src/MSBuild/XMake.cs:57

• Microsoft.IO is only referenced when FeatureMSIORedist is enabled (via FEATURE_MSIOREDIST and conditional PackageReference). Guard these Microsoft.IO using-aliases with #if FEATURE_MSIOREDIST (and fall back to System.IO) rather than #if NETFRAMEWORK, otherwise .NET Framework builds with DotNetBuildSourceOnly=true will fail to compile due to missing Microsoft.IO.Redist reference.

#if NETFRAMEWORK
// Use I/O operations from Microsoft.IO.Redist which is generally higher perf
// and also works around https://github.com/dotnet/msbuild/issues/10540.
// Unnecessary on .NET 6+ because the perf improvements are in-box there.
using Microsoft.IO;
using Directory = Microsoft.IO.Directory;
using File = Microsoft.IO.File;
using FileInfo = Microsoft.IO.FileInfo;
using Path = Microsoft.IO.Path;
#endif

src/Shared/FileSystem/WindowsFileSystem.cs:72

• The #if NETFRAMEWORK branch calls into Microsoft.IO.File, but Microsoft.IO.Redist is only referenced when FEATURE_MSIOREDIST is enabled. This should be conditioned on FEATURE_MSIOREDIST (or ensure the package is always referenced for this build) to avoid compile-time breaks when building .NET Framework without the redist package.

#if NETFRAMEWORK
            return Microsoft.IO.File.Exists(path);
#else
            return File.Exists(path);
#endif