jstedfast/MailKit · error · ArgumentNullException

visitor

Error message

visitor

What it means

BodyPartText.Accept(BodyPartVisitor) throws ArgumentNullException when the visitor parameter is null. The method immediately delegates to visitor.VisitBodyPartText, so a null visitor is rejected up front with ArgumentNullException per .NET conventions. A visitor is mandatory for the visitor-pattern traversal of text body parts.

Solutions

1. Pass a properly initialized BodyPartVisitor instance.
2. Guard the call: only invoke Accept when the visitor is non-null.
3. Initialize the visitor eagerly at the start of the traversal routine rather than lazily.

Example fix

// before
BodyPartVisitor visitor = null;
if (fetchText) visitor = new MyVisitor();
part.Accept(visitor);
// after
var visitor = new MyVisitor(); // always create, control behavior with flags inside
part.Accept(visitor);

if (visitor is null)
    visitor = new DefaultBodyPartVisitor();
textPart.Accept(visitor);

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

try {
    part.Accept(visitor);
} catch (ArgumentNullException ex) when (ex.ParamName == "visitor") {
    // visitor was null; create one and retry or skip
    part.Accept(new DefaultBodyPartVisitor());
}

When it happens

Trigger: Calling textBodyPart.Accept(null), commonly when the visitor is built conditionally (e.g. only created when downloading text bodies) and Accept is invoked unconditionally afterwards.

Common situations: En MimeMessage.BodyParts iteration where a developer's visitor variable is set inside an if-block but Accept is called outside it; or after refactoring removed the visitor initialization.

Related errors

• visitor
• visitor
• array
• uri
• bodyParts

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/044e092079b3d65c. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/BodyPartText.cs:123

		/// <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.VisitBodyPartText (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, Lines);
		}

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