{"record":{"id":"c4fc50a40f36ac29","repo":"jstedfast/MailKit","slug":"the-alias-cannot-be-empty","errorCode":null,"errorMessage":"The alias cannot be empty.","messagePattern":"The alias cannot be empty\\.","errorType":"validation","errorClass":"ArgumentException","httpStatus":null,"severity":"error","filePath":"MailKit/Net/Smtp/SmtpClient.cs","lineNumber":2681,"sourceCode":"\t\t/// <exception cref=\"SmtpProtocolException\">\n\t\t/// An SMTP protocol exception occurred.\n\t\t/// </exception>\n\t\tpublic override string Send (FormatOptions options, MimeMessage message, MailboxAddress sender, IEnumerable<MailboxAddress> recipients, CancellationToken cancellationToken = default, ITransferProgress? progress = null)\n\t\t{\n\t\t\tvar rcpts = ValidateArguments (options, message, sender, recipients);\n\n\t\t\treturn Send (options, message, sender, rcpts, cancellationToken, progress);\n\t\t}\n\n\t\t#endregion\n\n\t\tstring CreateExpandCommand (string alias)\n\t\t{\n\t\t\tif (alias == null)\n\t\t\t\tthrow new ArgumentNullException (nameof (alias));\n\n\t\t\tif (alias.Length == 0)\n\t\t\t\tthrow new ArgumentException (\"The alias cannot be empty.\", nameof (alias));\n\n\t\t\tif (alias.IndexOfAny (NewLineCharacters) != -1)\n\t\t\t\tthrow new ArgumentException (\"The alias cannot contain newline characters.\", nameof (alias));\n\n\t\t\tCheckDisposed ();\n\n\t\t\tif (!IsConnected)\n\t\t\t\tthrow new ServiceNotConnectedException (\"The SmtpClient is not connected.\");\n\n\t\t\treturn string.Format (\"EXPN {0}\\r\\n\", alias);\n\t\t}\n\n\t\tstatic InternetAddressList ParseExpandResponse (SmtpResponse response)\n\t\t{\n\t\t\tif (response.StatusCode != SmtpStatusCode.Ok)\n\t\t\t\tthrow new SmtpCommandException (SmtpErrorCode.UnexpectedStatusCode, response.StatusCode, response.Response);\n\n\t\t\tvar lines = response.Response.Split ('\\n');","sourceCodeStart":2663,"sourceCodeEnd":2699,"githubUrl":"https://github.com/jstedfast/MailKit/blob/9d3859a7855e3e17582c07fd01972b8e262bf176/MailKit/Net/Smtp/SmtpClient.cs#L2663-L2699","documentation":"SmtpClient.Expand/CreateExpandCommand throws ArgumentException when the alias is the empty string. An empty alias would produce the invalid command 'EXPN \\r\\n' and cannot be resolved by any SMTP server, so it is rejected up front.","triggerScenarios":"Calling Expand/ExpandAsync with \"\" — e.g. a trimmed user input that was empty, or string.Split producing an empty element that is passed straight through.","commonSituations":"User submitted a blank mailing-list name; Split(',') on an input like \"a,,b\" yielding empty entries; config key present but set to empty string.","solutions":["Check alias.Length > 0 (or !string.IsNullOrWhiteSpace) before calling Expand","Trim and validate user-supplied aliases; reject empty input with a user-facing error","Skip empty entries when iterating over a split alias list"],"exampleFix":"// before\nforeach (var alias in input.Split(','))\n    await smtpClient.ExpandAsync(alias);\n// after\nforeach (var alias in input.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries))\n    await smtpClient.ExpandAsync(alias);","handlingStrategy":"validation","validationCode":"if (string.IsNullOrWhiteSpace(alias))\n    throw new ArgumentException(\"Alias must not be empty\", nameof(alias));\nawait smtpClient.ExpandAsync(alias);","typeGuard":"bool IsValidAlias(string a) => !string.IsNullOrWhiteSpace(a);","tryCatchPattern":"try {\n    var list = await smtpClient.ExpandAsync(alias);\n} catch (ArgumentException) {\n    logger.LogWarning(\"EXPN called with empty alias\");\n}","preventionTips":["Trim user input before passing to SMTP commands","Use RemoveEmptyEntries when splitting alias lists","Treat blank form fields as validation errors before touching the client"],"tags":["smtp","validation","mailkit"],"backgroundTag":"empty-required-field","analyzedSha":"9d3859a7855e3e17582c07fd01972b8e262bf176","analyzedAt":"2026-09-15T15:46:11.592Z","contentChangedAt":"2026-09-15T15:46:11.592Z","schemaVersion":2},"datasetVersion":"2026-09-23T08:17:48.524Z"}