Umbrella for adopting .NET 11 and C# 15 in the 2027.0 line.
Summary of the analysis
Runtime async is the change with the largest potential impact, and it is the only one that is not
routine adoption work. It is present in .NET 11 and the framework libraries are compiled with it, but
it is not enabled for user code: a project needs <Features>runtime-async=on</Features> to get it.
That keeps it off the critical path, without making it safe to ignore, because opting in is one
silent MSBuild property that can be set repository-wide.
The C# 15 language features produce no work at all. Union types, closed hierarchies, extension
indexers, collection expression arguments, labeled break and continue, and the pointer
relaxations were each compiled against the .NET 11 SDK and their metadata inspected. Every one maps
onto a construct PostSharp already handles. Outside runtime async there are no known defects; the
remaining work is platform support, build and tooling updates.
Findings were verified against Preview 7 and re-verified against release candidate 1. All results
reproduced unchanged, including the exact IL bytes of a runtime-async method and the generated type
names for C# 15 extension blocks. The one finding that has not converged is whether runtime async is
enabled by default, where Microsoft documentation and the shipped bits disagree.
Analysis: Documentation/DotNet11-CSharp15-Impact.md in postsharp/PostSharp.
— Claude for Gael
Umbrella for adopting .NET 11 and C# 15 in the 2027.0 line.
Summary of the analysis
Runtime async is the change with the largest potential impact, and it is the only one that is not
routine adoption work. It is present in .NET 11 and the framework libraries are compiled with it, but
it is not enabled for user code: a project needs
<Features>runtime-async=on</Features>to get it.That keeps it off the critical path, without making it safe to ignore, because opting in is one
silent MSBuild property that can be set repository-wide.
The C# 15 language features produce no work at all. Union types, closed hierarchies, extension
indexers, collection expression arguments, labeled
breakandcontinue, and the pointerrelaxations were each compiled against the .NET 11 SDK and their metadata inspected. Every one maps
onto a construct PostSharp already handles. Outside runtime async there are no known defects; the
remaining work is platform support, build and tooling updates.
Findings were verified against Preview 7 and re-verified against release candidate 1. All results
reproduced unchanged, including the exact IL bytes of a runtime-async method and the generated type
names for C# 15 extension blocks. The one finding that has not converged is whether runtime async is
enabled by default, where Microsoft documentation and the shipped bits disagree.
Analysis:
Documentation/DotNet11-CSharp15-Impact.mdinpostsharp/PostSharp.— Claude for Gael