System.Text.Json.JsonSerializer: boxing when serialize structs

[vvdev]

Description

I noticed that JsonSerializer produces at least one boxing per every serialized struct with public properties/fields.
And maybe because of that, serialization of class instances is noticeably faster than serialization of structs with similar structure.

I was trying to implement some allocation-free processing with buffers and Utf8JsonWriter instances pooling, and found that it anyway impossible with current JsonSerializer implementation because of the boxing.
(because if i make my types i need to serialize classes, then i get rid of boxing, but instances i need to create and serialize becoming the source of allocations).

Configuration

Tested on .NET7 and .NET8

Data

Any generic or non-generic struct with public properties/fields, for example PointF or something like:

	public struct Test
	{
		private string _value;
		public string Value { get => _test; set => value; }
	}

or

	public private struct Test<T>
	{
		private T _value;
		public T Value { get => _test; set => value; }
	}

Analysis

Obviously, boxing happens (at least) in ObjectDefaultConverter, method OnTryWrite:

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Object/ObjectDefaultConverter.cs

Line 313 in 479df47

object obj = value; // box once

Wouldn't it be good to make this and related code boxing-free?

[ghost]

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Issue Details

---

Description

I noticed that JsonSerializer produces at least one boxing per every serialized struct with public properties/fields.
And maybe because of that, serialization of class instances is noticeably faster than serialization of structs with similar structure.

I was trying to implement some allocation-free processing with buffers and Utf8JsonWrite instances pooling, and found that it anyway impossible with current JsonSerializer implementation because of the boxing.
(because if i make my types i need to serialize classes, then i get rid of boxing, but instances i need to create and serialize becoming the source of allocations).

Configuration

Tested on .NET7 and .NET8

Data

Any generic or non-generic struct with public properties/fields, for example PointF or something like:

	private struct Test
	{
		private string _value;
		public string Value { get => _test; set => value; }
	}

or

	private struct Test<T>
	{
		private T _value;
		public T Value { get => _test; set => value; }
	}

Analysis

Obviously, boxing happens (at least) in ObjectDefaultConverter, method OnTryWrite:

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Object/ObjectDefaultConverter.cs

Line 313 in 479df47

object obj = value; // box once

Wouldn't it be good to make this and related code boxing-free?

Author:	vvdev
Assignees:	-
Labels:	area-System.Text.Json, tenet-performance, untriaged
Milestone:	-

[eiriktsarpalis]

This is a side-effect of the current design of the JsonPropertyInfo class, which is generic only on the property type but not the declaring type. We have this issue tracking a potential improvement however there are trade-offs:

1. It would regress the static footprint of Native AOT applications due to more generic specializations being necessary and
2. The current design has leaked to public APIs in the case of the source generator, so we would need to author replacement APIs to make this possible.

Duplicate of #82720

[vvdev]

@eiriktsarpalis, thanks for explanation.