What causes "Option %s: invalid form " in dotnet/runtime?

Remove the lone '-' or replace it with the intended option (e.g. -d / --debug).

Option %s: invalid form

Error message

Option %s: invalid form

What it means

Printed by parse_args() when an argv token starts with an option delimiter (is_cli_option true) but has length 1 — i.e. the token is exactly '-' (or '/' on Windows). Such a token has no option name and is rejected as invalid form. Parsing then breaks out of the loop and returns false, causing EXIT_FAILURE in MAIN.

Source

Thrown at src/coreclr/hosts/corerun/corerun.cpp:769

            i++; // Move to next argument.

            config.entry_assembly_argc = argc - i;
            config.entry_assembly_argv = (const char_t**)::malloc(config.entry_assembly_argc * sizeof(const char_t*));
            assert(config.entry_assembly_argv != nullptr);
            for (int c = 0; c < config.entry_assembly_argc; ++c)
            {
                config.entry_assembly_argv[c] = pal::strdup(argv[i + c]);
            }

            // Successfully parsed arguments.
            return true;
        }

        const char_t* arg = argv[i];
        size_t arg_len = pal::strlen(arg);
        if (arg_len == 1)
        {
            pal::fprintf(stderr, W("Option %s: invalid form\n"), arg);
            break; // Invalid option
        }

        const char_t* option = arg + 1;
        if (option[0] == W('-')) // Handle double '--'
            option++;

        // Path to core_root
        if (pal::strcmp(option, W("c")) == 0 || (pal::strcmp(option, W("clr-path")) == 0))
        {
            i++;
            if (i < argc)
            {
                config.clr_path = argv[i];
            }
            else
            {
                pal::fprintf(stderr, W("Option %s: missing path\n"), arg);

View on GitHub (pinned to 290d5ab72c)

Solutions

Remove the lone '-' or replace it with the intended option (e.g. -d / --debug).
Run 'corerun --help' (or -h / -?) to see the valid options.
Validate the command line in your build script with a check that no token is a single '-'.

Example fix

# before
corerun - App.dll
# after
corerun -d App.dll

Defensive patterns

Strategy: validation

Validate before calling

# reject a lone '-' / '/' before launching corerun
for a in "$@"; do
  if [ "$a" = "-" ] || [ "$a" = "/" ]; then
    echo "invalid lone option token: $a"; exit 2
  fi
done

Prevention

Double-check options that begin with '-' actually have a name after the dash.
Run 'corerun --help' when in doubt.
In scripts, fail fast on a single-dash token.

When it happens

Trigger: User runs: corerun - App.dll (a bare '-' as first token), or 'corerun App.dll -' where the lone '-' is hit before the assembly token... but actually the assembly is taken as soon as a non-option token appears, so the trigger is specifically a leading lone '-' / '/' before the assembly.

Common situations: Typos like 'corerun -' meaning to type '-d'; copy-paste of an empty option; shell quoting that collapsed an option to a single dash; using '/' alone on Windows.

Related errors

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/78d02a04c1d83ec9. Report an issue: GitHub.