jstedfast/MailKit · error · ArgumentNullException

visitor

Error message

visitor

What it means

BodyPartMultipart.Accept(BodyPartVisitor) throws ArgumentNullException when the visitor parameter is null. Accept dispatches to visitor.VisitBodyPartMultipart, which requires a live visitor object; a null argument would crash inside the call, so the method validates immediately. Every concrete BodyPart subtype follows this same pattern.

Solutions

1. Ensure a non-null BodyPartVisitor is constructed before traversing.
2. Add a null check before calling Accept when the visitor is optional in your flow.
3. Validate visitor at your own public API boundary so nulls never reach library calls.

Example fix

// before
foreach (var part in bodyParts)
    part.Accept(visitor); // may be null
// after
ArgumentNullException.ThrowIfNull(visitor);
foreach (var part in bodyParts)
    part.Accept(visitor);

if (visitor is null)
    return; // or create a default visitor
multipart.Accept(visitor);

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

try {
    multipart.Accept(visitor);
} catch (ArgumentNullException ex) when (ex.ParamName == "visitor") {
    visitor = new DefaultBodyPartVisitor();
    multipart.Accept(visitor);
}

When it happens

Trigger: Calling multipartBodyPart.Accept(null) while walking a parsed BODYSTRUCTURE tree, usually because the visitor variable was never assigned or a factory method returned null.

Common situations: Generic traversal code that receives the visitor as a parameter itself (possibly null from an outer caller) and forwards it to Accept without checking.

Related errors

• visitor
• visitor
• array
• uri
• bodyParts

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/8d910c79d99be7e2. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/BodyPartMultipart.cs:156

		/// <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.VisitBodyPartMultipart (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)
		{
			Encode (builder, BodyParts);
			builder.Append (' ');
			Encode (builder, ContentType.MediaSubtype);
			builder.Append (' ');
			Encode (builder, ContentType.Parameters);

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