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Notes about MSBuild machinery

How is a rebuild determined?

Rebuilding is determined by:

  • the hash of the version of myriad
  • reference paths
  • every attributed file's content, output path, config key and params

The MSBuild implementation relies on _MyriadSdkCodeGenInputCache property which is defined in Myriad.Sdk.targets - one combined hash over every codegen input, not a per-file hash. Because of that, MyriadSdkGenerateCode's own up-to-date check is a single stamp file (_MyriadSdkGenerateStamp) covering the whole project, not one output-file check per attributed file: editing any one attributed file already invalidates the combined hash and forces every file to regenerate, so a per-file check would only have been misleading, not actually finer-grained. Confirmed by editing a single file and observing every generated file get rewritten, not just the one that depended on it.

Why one process, not one per file

MyriadSdkGenerateCode runs Myriad exactly once per build (when anything needs regenerating), for every attributed file in the project together, via a --manifest <path-to-toml> argument rather than one --inputfile/--outputfile/--configkey invocation per file. The manifest is a small TOML file, one [[unit]] table per file, written by MSBuild into $(IntermediateOutputPath)myriad.manifest.toml just before the <Exec>. Program.fs loops over every unit in one process, loading plugins once rather than once per file. The CLI's single-file flags (--inputfile, --outputfile, --configkey, --additionalparams, --generator-filter, --inlinegeneration) still work unchanged for direct/non-MSBuild invocation; --manifest is an alternative to that whole group, not an addition to it.

Per-item metadata (FlattenedParams, GeneratorsPipeDelimited) needed for the manifest is computed in a separate target, _MyriadSdkFlattenParams, batched per MyriadCodegen item over %(MyriadCodegen.OutputPath) (the one metadata value guaranteed unique per attributed file - batching on the input file's identity instead undercounts whenever several files share the same MyriadFile). ; is swapped for | in both values before they're embedded in the manifest, because the manifest-building step assigns them into an MSBuild Include attribute, which silently re-splits any unescaped ; into separate items.

A target's Condition runs before its own dependencies

MyriadSdkGenerateCode does not gate on '@(MyriadCodegen)' != '' at the target level, even though an empty project should skip codegen entirely. A target's own Condition is evaluated before its DependsOnTargets/BeforeTargets chain has run, so @(MyriadCodegen) - populated by _MyriadSdkFilesList, one of that chain's own targets - always reads as empty at that point and the target would skip unconditionally, every build. The guard instead sits on the <Exec> (and its follow-up <Touch>) inside the target body, evaluated after the dependency chain has actually populated the item list.