jstedfast/MailKit · error · ArgumentException

Must supply at least one folder.

Error message

Must supply at least one folder.

What it means

ImapEventGroup.Mailboxes.CreateEventGroup-style constructor requires a non-empty list of folders; passing an empty IList<IMailFolder> throws ArgumentException("Must supply at least one folder."). A mailboxes event group with zero folders would produce a meaningless NOTIFY filter.

Solutions

1. Check folders.Count > 0 before calling Mailboxes and skip or short-circuit the NOTIFY setup if empty
2. Fall back to ImapEventGroup.Personal or Selected when no specific folders are available
3. Fix the upstream query that produced the empty folder list if it should have returned folders

Example fix

// before
var group = ImapEventGroup.Mailboxes("notify", matchedFolders);

// after
if (matchedFolders.Count == 0) {
    var group = ImapEventGroup.Personal();
} else {
    var group = ImapEventGroup.Mailboxes("notify", matchedFolders);
}

if (folders == null || folders.Count == 0)
    throw new ArgumentException("Must supply at least one folder before building a Mailboxes event group.");

bool HasFolders(IList<IMailFolder> folders) => folders is { Count: > 0 };

try {
    var group = ImapEventGroup.Mailboxes(name, folders);
} catch (ArgumentException ex) when (ex.ParamName == "folders") {
    // fall back to Personal/Selected group or skip NOTIFY
}

When it happens

Trigger: Calling ImapEventGroup.Mailboxes (the static factory/constructor for a folders-based group) with an empty List<IMailFolder>, e.g. when a folder-search produced no results before constructing the group.

Common situations: Computing the folder list dynamically (e.g. from a UI selection or GetFolders result) and passing it unfiltered; empty search results silently flowing into the NOTIFY setup.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

• All folders must be ImapFolders.
• Annotation entry paths must not end with '/'.
• Annotation entry paths must not end with '.'.
• Cannot rename a folder using itself as the new parent…
• Cannot search for an empty set of unique identifiers.

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/f749393bd80cc5ac. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/Net/Imap/ImapEventGroup.cs:309

			/// </remarks>
			/// <param name="name">The name of the mailbox filter.</param>
			/// <param name="folders">The list of folders to watch for events.</param>
			/// <exception cref="System.ArgumentNullException">
			/// <paramref name="folders"/> is <see langword="null" />.
			/// </exception>
			/// <exception cref="System.ArgumentException">
			/// <para>The list of <paramref name="folders"/> is empty.</para>
			/// <para>-or-</para>
			/// <para>The list of <paramref name="folders"/> contains folders that are not of
			/// type <see cref="ImapFolder"/>.</para>
			/// </exception>
			internal Mailboxes (string name, IList<IMailFolder> folders) : base (name)
			{
				if (folders == null)
					throw new ArgumentNullException (nameof (folders));

				if (folders.Count == 0)
					throw new ArgumentException ("Must supply at least one folder.", nameof (folders));

				this.folders = new ImapFolder[folders.Count];
				for (int i = 0; i < folders.Count; i++) {
					if (folders[i] is not ImapFolder folder)
						throw new ArgumentException ("All folders must be ImapFolders.", nameof (folders));

					this.folders[i] = folder;
				}
			}

			/// <summary>
			/// Format the IMAP NOTIFY command for this particular IMAP mailbox filter.
			/// </summary>
			/// <remarks>
			/// Formats the IMAP NOTIFY command for this particular IMAP mailbox filter.
			/// </remarks>
			/// <param name="engine">The IMAP engine.</param>
			/// <param name="command">The IMAP command builder.</param>

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