Feature request: treat Memory/ReadOnlyMemory<T> as T[] in System.Text.Json

[sandersaares]

EDIT: see #86442 (comment) for API proposal.

Original Proposal

I would like System.Text.Json to serialize/deserialize Memory/ReadOnlyMemory<T> using the same logic as it uses for T[].

This implies:

• Memory<byte> should be base64-encoded just like byte[] is
• Memory<FooBar> should be serialized as appropriate for the type FooBar, same as FooBar[] would.

The main motivation is that I have an array from ArrayPool<T>.Shared, which can have space for more items than I actually need. To serialize only a segment of the array, I would like to use Memory<T> to identify only use "used" part of the array.

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Issue Details

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I would like System.Text.Json to serialize/deserialize Memory/ReadOnlyMemory<T> using the same logic as it uses for T[].

This implies:

• Memory<byte> should be base64-encoded just like byte[] is
• Memory<FooBar> should be serialized as appropriate for the type FooBar, same as FooBar[] would.

The main motivation is that I have an array from ArrayPool<T>.Shared, which can have space for more items than I actually need. To serialize only a segment of the array, I would like to use Memory<T> to identify only use "used" part of the array.

Author:	sandersaares
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[eiriktsarpalis]

Also related to #73500, #81970 and #60780. One concern that has prevented us from adding built-in support for more types is trimmed application size, although this has become less of an issue for apps that only source generators.

[eiriktsarpalis]

Provisional API proposal:

namespace System.Text.Json.Serialization.Metadata;

public partial class JsonMetadataServices
{
    // Similar to byte[] serializes as Base64 strings
    public static JsonConverter<Memory<byte>> MemoryByteConverter { get; }
    public static JsonConverter<ReadOnlyMemory<byte>> ReadOnlyMemoryByteConverter { get; }

    // Other element types serialize as JSON arrays
    public static JsonTypeInfo<Memory<TElement>> CreateMemoryInfo<TElement>(JsonSerializerOptions options, JsonCollectionInfoValues<Memory<TElement>> collectionInfo);
    public static JsonTypeInfo<ReadOnlyMemory<TElement>> CreateReadOnlyMemoryInfo<TElement>(JsonSerializerOptions options, JsonCollectionInfoValues<ReadOnlyMemory<TElement>> collectionInfo);
}