jstedfast/MailKit · error · ArgumentNullException
visitor
Error message
visitor
What it means
BodyPartBasic.Accept(BodyPartVisitor) throws ArgumentNullException when the visitor parameter is null. This method is part of the visitor pattern used to walk a BODYSTRUCTURE tree; a null visitor would make the dispatch a no-op, so MailKit rejects it eagerly. The message is the parameter name "visitor".
Solutions
- Instantiate the visitor before calling Accept: part.Accept(new MyVisitor())
- Guard with a null check and skip traversal when no visitor is needed
- Fix the visitor factory/DI registration so it never returns null
Example fix
// before BodyPartVisitor visitor = GetVisitor(); // returned null part.Accept(visitor); // after var visitor = GetVisitor() ?? new DefaultBodyPartVisitor(); part.Accept(visitor);
Defensive patterns
Strategy: validation
Validate before calling
if (visitor == null) throw new InvalidOperationException("A BodyPartVisitor must be provided to traverse the body structure."); Type guard
static bool HasVisitor(BodyPartVisitor? v) => v is not null;
Try / catch
try { part.Accept(visitor); }
catch (ArgumentNullException ex) when (ex.ParamName == "visitor") { /* construct or fix the visitor, then retry */ } Prevention
- Always pass a concrete visitor instance: part.Accept(new MyVisitor())
- Check DI/factory code that supplies visitors so it never returns null
- Make visitor fields readonly and initialize them at declaration
When it happens
Trigger: Calling part.Accept(null) on any BodyPart (including BodyPartBasic) — usually when the visitor variable was never initialized or a factory returned null.
Common situations: Building a BodyPartVisitor subclass conditionally and forgetting to instantiate it; dependency-injection returning null in tests; refactoring that removed the new MyVisitor() call.
Related errors
AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15).
Data as JSON: /api/errors/616ae8fb89c91823.
Report an issue: GitHub.
Appendix: source
Thrown at MailKit/BodyPartBasic.cs:231
/// <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.VisitBodyPartBasic (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, ContentType);
builder.Append (' ');
Encode (builder, ContentId);
builder.Append (' ');
Encode (builder, ContentDescription);View on GitHub (pinned to 9d3859a785)