What causes "ArgumentNullException" in jstedfast/MailKit?

Pass the actual folder name string (use string.Empty if genuinely unknown is unacceptable — prefer a real value)

jstedfast/MailKit · error · ArgumentNullException

ArgumentNullException

Error message

ArgumentNullException

What it means

The FolderNotFoundException(string message, string folderName, Exception innerException) constructor throws ArgumentNullException when folderName is null. The exception object exists to carry the missing folder's name, so a null name defeats its purpose and is rejected per its documented contract.

Solutions

Pass the actual folder name string (use string.Empty if genuinely unknown is unacceptable — prefer a real value)
Capture the folder name at the throw site before wrapping the inner exception
If the name is unavailable, use a different exception type such as InvalidOperationException

Example fix

// before
throw new FolderNotFoundException (msg, null, inner);
// after
throw new FolderNotFoundException (msg, folder?.Path ?? string.Empty, inner);

Defensive patterns

Strategy: validation

Validate before calling

if (folderName == null)
    throw new InvalidOperationException ("folder name required");
throw new FolderNotFoundException (message, folderName, innerException);

Type guard

bool HasName (IMailFolder f) => f != null && f.FullName != null;

Try / catch

try {
    throw new FolderNotFoundException (message, folderName, inner);
} catch (ArgumentNullException ex) when (ex.ParamName == "folderName") {
    // supply a real name and rethrow
}

Prevention

Capture folder names at the failure site before wrapping exceptions
Never pass null for documented non-null exception parameters
Use the folder's FullName/Path as the canonical name

When it happens

Trigger: Constructing FolderNotFoundException with (message, null, innerException) — e.g. rethrowing code that lost the folder name variable.

Common situations: Wrapping lower-level IO/IMAP errors where the folder name was not captured; logging helpers building exceptions generically; tests of exception constructors.

Related errors

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/d080310d90f00b46. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/FolderNotFoundException.cs:79

		}
#endif

		/// <summary>
		/// Initializes a new instance of the <see cref="MailKit.FolderNotFoundException"/> class.
		/// </summary>
		/// <remarks>
		/// Creates a new <see cref="FolderNotFoundException"/>.
		/// </remarks>
		/// <param name="message">The error message.</param>
		/// <param name="folderName">The name of the folder.</param>
		/// <param name="innerException">The inner exception.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="folderName"/> is <see langword="null" />.
		/// </exception>
		public FolderNotFoundException (string message, string folderName, Exception innerException) : base (message, innerException)
		{
			if (folderName == null)
				throw new ArgumentNullException (nameof (folderName));

			FolderName = folderName;
		}

		/// <summary>
		/// Initializes a new instance of the <see cref="MailKit.FolderNotFoundException"/> class.
		/// </summary>
		/// <remarks>
		/// Creates a new <see cref="FolderNotFoundException"/>.
		/// </remarks>
		/// <param name="message">The error message.</param>
		/// <param name="folderName">The name of the folder.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="folderName"/> is <see langword="null" />.
		/// </exception>
		public FolderNotFoundException (string message, string folderName) : base (message)
		{
			if (folderName == null)

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