System.Text.Json is a high-performance JSON API added in .NET Core 3.

This looks at how one might use its various features.

Sample data used: http://data.consumerfinance.gov/api/views.json

TODO.

TODO.

Utf8JsonReader is a pull parser similar to XmlReader. It is a bit low-level, presenting the document as a stream of tokens that you must interpret.

Complaint.Read implements a synchronous parser on top of ReadOnlySpan.

This is the simplest parser you can write on top of Utf8JsonReader, with each "object" being parsed in its own method and a clear call stack as you go through the document.

Complaint.ReadAsync implements an asynchronous parser on top of Stream.

This does not load the entire Stream into memory: instead, it is parsed in reasonably sized chunks. We must handle the case where Utf8JsonReader has exhausted the current buffer, and read the next chunk. Because Utf8JsonReader is a ref struct, we can not use it in an async method and must instead implement our own state machine manually. This makes things a bit harder to follow.

This is implemented via a resumable, I/O-agnostic parser ComplaintParser that is passed to one of three methods which handle the I/O:

• See JsonParser.Memory.cs for a simple Span-based implementation that uses that parser.
• See JsonParser.ParseSimpleAsync.cs for a Stream-based implementation that implements trivial copy & grow buffering.
• See Jsonparser.ParseNoCopyAsync.cs for a Stream-based implementation that implements sequences to avoid copying when growing buffers.