jstedfast/MailKit · error · ArgumentException

Cannot search for an empty string.

Error message

Cannot search for an empty string.

What it means

SearchQuery.HasGMailLabel(string label) rejects an empty-string label with ArgumentException("Cannot search for an empty string.") because IMAP/GMail label searches require non-empty text; an empty term would produce an invalid search key.

Solutions

1. Check string.IsNullOrEmpty(label) before calling HasGMailLabel and skip the query when empty.
2. Trim and validate user/config-supplied label values at input time.
3. Filter out empty entries when building labels from a list.

Example fix

// before
var query = SearchQuery.HasGMailLabel(label); // label may be ""
// after
var trimmed = label?.Trim();
var query = string.IsNullOrEmpty(trimmed) ? null : SearchQuery.HasGMailLabel(trimmed);

if (string.IsNullOrWhiteSpace(label)) throw new ArgumentException("GMail label cannot be empty.", nameof(label));
var query = SearchQuery.HasGMailLabel(label.Trim());

static bool IsValidLabel(string label) => !string.IsNullOrWhiteSpace(label);

try { var q = SearchQuery.HasGMailLabel(label); } catch (ArgumentException) { /* skip label filter */ }

When it happens

Trigger: Calling SearchQuery.HasGMailLabel("") — usually from a config value that is present but blank, or string.Split/trimming results yielding "".

Common situations: Blank label entries in settings files; user leaving a label field empty in UI; parsing label lists where an empty token appears.

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

• Cannot search for an empty string.
• The host name cannot be empty.
• The language code cannot be empty.
• A metadata tag identifier cannot be empty.
• Annotation attribute specifiers cannot be empty.

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/d2303dbae9900870. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/Search/SearchQuery.cs:1179

		/// for use with IMAP servers that support the <c>X-GM-EXT1</c> extension.</note>
		/// <seealso cref="MailKit.Net.Imap.ImapCapabilities.GMailExt1"/>
		/// <seealso cref="MailKit.Net.Imap.ImapClient.Capabilities"/>
		/// </remarks>
		/// <returns>A <see cref="TextSearchQuery"/>.</returns>
		/// <param name="label">The GMail label.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="label"/> is <see langword="null" />.
		/// </exception>
		/// <exception cref="System.ArgumentException">
		/// <paramref name="label"/> is empty.
		/// </exception>
		public static TextSearchQuery HasGMailLabel (string label)
		{
			if (label == null)
				throw new ArgumentNullException (nameof (label));

			if (label.Length == 0)
				throw new ArgumentException ("Cannot search for an empty string.", nameof (label));

			return new TextSearchQuery (SearchTerm.GMailLabels, label);
		}

		/// <summary>
		/// Match messages using the GMail search expression.
		/// </summary>
		/// <remarks>
		/// <para>Matches messages using Google's custom message search syntax.</para>
		/// <note type="note">This is equivalent to the <c>X-GM-RAW</c> search key as defined in <a href="https://developers.google.com/gmail/imap_extensions">Google's IMAP extensions</a> and is therefor only available
		/// for use with IMAP servers that support the <c>X-GM-EXT1</c> extension.</note>
		/// <seealso cref="MailKit.Net.Imap.ImapCapabilities.GMailExt1"/>
		/// <seealso cref="MailKit.Net.Imap.ImapClient.Capabilities"/>
		/// </remarks>
		/// <returns>A <see cref="TextSearchQuery"/>.</returns>
		/// <param name="expression">The raw GMail search text.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="expression"/> is <see langword="null" />.

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