System.Text.Json: Add option to write trailing commas on serialization.

[Drake53]

Background and motivation

Currently trailing commas are only supported on deserialization using JsonSerializerOptions.AllowTrailingCommas, it'd be nice if they can be supported on serialization as well.

One of the benefits of this feature would be improving diff views (for example with this library: https://github.com/mmanela/diffplex), since diffs look better when the JSON texts have trailing commas.

API Proposal

namespace System.Text.Json;

public class JsonSerializerOptions
{
    public bool WriteTrailingCommas { get; set; }
}

API Usage

var options = new JsonSerializerOptions
{
    WriteTrailingCommas = true,
    WriteIndented = true,
};

var oldText = JsonSerializer.Serialize(oldObject, options);
var newText = JsonSerializer.Serialize(newObject, options);

var diffView = new DiffViewer
{
    Dock = DockStyle.Fill,
    OldText = oldText,
    NewText = newText,
};

Alternative Designs

No response

Risks

No response

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Background and motivation

Currently trailing commas are only supported on deserialization using JsonSerializerOptions.AllowTrailingCommas, it'd be nice if they can be supported on serialization as well.

One of the benefits of this feature would be improving diff views (for example with this library: https://github.com/mmanela/diffplex), since diffs look better when the JSON texts have trailing commas.

API Proposal

namespace System.Text.Json;

public class JsonSerializerOptions
{
    public bool WriteTrailingCommas { get; set; }
}

API Usage

var options = new JsonSerializerOptions
{
    WriteTrailingCommas = true,
    WriteIndented = true,
};

var oldText = JsonSerializer.Serialize(oldObject, options);
var newText = JsonSerializer.Serialize(newObject, options);

var diffView = new DiffViewer
{
    Dock = DockStyle.Fill,
    OldText = oldText,
    NewText = newText,
};

Alternative Designs

No response

Risks

No response

Author:	Drake53
Assignees:	-
Labels:	api-suggestion, area-System.Text.Json
Milestone:	-

[fbrosseau]

Allowing to read non-standard extensions like commas (and probably much more often, comments) is very different from allowing to generate non-standard output.

Being able to read allows you to create a well-behaving application that happens to tolerate slightly invalid input (again, mostly comments) without having to reinvent systemtextjson, or without having to preprocess user input.

Being able to write would make dotnet generate objectively invalid output, which is probably not something you want as a framework.

since diffs look better when the JSON texts have trailing commas.

This is probably something for difftools to improve on. A Json-aware diff tool could offer to hide those details, the same way most difftools offer to hide insignificant whitespace.

[eiriktsarpalis]

Even though writing invalid JSON is something we would never consider for STJ, it might be possible to include opt-in support for reading certain forms of invalid JSON. This is tracked by #32291.