jstedfast/MailKit · error · ArgumentNullException

Value cannot be null. (Parameter 'keywords')

Error message

Value cannot be null. (Parameter 'keywords')

What it means

The MessageFlagsChangedEventArgs(int index, MessageFlags flags, IReadOnlySetOfStrings keywords) constructor requires a non-null keywords set. ArgumentNullException is thrown when keywords is null. Pass an empty set instead when there are no keywords.

Solutions

1. Pass an empty set (e.g. new HashSet<string>() as IReadOnlySetOfStrings) instead of null
2. Check upstream data and default null keywords to an empty collection before constructing the args
3. In library internals, report the null to the MailKit maintainers with a stack trace

Example fix

// before
new MessageFlagsChangedEventArgs(index, flags, null);
// after
var keywords = (IReadOnlySetOfStrings)(rawKeywords ?? new HashSet<string>());
new MessageFlagsChangedEventArgs(index, flags, keywords);

if (keywords == null) keywords = new HashSet<string>();
var args = new MessageFlagsChangedEventArgs(index, flags, keywords);

IReadOnlySetOfStrings NonNull(IReadOnlySetOfStrings set) => set ?? new HashSet<string>();

try { args = new MessageFlagsChangedEventArgs(index, flags, keywords); }
catch (ArgumentNullException ex) when (ex.ParamName == "keywords") { args = new MessageFlagsChangedEventArgs(index, flags, new HashSet<string>()); }

When it happens

Trigger: Raising or constructing MessageFlagsChangedEventArgs manually (e.g. in custom folder implementations or tests) with keywords = null.

Common situations: Implementing a custom IMailFolder or proxy that forwards flag-change notifications but passes through a null keywords collection from upstream code.

Related errors

• Value cannot be null. (Parameter 'labels')
• Specified argument was out of the range of valid values…
• A metadata tag identifier cannot be empty.
• ArgumentNullException
• ArgumentOutOfRangeException

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/ff3bdba13037521a. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/MessageFlagsChangedEventArgs.cs:89

		/// <summary>
		/// Initializes a new instance of the <see cref="MailKit.MessageFlagsChangedEventArgs"/> class.
		/// </summary>
		/// <remarks>
		/// Creates a new <see cref="MessageFlagsChangedEventArgs"/>.
		/// </remarks>
		/// <param name="index">The message index.</param>
		/// <param name="flags">The message flags.</param>
		/// <param name="keywords">The user-defined keywords.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="keywords"/> is <see langword="null" />.
		/// </exception>
		/// <exception cref="System.ArgumentOutOfRangeException">
		/// <paramref name="index"/> is out of range.
		/// </exception>
		public MessageFlagsChangedEventArgs (int index, MessageFlags flags, IReadOnlySetOfStrings keywords) : base (index)
		{
			if (keywords == null)
				throw new ArgumentNullException (nameof (keywords));

			Keywords = keywords;
			Flags = flags;
		}

		/// <summary>
		/// Initializes a new instance of the <see cref="MailKit.MessageFlagsChangedEventArgs"/> class.
		/// </summary>
		/// <remarks>
		/// Creates a new <see cref="MessageFlagsChangedEventArgs"/>.
		/// </remarks>
		/// <param name="index">The message index.</param>
		/// <param name="flags">The message flags.</param>
		/// <param name="modseq">The modification sequence value.</param>
		/// <exception cref="System.ArgumentOutOfRangeException">
		/// <paramref name="index"/> is out of range.
		/// </exception>
		public MessageFlagsChangedEventArgs (int index, MessageFlags flags, ulong modseq) : base (index)

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