jstedfast/MailKit · error · ArgumentNullException

visitor

Error message

visitor

What it means

BodyPartMessage.Accept(BodyPartVisitor) throws ArgumentNullException when the visitor parameter is null. The visitor-pattern dispatch delegates directly to visitor.VisitBodyPartMessage, so a null visitor would cause a NullReferenceException later; the library validates up front. A non-null visitor is required to traverse this message body part.

Solutions

1. Pass a valid BodyPartVisitor instance to Accept.
2. If a visitor is optional in your logic, guard with an if (visitor != null) check before calling Accept.
3. Fix the visitor construction path so it never yields null (throw at construction time with a clear message instead).

Example fix

// before
part.Accept(visitor); // visitor may be null
// after
if (visitor != null)
    part.Accept(visitor);

if (visitor is null)
    throw new InvalidOperationException("No visitor configured for body structure traversal.");
messageBodyPart.Accept(visitor);

bool CanAccept(BodyPart part, BodyPartVisitor? visitor) => part is BodyPartMessage && visitor is not null;

try {
    part.Accept(visitor);
} catch (ArgumentNullException ex) when (ex.ParamName == "visitor") {
    logger.LogWarning("Traversal skipped: visitor was null");
}

When it happens

Trigger: Calling bodyPart.Accept(null) on a BodyPartMessage, typically when the visitor was conditionally created (e.g. null returned from a factory or not yet initialized).

Common situations: Constructing a visitor based on options (like a BodyPartVisitor subclass for HTML/text extraction) where a config branch leaves the variable null, then unconditionally calling Accept during body-structure traversal.

Related errors

• visitor
• visitor
• array
• uri
• bodyParts

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/f783a33463fe6784. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/BodyPartMessage.cs:120

		/// <summary>
		/// Dispatches to the specific visit method for this MIME body part.
		/// </summary>
		/// <remarks>
		/// This default implementation for <see cref="MailKit.BodyPart"/> nodes
		/// calls <see cref="MailKit.BodyPartVisitor.VisitBodyPart"/>. Override this
		/// method to call into a more specific method on a derived visitor class
		/// of the <see cref="MailKit.BodyPartVisitor"/> class. However, it should still
		/// support unknown visitors by calling
		/// <see cref="MailKit.BodyPartVisitor.VisitBodyPart"/>.
		/// </remarks>
		/// <param name="visitor">The visitor.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="visitor"/> is <see langword="null" />.
		/// </exception>
		public override void Accept (BodyPartVisitor visitor)
		{
			if (visitor == null)
				throw new ArgumentNullException (nameof (visitor));

			visitor.VisitBodyPartMessage (this);
		}

		/// <summary>
		/// Encodes the <see cref="BodyPart"/> into the <see cref="System.Text.StringBuilder"/>.
		/// </summary>
		/// <remarks>
		/// Encodes the <see cref="BodyPart"/> into the <see cref="System.Text.StringBuilder"/>.
		/// </remarks>
		/// <param name="builder">The string builder.</param>
		protected override void Encode (StringBuilder builder)
		{
			base.Encode (builder);

			builder.Append (' ');
			Encode (builder, Envelope);
			builder.Append (' ');

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