!;
+JsonNode firstDate = datesNode[0]!.GetValue<DateTime>();
+Console.WriteLine($"firstDate={ firstDate}");
+//output:
+//firstDate = "2019-08-01T00:00:00"
+```
+
+Using the low-level JSON reader/writer types
+```csharp
+using System;
+using System.IO;
+using System.Text;
+using System.Text.Json;
+
+var writerOptions = new JsonWriterOptions
+{
+    Indented = true
+};
+
+using var stream = new MemoryStream();
+using var writer = new Utf8JsonWriter(stream, writerOptions);
+
+writer.WriteStartObject();
+writer.WriteString("date", DateTimeOffset.Parse("8/2/2023 9:00 AM"));
+writer.WriteNumber("temp", 42);
+writer.WriteEndObject();
+writer.Flush();
+
+var jsonBytes = stream.ToArray();
+string json = Encoding.UTF8.GetString(jsonBytes);
+Console.WriteLine(json);
+// {
+//   "date": "2023-08-02T09:00:00+00:00"
+//   "temp": 42
+// }
+
+var readerOptions = new JsonReaderOptions
+{
+    AllowTrailingCommas = true,
+    CommentHandling = JsonCommentHandling.Skip
+};
+var reader = new Utf8JsonReader(jsonBytes, readerOptions);
+
+while (reader.Read())
+{
+    Console.Write(reader.TokenType);
+
+    switch (reader.TokenType)
+    {
+        case JsonTokenType.PropertyName:
+        case JsonTokenType.String:
+            {
+                string? text = reader.GetString();
+                Console.Write(" ");
+                Console.Write(text);
+                break;
+            }
+
+        case JsonTokenType.Number:
+            {
+                int intValue = reader.GetInt32();
+                Console.Write(" ");
+                Console.Write(intValue);
+                break;
+            }
+
+            // Other token types elided for brevity
+    }
+    Console.WriteLine();
+}
+// StartObject
+// PropertyName date
+// String 2023-08-02T09:00:00+00:00
+// PropertyName temp
+// Number 42
+// EndObject
+```
+
+## Main Types
+
+<!-- The main types provided in this library -->
+
+The main types provided by this library are:
+
+* `System.Text.Json.Utf8JsonWriter`
+* `System.Text.Json.Utf8JsonReader`
+* `System.Text.Json.JsonSerializer`
+* `System.Text.Json.JsonConverter`
+* `System.Text.Json.JsonDocument`
+* `System.Text.Json.Nodes.JsonNode`
+* `System.Text.Json.Serialization.Metadata.JsonTypeInfo`
+
+## Addtional Documentation
+
+* [Conceptual documentation](https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/overview)
+* [API documentation](https://learn.microsoft.com/en-us/dotnet/api/system.text.json)
+
+## Related Packages
+
+<!-- The related packages associated with this package -->
+
+* Lightweight data formats abstraction: [System.Memory.Data](https://www.nuget.org/packages/System.Memory.Data/)
+* Serialization of HttpContent: [System.Net.Http.Json](https://www.nuget.org/packages/System.Net.Http.Json/)
+
+
+## Feedback & Contributing
 
-For more information, see the documentation:
+<!-- How to provide feedback on this package and contribute to it -->
 
-- [JSON serialization and deserialization in .NET](https://docs.microsoft.com/dotnet/standard/serialization/system-text-json-overview)
-- [How to serialize and deserialize JSON in .NET](https://docs.microsoft.com/dotnet/standard/serialization/system-text-json-how-to)
-- [System.Text.Json API reference](https://docs.microsoft.com/dotnet/api/system.text.json)
+System.Text.Json is released as open source under the [MIT license](https://licenses.nuget.org/MIT). Bug reports and contributions are welcome at [the GitHub repository](https://github.com/dotnet/runtime).