Pipe Reader

Json+PipeReader deserialization support in MVC and Minimal APIs

PR: https://github.com/dotnet/aspnetcore/pull/62895

See https://github.com/dotnet/core/blob/main/release-notes/10.0/preview/preview7/libraries.md#pipereader-support-for-json-serializer

MVC, Minimal APIs, and the HttpRequestJsonExtensions.ReadFromJsonAsync methods have all been updated to use the new Json+PipeReader support without requiring any code changes from applications.

For most applications, the addition of this support has no effect on their behavior. However, if the application is using a custom xref:System.Text.Json.Serialization.JsonConverter, there's a chance that the converter doesn't handle xref:System.Text.Json.Utf8JsonReader.HasValueSequence%2A?displayProperty=nameWithType correctly. This can result in missing data and errors, such as xref:System.ArgumentOutOfRangeException, when deserializing.

The quick workaround (especially if you don't own the custom JsonConverter being used) is to set the "Microsoft.AspNetCore.UseStreamBasedJsonParsing" xref:System.AppContext switch to "true". This should be a temporary workaround, and the JsonConverter should be updated to support xref:System.Text.Json.Utf8JsonReader.HasValueSequence%2A.

To fix JsonConverter implementations, there's a quick fix that allocates an array from the ReadOnlySequence and would look like the following example:

public override T? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
    var span = reader.HasValueSequence ? reader.ValueSequence.ToArray() : reader.ValueSpan;
    // previous code
}

There's also a more complicated (but performant) fix, which would involve having a separate code path for the ReadOnlySequence handling:

public override T? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
    if (reader.HasValueSequence)
    {
        reader.ValueSequence;
        // ReadOnlySequence optimized path
    }
    else
    {
        reader.ValueSpan;
        // ReadOnlySpan optimized path
    }
}