copy-to-output-directory

Use PreserveNewest for normal source files you edit, IfDifferent when the destination file gets mutated between builds and must be reset to the source, and Always only when you truly need a copy on every build. Never is the default for files not needed at runtime.

Always re-copies the file on every build, including otherwise-clean incremental builds, which is a measurable recurring cost. Switch to IfDifferent or set SkipUnchangedFilesOnCopyAlways to true so copies happen only when files actually differ.

IfDifferent and SkipUnchangedFilesOnCopyAlways require MSBuild 17.13 or later, which ships with .NET SDK 9.0.2xx and Visual Studio 2022 17.13. On older toolsets the value is not recognized and the item is silently not copied.

Set CopyToOutputDirectory to IfDifferent on the item. MSBuild compares timestamp and size, and re-copies the source over the mutated destination on the next build, restoring the known-good version without copying on every build.

Yes. Setting SkipUnchangedFilesOnCopyAlways to true makes the Always copy target pass SkipUnchangedFiles to the Copy task, so items copy only when they differ. Set it in Directory.Build.props to opt an entire repo in at once.