jstedfast/MailKit · error · ArgumentException

The filter name cannot be empty.

Error message

The filter name cannot be empty.

What it means

FilterSearchQuery throws ArgumentException when the filter name is the empty string. An empty name cannot match any server-side filter metadata tag, so the library rejects it eagerly rather than producing a query that always fails.

Solutions

1. Supply the real filter name from the IMAP server (must be non-empty).
2. Validate with string.IsNullOrWhiteSpace before constructing; surface a config error instead.
3. Fix whatever produced the empty value (config key missing, blank form field).

Example fix

// before
var query = SearchQuery.Filter(filterName.Trim());
// after
var name = filterName?.Trim();
if (string.IsNullOrEmpty(name)) throw new InvalidOperationException("filter name is required");
var query = SearchQuery.Filter(name);

if (string.IsNullOrWhiteSpace(filterName)) throw new InvalidOperationException("filter name must be non-empty");

bool HasValidFilterName(string name) => !string.IsNullOrWhiteSpace(name);

try { var q = SearchQuery.Filter(name); } catch (ArgumentException ex) { /* report empty filter name */ }

When it happens

Trigger: Calling new FilterSearchQuery("") or SearchQuery.Filter("") — commonly when a config value or Trim()ed user input is empty.

Common situations: Empty filter-name fields in settings files; trimming removed whitespace leaving an empty string; defaults never filled in.

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

• The uid is invalid.
• Only the "value", "value.priv", and "value.shared"…
• Metadata tag does not reference a valid filter.
• ArgumentOutOfRangeException
• ArgumentNullException

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/23ba897bebd7d241. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/Search/FilterSearchQuery.cs:58

		/// Initializes a new instance of the <see cref="T:MailKit.Search.FilterSearchQuery"/> class.
		/// </summary>
		/// <remarks>
		/// A search query that references a predefined filter.
		/// </remarks>
		/// <param name="name">The name of the filter.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="name"/> is <see langword="null" />.
		/// </exception>
		/// <exception cref="System.ArgumentException">
		/// <paramref name="name"/> is empty.
		/// </exception>
		public FilterSearchQuery (string name) : base (SearchTerm.Filter)
		{
			if (name == null)
				throw new ArgumentNullException (nameof (name));

			if (name.Length == 0)
				throw new ArgumentException ("The filter name cannot be empty.", nameof (name));

			Name = name;
		}

		/// <summary>
		/// Initializes a new instance of the <see cref="T:MailKit.Search.FilterSearchQuery"/> class.
		/// </summary>
		/// <remarks>
		/// A search query that references a predefined filter.
		/// </remarks>
		/// <param name="filter">The metadata tag representing the filter.</param>
		/// <exception cref="System.ArgumentException">
		/// <paramref name="filter"/> does not reference a valid filter.
		/// </exception>
		public FilterSearchQuery (MetadataTag filter) : base (SearchTerm.Filter)
		{
			if (filter.Id.StartsWith ("/private/filters/values/", StringComparison.Ordinal))
				Name = filter.Id.Substring ("/private/filters/values/".Length);

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