incremental-build

Build twice with binary logs using dotnet build /bl:first.binlog and /bl:second.binlog, then search the second log for 'Building target completely' versus 'Skipping target' messages. Search for 'is newer than output' to find the exact input file that triggered the rebuild.

Add both Inputs and Outputs attributes to the target so MSBuild compares file timestamps and skips it when outputs are newer. Use stable paths like $(IntermediateOutputPath), register generated files in the FileWrites item group, and avoid timestamps or GUIDs in output paths.

Visual Studio uses its own Fast Up-to-Date Check that is separate from MSBuild's Inputs/Outputs mechanism and can be wrong with custom targets or non-standard item types. Set DisableFastUpToDateCheck to true or enable verbose up-to-date check logging to see which file FUTDC considers out of date.

FileWrites tracks files generated during the build so dotnet clean can remove them. Files created by custom targets but not registered accumulate across builds, are never cleaned, and can confuse incremental up-to-date checks.

Use Returns when you only need to pass items to calling targets without affecting incremental build checks. Outputs serves double duty as both the incremental check and the return value, while Returns only defines the return value.