jstedfast/MailKit · error · InvalidOperationException

Cannot create child folders.

Error message

Cannot create child folders.

What it means

This InvalidOperationException is thrown by QueueCreateCommand when attempting to create a child folder under a parent whose DirectorySeparator is '\0' - meaning the server (or this folder) does not support a hierarchy separator, so child folders cannot exist under it.

Solutions

1. Ensure the parent folder's children were loaded (folder.Open + get subfolders) so DirectorySeparator is known before creating children.
2. Create the folder with its full hierarchical path from the top-level (root) folder instead of nesting: root.CreateFolder("Parent/Child") with the correct separator.
3. Check folder.DirectorySeparator before attempting nested creation and fall back to flat names if '\0'.
4. Some servers (or folder objects not opened as selectable) never expose a separator; verify server hierarchy support (LIST "" "%%").

Example fix

// before
var parent = client.GetFolder("Parent");
parent.CreateFolder("Child"); // throws if separator unknown

// after
var parent = client.GetFolder("Parent");
if (parent.DirectorySeparator == '\0')
    client.GetFolder(Cancellation).CreateFolder("Parent" + sep + "Child");
else
    parent.CreateFolder("Child");

if (string.IsNullOrEmpty(folder.FullName) || folder.DirectorySeparator != '\0') { folder.CreateFolder(name); } else { /* create via full path from root */ }

try { parent.CreateFolder(name); } catch (InvalidOperationException) { root.CreateFolder(parent.FullName + sep + name); }

When it happens

Trigger: Calling childFolder.CreateFolder(name) where childFolder has a non-empty FullName but DirectorySeparator == '\0' - typical when the folder was opened without its children (children collection not loaded) or the server reports no separator for that mailbox.

Common situations: Creating subfolders under a folder whose separator is unknown because the folder was never opened/its children not initialized; servers that report no hierarchy delimiter for that mailbox; using a stale folder reference where DirectorySeparator was never populated.

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

• An ImapFolder has not been opened.
• Annotation attribute specifiers cannot be empty.
• Annotation attribute specifiers cannot contain '*' or '%'.
• Annotation entry paths cannot be empty.
• Annotation entry paths must begin with '/'.

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/045a11057c086b98. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/Net/Imap/ImapFolder.cs:838

				Engine.OnFolderCreated (folder);
			}

			return folder;
		}

		ImapCommand QueueCreateCommand (string name, bool isMessageFolder, CancellationToken cancellationToken, out string encodedName)
		{
			if (name == null)
				throw new ArgumentNullException (nameof (name));

			if (!ImapEngine.IsValidMailboxName (name, DirectorySeparator))
				throw new ArgumentException ("The name is not a legal folder name.", nameof (name));

			CheckState (false, false);

			if (!string.IsNullOrEmpty (FullName) && DirectorySeparator == '\0')
				throw new InvalidOperationException ("Cannot create child folders.");

			var fullName = !string.IsNullOrEmpty (FullName) ? FullName + DirectorySeparator + name : name;
			encodedName = Engine.EncodeMailboxName (fullName);
			var createName = encodedName;

			if (!isMessageFolder && Engine.QuirksMode != ImapQuirksMode.GMail)
				createName += DirectorySeparator;

			return Engine.QueueCommand (cancellationToken, null, "CREATE %S\r\n", createName);
		}

		MailboxIdResponseCode? ProcessCreateResponse (ImapCommand ic)
		{
			ProcessResponseCodes (ic, null);

			if (ic.Response != ImapCommandResponse.Ok && ic.GetResponseCode (ImapResponseCodeType.AlreadyExists) == null)
				throw ImapCommandException.Create ("CREATE", ic);

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