[WebPubSub] Upgrade MessagePack from 2.5.192 to 3.x

[jsquire]

Upgrade MessagePack from 2.5.192 to 3.x

Summary

The MessagePack package is currently pinned at version 2.5.192 in eng/Packages.Data.props. Version 3.x is now available with significant improvements but includes breaking changes that require a dedicated migration effort.

Current State

• Current Version: 2.5.192
• Latest Version: 3.1.4
• Location: eng/Packages.Data.props line ~280

Why Upgrade?

MessagePack 3.x introduces:

• AOT source generation by default using Roslyn source generators (improved startup performance, better debugging)
• Support for serializing private members without DynamicObjectResolverAllowPrivate
• Analyzers enabled by default with new diagnostics
• Custom formatters automatically discovered in same assembly
• New [CompositeResolver] attribute for faster runtime alternative
• .NET 9 support with Int128, UInt128, Rune, OrderedDictionary<T,V>, ReadOnlySet<T>

Breaking Changes

Major Changes

1. mpc tool is no longer available - replaced by Roslyn source generators
2. AOT source generation is the default - dynamic formatters still exist but are secondary
3. Custom formatters auto-discovery - formatters in same assembly are automatically used unless attributed with [ExcludeFormatterFromSourceGeneratedResolver]
4. Unity distribution changed - now via NuGetForUnity and UPM instead of .unitypackage

Migration Required

• Review all [MessagePackObject] types for compatibility
• Update any code that relied on mpc generation
• Test serialization behavior with new source generator
• Review analyzer warnings (now on by default)

Migration Guide

• Official migration guide

• Detailed blog

[github-actions]

Thanks for the feedback! We are routing this to the appropriate team for follow-up. cc @JialinXin @vicancy.

[Y-Sindo]

Hi @jsquire ,

MessagePack is a dependency brought in by the SignalR Functions extension library, Microsoft.Azure.WebJobs.Extensions.SignalRService.

Currently, the latest MessagePack version referenced by ASP.NET Core SignalR is 2.5.187. You can see this in the NuGet dependency list for Microsoft.AspNetCore.SignalR.Protocols.MessagePack:
https://www.nuget.org/packages/Microsoft.AspNetCore.SignalR.Protocols.MessagePack/11.0.0-preview.1.26104.118#dependencies-body-tab

Since our implementation is based on ASP.NET Core SignalR, we need to stay aligned with its dependency versions to avoid compatibility risks. Once ASP.NET Core SignalR updates its MessagePack dependency, we will follow accordingly.