jstedfast/MailKit · error · FormatException

An annotation entry cannot be empty.

Error message

An annotation entry cannot be empty.

What it means

Validation inside AnnotationEntry.Parse: after the null check, an empty string is rejected with FormatException because an annotation entry must contain a mailbox path, entry name and value; there is nothing to parse from an empty string.

Solutions

1. Skip empty strings before calling Parse (filter after splitting)
2. Validate entry.Length > 0 and handle the empty case explicitly
3. Fix the source so a real entry string is provided

Example fix

// before
foreach (var s in raw.Split (','))
    entries.Add (AnnotationEntry.Parse (s));
// after
foreach (var s in raw.Split (',', StringSplitOptions.RemoveEmptyEntries))
    entries.Add (AnnotationEntry.Parse (s));

var tokens = raw.Split (',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
foreach (var t in tokens)
    entries.Add (AnnotationEntry.Parse (t));

bool CanParse (string? entry) => !string.IsNullOrEmpty (entry);

try {
    var annotation = AnnotationEntry.Parse (entry);
} catch (FormatException) {
    logger.LogWarning ("Skipped empty or invalid annotation entry");
}

When it happens

Trigger: Calling AnnotationEntry.Parse("") or Parse of a string that trimmed to empty — e.g. a config value that is an empty string, or string.Split producing empty tokens fed to Parse.

Common situations: Empty environment/config values, splitting an entry list on separators that yields blank items, initializing strings to "" and never populating them.

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

• Value cannot be null. (Parameter 'entry')
• An annotation entry must begin with a '/' character.
• Invalid annotation entry.
• Value cannot be null. (Parameter 'entry')
• Annotation entry paths must not end with '/'.

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/87ad7f735a99c208. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/AnnotationEntry.cs:430

		/// </summary>
		/// <remarks>
		/// Parses an annotation entry.
		/// </remarks>
		/// <param name="entry">The annotation entry.</param>
		/// <returns>The parsed annotation entry.</returns>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="entry"/> is <see langword="null" />.
		/// </exception>
		/// <exception cref="System.FormatException">
		/// <paramref name="entry"/> does not conform to the annotation entry syntax.
		/// </exception>
		public static AnnotationEntry Parse (string entry)
		{
			if (entry == null)
				throw new ArgumentNullException (nameof (entry));

			if (entry.Length == 0)
				throw new FormatException ("An annotation entry cannot be empty.");

			if (entry[0] != '/' && entry[0] != '*' && entry[0] != '%')
				throw new FormatException ("An annotation entry must begin with a '/' character.");

			var scope = AnnotationScope.Both;
			int startIndex = 0, endIndex;
			string? partSpecifier = null;
			var component = 0;
			var pc = entry[0];
			string path;

			for (int i = 1; i < entry.Length; i++) {
				char c = entry[i];

				if (c >= '0' && c <= '9' && pc == '/') {
					if (component > 0)
						throw new FormatException ("Invalid annotation entry.");

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