jstedfast/MailKit · error · ArgumentException

A metadata tag identifier cannot be empty.

Error message

A metadata tag identifier cannot be empty.

What it means

The MetadataTag(string id) constructor rejects an empty identifier because IMAP metadata tags (RFC 5464 entries like "/private/comment") must be non-empty strings. Null is rejected separately with ArgumentNullException.

Solutions

1. Pass a non-empty tag id such as "/private/comment" or "/shared/vendor/vendor.example.key"
2. Trim and validate the source string before constructing the MetadataTag
3. Fix the config/input source that produced the empty identifier

Example fix

// before
var tag = new MetadataTag(configEntry.Name); // Name may be ""
// after
if (string.IsNullOrWhiteSpace(configEntry.Name))
	throw new InvalidOperationException("Metadata tag name is not configured.");
var tag = new MetadataTag(configEntry.Name.Trim());

if (string.IsNullOrEmpty(id))
	throw new ArgumentException("Metadata tag id must be non-empty.", nameof(id));

bool IsValidMetadataTagId(string id) => !string.IsNullOrWhiteSpace(id);

try {
	var tag = new MetadataTag(id);
} catch (ArgumentException) {
	// skip or log the invalid tag id
}

When it happens

Trigger: Calling `new MetadataTag("")` or `new MetadataTag(someString)` where someString came from an empty config value, a split() result, or an uninitialized variable.

Common situations: Reading metadata tag names from user input or configuration files that are blank; parsing annotation names from server responses with a delimiter that yields empty tokens.

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 host name cannot be empty.
• The keyword cannot be an empty string.
• Cannot search for null or empty keywords.
• Cannot search an empty header field name.
• Annotation attribute specifiers cannot be empty.

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/9088f6f6243f5b53. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/MetadataTag.cs:91

		/// Initializes a new instance of the <see cref="MailKit.MetadataTag"/> struct.
		/// </summary>
		/// <remarks>
		/// Creates a new <see cref="MetadataTag"/>.
		/// </remarks>
		/// <param name="id">The metadata tag identifier.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="id"/> is <see langword="null" />.
		/// </exception>
		/// <exception cref="System.ArgumentException">
		/// <paramref name="id"/> is an empty string.
		/// </exception>
		public MetadataTag (string id)
		{
			if (id == null)
				throw new ArgumentNullException (nameof (id));

			if (id.Length == 0)
				throw new ArgumentException ("A metadata tag identifier cannot be empty.");

			Id = id;
		}

		/// <summary>
		/// Get the metadata tag identifier.
		/// </summary>
		/// <remarks>
		/// Gets the metadata tag identifier.
		/// </remarks>
		/// <value>The metadata tag identifier.</value>
		public string Id {
			get; private set;
		}

		/// <summary>
		/// Determine whether the specified <see cref="System.Object"/> is equal to the current <see cref="MailKit.MetadataTag"/>.
		/// </summary>

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