Drop net9.0: target net10.0 only (single TargetFramework)

[credfeto]

Summary

This repository has one or more projects multi-targeting net9.0;net10.0. We're moving to support .NET 10 only, so drop net9.0 and target net10.0 exclusively.

Change

For every .csproj currently containing:

<TargetFrameworks>net9.0;net10.0</TargetFrameworks>

replace the element with a single TargetFramework:

<TargetFramework>net10.0</TargetFramework>

Treat this as a repository-wide rule (grep for the pattern above at the start of work), not a fixed file list: the set below reflects a scan of main at the time this issue was raised and may have drifted.

Known affected projects (at time of writing)

• src/FunFair.CodeAnalysis.Benchmark.Tests/FunFair.CodeAnalysis.Benchmark.Tests.csproj
• src/FunFair.CodeAnalysis.Tests/FunFair.CodeAnalysis.Tests.csproj

Also check

• TFM-specific conditions that reference net9.0 directly (e.g. Condition="'$(TargetFramework)' == 'net9.0'", #if NET9_0 (not _OR_GREATER), or PackageReference/ItemGroup blocks scoped to net9.0) become dead code once net9.0 is no longer a target; remove them per the Conditional Compilation and Dead Code rule in dotnet.instructions.md.
• #if NET9_0_OR_GREATER-style guards remain valid (net10.0 still satisfies _OR_GREATER); no action needed there.
• global.json and any CI workflow files pinning a net9-specific SDK version.
• Shared Directory.Build.props/Directory.Build.targets or solution-level MSBuild files referencing net9.0.
• Run dotnet buildcheck, dotnet build, and dotnet test after the change; fix anything that surfaces.

Origin

Raised directly by a human instruction (not an ad-hoc/blocked AI-initiated issue): "for all the writable credfeto funfair-tech repos that have dotnet projects that have a net9.0;net10.0 in main; switch the project to net10 so we just support dotnet 10."

[credfeto]

Implementation Plan

Files to change

• src/FunFair.CodeAnalysis.Benchmark.Tests/FunFair.CodeAnalysis.Benchmark.Tests.csproj: change <TargetFrameworks>net9.0;net10.0</TargetFrameworks> to <TargetFramework>net10.0</TargetFramework>
• src/FunFair.CodeAnalysis.Tests/FunFair.CodeAnalysis.Tests.csproj: change <TargetFrameworks>net9.0;net10.0</TargetFrameworks> to <TargetFramework>net10.0</TargetFramework>

(Re-grep for <TargetFrameworks>net9.0;net10.0</TargetFrameworks> at the start of work in case the set has drifted since this issue was raised.)

Approach

Change every matching .csproj's <TargetFrameworks>net9.0;net10.0</TargetFrameworks> element to a single <TargetFramework>net10.0</TargetFramework> element. Then search the same projects (and shared Directory.Build.props/CI workflow files) for any net9.0-specific conditions (MSBuild Condition attributes, #if NET9_0 non-_OR_GREATER guards) and remove them as dead code now that net9.0 is no longer a target; _OR_GREATER guards remain valid under net10.0 and need no change. Run dotnet buildcheck, dotnet build, and dotnet test to confirm everything still passes with the single target framework.

Test strategy

Run dotnet buildcheck -solution <solution> first (repository health check), then dotnet build and dotnet test for every project/solution in the repo. No new tests are needed since this is a target-framework change, not a behaviour change; existing tests must continue to pass unmodified under net10.0.

Assumptions

• Every affected project already builds and passes tests under net10.0 today (it is already multi-targeted), so removing net9.0 should not surface new failures beyond dead-code cleanup.
• No consumer of this package's published artifacts still requires a net9.0 asset.

Open questions

None, ready to proceed pending approval.