jstedfast/MailKit · error · ArgumentOutOfRangeException

ArgumentOutOfRangeException

Error message

ArgumentOutOfRangeException

What it means

The FolderNamespaceCollection indexer getter (this[int index]) throws ArgumentOutOfRangeException when index is negative or >= namespaces.Count. This is the documented out-of-range guard for reading a namespace by position.

Solutions

1. Check index against collection.Count before indexing
2. Check Count > 0 before accessing element 0
3. Use foreach enumeration instead of positional access when processing all namespaces

Example fix

// before
var ns = namespaces[0];
// after
var ns = namespaces.Count > 0 ? namespaces[0] : null;

FolderNamespace GetNs (FolderNamespaceCollection c, int i) =>
    (i >= 0 && i < c.Count) ? c[i] : null;

bool InRange (int i, int count) => i >= 0 && i < count;

try {
    ns = namespaces[index];
} catch (ArgumentOutOfRangeException) {
    ns = null; // collection empty or index invalid
}

When it happens

Trigger: Reading collection[index] with index < 0 or index >= collection.Count, e.g. hard-coded index 0 on an empty collection returned by ImapClient.GetFolders or the NAMESPACE response.

Common situations: Assuming the server always returns at least one personal namespace; looping with an off-by-one bound (<= Count); copying code that assumed a non-empty namespace list.

Related errors

• ArgumentNullException
• One or more of the messages is null.
• The number of messages and the number of flags must be…
• The uid is invalid.
• part

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/a54f42129b0be860. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/FolderNamespaceCollection.cs:155

		/// <summary>
		/// Gets the <see cref="MailKit.FolderNamespace"/> at the specified index.
		/// </summary>
		/// <remarks>
		/// Gets the <see cref="MailKit.FolderNamespace"/> at the specified index.
		/// </remarks>
		/// <value>The folder namespace at the specified index.</value>
		/// <param name="index">The index.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="value"/> is <see langword="null" />.
		/// </exception>
		/// <exception cref="System.ArgumentOutOfRangeException">
		/// <paramref name="index"/> is out of range.
		/// </exception>
		public FolderNamespace this [int index] {
			get {
				if (index < 0 || index >= namespaces.Count)
					throw new ArgumentOutOfRangeException (nameof (index));

				return namespaces[index];
			}
			set {
				if (index < 0 || index >= namespaces.Count)
					throw new ArgumentOutOfRangeException (nameof (index));

				if (value == null)
					throw new ArgumentNullException (nameof (value));

				namespaces[index] = value;
			}
		}

		#endregion

		#region IEnumerable implementation

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