What causes "No .mc files generated." in dotnet/runtime?

Re-run with verbose logging and inspect the collection tool's stderr/return code (pmi/crossgen2/ilc) for the real failure.

dotnet/runtime · error · RuntimeError

No .mc files generated.

Error message

No .mc files generated.

What it means

Raised at the end of `__collect_mc_files__` (superpmi.py:1348-1350) when no `*.mc` files were produced in the temp directory after the pmi/crossgen2/nativeaot collection step. The `.mc` files are the per-assembly SuperPMI method-context output that later get merged into an MCH; zero of them means the underlying collection tool produced nothing usable.

Source

Thrown at src/coreclr/scripts/superpmi.py:1350

                # Set environment variables.
                nativeaot_command_env = env_copy.copy()
                set_and_report_env(nativeaot_command_env, root_env)

                old_env = os.environ.copy()
                os.environ.update(nativeaot_command_env)

                ilc_rsps = list(filter(lambda ilc_rsp: ilc_rsp.endswith(".ilc.rsp"), ilc_rsps))
                helper = AsyncSubprocessHelper(ilc_rsps, verbose=True)
                helper.run_to_completion(run_nativeaot, self)

                os.environ.clear()
                os.environ.update(old_env)
            ################################################################################################ end of "self.coreclr_args.nativeaot is True"

        mc_files = [os.path.join(self.temp_location, item) for item in os.listdir(self.temp_location) if item.endswith(".mc")]
        if len(mc_files) == 0:
            raise RuntimeError("No .mc files generated.")

    def __merge_mc_files__(self):
        """ Merge the mc files that were generated

        Notes:
            mcs -merge <s_baseMchFile> <s_tempDir>\\*.mc -recursive -dedup -thin

        """

        logging.info("Merging MC files")

        pattern = os.path.join(self.temp_location, "*.mc")

        command = [self.mcs_path, "-merge", self.base_mch_file, pattern, "-recursive", "-dedup", "-thin"]
        run_and_log(command)

        if not os.path.isfile(self.base_mch_file):
            raise RuntimeError("MCH file failed to be generated at: %s" % self.base_mch_file)

View on GitHub (pinned to 290d5ab72c)

Solutions

Re-run with verbose logging and inspect the collection tool's stderr/return code (pmi/crossgen2/ilc) for the real failure.
Verify `-assemblies` resolves to real managed dlls and that `-exclude` is not removing all of them.
Confirm Core_Root contains the matching clrjit/corerun for the chosen arch and build type.
Check the temp directory is writable and not being cleaned by AV/CI between writes.

Defensive patterns

Strategy: try-catch

Validate before calling

# After the collection driver runs, verify .mc output exists before merging
import glob
mc_files = glob.glob(os.path.join(temp_location, '*.mc'))
if not mc_files:
    raise SystemExit('No .mc files produced; inspect the pmi/crossgen2/ilc stderr above.')

Try / catch

try:
    collection.__collect_mc_files__()
except RuntimeError as e:
    if 'No .mc files' in str(e):
        logging.error('Collection produced no MC files; re-run verbose and check the JIT/Core_Root.')
        raise
    raise

Prevention

Always inspect the collection driver's return code/stderr before trusting MC output.
Confirm Core_Root and assemblies paths are correct and arch/build-type matched.

When it happens

Trigger: The collection driver (pmi.dll, crossgen2, or ilc) ran but wrote no `.mc` files into temp_location. Common causes: the collection tool exited non-zero or crashed, assemblies path was wrong/empty, Core_Root/JIT mismatch, env vars (DOTNET_*) misconfigured, or the exclude list removed everything.

Common situations: Pointing `-assemblies` at a path with no managed dlls; using a Core_Root missing clrjit/corerun; an arch/build-type mismatch between the JIT and the runtime; antimalware or permission issues deleting the `.mc` files as they are written.

Related errors

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/0c880c2771598baf. Report an issue: GitHub.