dotnet/runtime · error

Error : Invalid Option: %s

Error message

Error : Invalid Option: %s

What it means

ilasm fell through to the `InvalidOption` label (main.cpp:528-531): the switch prefix (`/` or `-`) was recognized but the option name or its argument did not match any known option, or a value-bearing option had no value. The offending argv element is printed so the user can see exactly which token was rejected.

Source

Thrown at src/coreclr/ilasm/main.cpp:530

                        if(pStr == NULL) goto InvalidOption;
                        pStr++;
                        const CHAR *pFmt = ((*pStr=='0')&&(*(pStr+1) == 'x'))? "%x" : "%d";
                        NarrowForNumberParsing str{pStr};
                        if(sscanf_s(str,pFmt,&g_dwTestRepeat)!=1) goto InvalidOption;
                    }
#endif
                    else if (!_stricmp(szOpt, "ANA"))
                    {
                        WCHAR *pEqualOrColon = EqualOrColon(argv[i]);
                        if (pEqualOrColon == NULL) goto InvalidOption;
                        MAKE_UTF8PTR_FROMWIDE_NOTHROW(anameOverride, pEqualOrColon + 1);
                        pAsm->m_pOverrideAssemblyName = _strdup(anameOverride);
                    }
                    else
                    {
                    InvalidOption:
                        MAKE_UTF8PTR_FROMWIDE_NOTHROW(invalidOpt, argv[i]);
                        fprintf(stderr, "Error : Invalid Option: %s\n", invalidOpt);
                        goto ErrorExit;
                    }
                }
                else
                {
                    if(u16_strlen(argv[i]) >= MAX_FILENAME_LENGTH)
                    {
                        printf("\nError: Input file name exceeds %d characters\n",MAX_FILENAME_LENGTH-1);
                        goto ErrorExit;
                    }
                    pwzInputFiles[NumFiles++] = argv[i];
                    if(NumFiles == 1)
                    {
                        MakeProperSourceFileName(argv[i], uCodePage, wzInputFilename, szInputFilename);
                    }
                }

            }

View on GitHub (pinned to 290d5ab72c)

Solutions

1. Run `ilasm /?` to list supported options and correct the flagged token.
2. Quote paths beginning with `/` or `-` and provide the missing `=` argument.
3. Ensure you are invoking ilasm (IL Assembler) and not another tool.

Example fix

// before
ilasm /target:library foo.il
// after
ilasm /DLL foo.il

// Whitelist known ilasm switches before assembling the command line.
const KNOWN = new Set(['DLL','EXE','OUT','DEBUG','OPT','CLOCK','DETERMINISTIC','QUIET','NOLOGO','X86','X64','ARM64','32BITPREFERRED','ALI','BAS','FLA','SUB','SSV','MSV','MDV','INC','KEY','KEYSOURCE','ANC','ANA']);
for (const opt of argv) if (opt.startsWith('/') && !KNOWN.has(opt.slice(1).split(/[=:]/)[0].toUpperCase())) throw new Error('Unknown ilasm option: ' + opt);

When it happens

Trigger: Typing an unrecognized flag (e.g. `/FOO`); a recognized flag missing its `=` value; a non-option token starting with `/` or `-` being parsed as a switch.

Common situations: Wrong tool in mind (passing csc/cl flags to ilasm); shell-quoting accident producing a stray token; version differences in supported switches.

Related errors

• Option %s: invalid form
• Error: Output file name exceeds %d characters
• File Alignment must be power of 2 from 0x200 to 0x10000
• Base address must be 0x10000-aligned
• Invalid Image Base specified for 32-bit target

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/4b1021865d8ebf02. Report an issue: GitHub.