jstedfast/MailKit · error · ArgumentNullException

ArgumentNullException

Error message

ArgumentNullException

What it means

IMailFolder.Append (extension) validates its parameters before touching the server. Passing a null FormatOptions throws ArgumentNullException because the options object controls message formatting and must always be supplied (or explicitly defaulted).

Solutions

1. Pass FormatOptions.Default instead of null.
2. Coalesce null options at the call site: options ?? FormatOptions.Default.
3. Fix the wrapper/helper that forwards a null options value.

Example fix

// before
folder.Append(null, messages, flags);
// after
folder.Append(FormatOptions.Default, messages, flags);

options ??= FormatOptions.Default;
folder.Append(options, messages, flags);

FormatOptions EnsureOptions(FormatOptions? o) => o ?? FormatOptions.Default;

try { folder.Append(options, messages, flags); } catch (ArgumentNullException ex) { /* ex.ParamName == "options": fix option source */ }

When it happens

Trigger: Calling folder.Append(null, messages, flags) where options was expected from a cached/shared variable that was never initialized.

Common situations: Refactors that introduced an options parameter but left call sites passing null; helper wrappers forwarding an optional options value that defaulted to null instead of FormatOptions.Default.

Related errors

• part
• ArgumentOutOfRangeException
• One or more of the messages is null.
• The number of messages and the number of flags must be…
• The uid is invalid.

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/afcc5fa74d76dde4. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/IMailFolderAppendExtensions.cs:935

		/// <exception cref="System.OperationCanceledException">
		/// The operation was canceled via the cancellation token.
		/// </exception>
		/// <exception cref="System.NotSupportedException">
		/// Internationalized formatting was requested but is not supported by the server.
		/// </exception>
		/// <exception cref="System.IO.IOException">
		/// An I/O error occurred.
		/// </exception>
		/// <exception cref="ProtocolException">
		/// The server's response contained unexpected tokens.
		/// </exception>
		/// <exception cref="CommandException">
		/// The command failed.
		/// </exception>
		public static IList<UniqueId> Append (this IMailFolder folder, FormatOptions options, IList<MimeMessage> messages, IList<MessageFlags> flags, CancellationToken cancellationToken = default, ITransferProgress? progress = null)
		{
			if (options == null)
				throw new ArgumentNullException (nameof (options));

			if (messages == null)
				throw new ArgumentNullException (nameof (messages));

			for (int i = 0; i < messages.Count; i++) {
				if (messages[i] == null)
					throw new ArgumentException ("One or more of the messages is null.");
			}

			if (flags == null)
				throw new ArgumentNullException (nameof (flags));

			if (messages.Count != flags.Count)
				throw new ArgumentException ("The number of messages and the number of flags must be equal.");

			var requests = new AppendRequest[messages.Count];
			for (int i = 0; i < messages.Count; i++) {
				requests[i] = new AppendRequest (messages[i], flags[i]) {

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