jstedfast/MailKit · error · InvalidOperationException

Indexes and '*' cannot be used while…

Error message

Indexes and '*' cannot be used while MessageNew/MessageExpunge is registered with NOTIFY for SELECTED.

What it means

This InvalidOperationException is thrown by ImapFolder.CheckAllowIndexes when a command uses message sequence numbers (indexes) or '*' while the NOTIFY extension has MessageNew/MessageExpunge registered for the SELECTED state. Under RFC 5465, such events make MSNs unstable, so MailKit refuses the operation to avoid acting on stale indexes.

Solutions

Switch to UID-based APIs: GetMessageUids/GetMessages(uid range), GetUidNext/GetUidValidity, or ImapFolder.Fetch by UID.
Reconfigure NOTIFY to not register MessageNew/MessageExpunge for SELECTED (adjust ImapClient.NotifyFlags), disabling this restriction.
Convert existing sequence-number logic to use UniqueId ranges obtained via GetUids/Search.
Use Search queries returning UniqueIds instead of '*' index expressions.

Example fix

// before
var summary = folder.GetMessage(0); // index with NOTIFY MessageNew/Expunge

// after
var uids = folder.Search(SearchQuery.All);
var summary = folder.GetMessage(new UniqueId(uids[0].Id));

Defensive patterns

Strategy: validation

Validate before calling

if (!client.Capabilities.HasFlag(ImapCapabilities.Quota) /* NOTIFY check */) { /* use UID APIs or skip */ }
bool canUseIndexes = !EngineNotifySelectedNewExpunge(client);

Prevention

Prefer UID-based APIs (GetMessage by UniqueId, Search returning UniqueIds) throughout the app.
Do not enable MessageNew/MessageExpunge in NOTIFY for SELECTED unless you only use UID APIs.
Centralize NOTIFY configuration so index APIs and notification flags are consistent.
Wrap index-based calls in try/catch for InvalidOperationException if NOTIFY may be active.

When it happens

Trigger: Calling GetMessages(index-based ranges), GetMessage by index, CopyTo/MoveTo with indexes, Fetch by index range, or Search queries using '*' after enabling NOTIFY with MessageNew/MessageExpunge for SELECTED (Engine.NotifySelectedNewExpunge true, e.g. via ImapClient.NotifyFlags/NotifyCommands).

Common situations: Apps that enable CONDSTORE/QRESYNC-style change notification (NotifyFlags with MessageNew|MessageExpunge) and then keep using index-based APIs; mixed legacy code using GetMessages(0, -1) with push notifications enabled.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/4da353fe8e06f8ea. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/Net/Imap/ImapFolder.cs:181

				throw new ServiceNotConnectedException ("The ImapClient is not connected.");

			if (Engine.State < ImapEngineState.Authenticated)
				throw new ServiceNotAuthenticatedException ("The ImapClient is not authenticated.");

			if (open) {
				var access = rw ? FolderAccess.ReadWrite : FolderAccess.ReadOnly;

				if (!IsOpen || Access < access)
					throw new FolderNotOpenException (FullName, access);
			}
		}

		void CheckAllowIndexes ()
		{
			// Indexes ("Message Sequence Numbers" or MSNs in the RFCs) and * are not stable while MessageNew/MessageExpunge is registered for SELECTED and therefore should not be used
			// https://tools.ietf.org/html/rfc5465#section-5.2
			if (Engine.NotifySelectedNewExpunge)
				throw new InvalidOperationException ("Indexes and '*' cannot be used while MessageNew/MessageExpunge is registered with NOTIFY for SELECTED.");
		}

		void CheckValidDestination (IMailFolder destination)
		{
			if (destination == null)
				throw new ArgumentNullException (nameof (destination));

			if (destination is not ImapFolder target || (target.Engine != Engine))
				throw new ArgumentException ("The destination folder does not belong to this ImapClient.", nameof (destination));
		}

		internal void Reset ()
		{
			// basic state
			((HashSet<string>) PermanentKeywords).Clear ();
			((HashSet<string>) AcceptedKeywords).Clear ();
			PermanentFlags = MessageFlags.None;
			AcceptedFlags = MessageFlags.None;

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