jstedfast/MailKit · error · ArgumentNullException

part

Error message

part

What it means

MailFolder.GetStream(UniqueId, BodyPart, ...) throws ArgumentNullException with param name "part" when the BodyPart argument is null. The library requires a non-null BodyPart to know which MIME part specifier to fetch.

Solutions

1. Ensure the BodyPart comes from a fetched BodyStructure (folder.GetBodyPart / message summary BodyParts) before passing it.
2. Null-check the result of any LINQ lookup (FirstOrDefault/SingleOrDefault) before calling GetStream.
3. Fall back to a default part such as the message body via GetStream(uid, "1") or GetMessage when the desired part is absent.

Example fix

// before
var part = summary.BodyParts.OfType<BodyPartText>().FirstOrDefault();
var stream = folder.GetStream(summary.UniqueId, part); // part may be null
// after
var part = summary.BodyParts.OfType<BodyPartText>().FirstOrDefault();
if (part == null) return;
var stream = folder.GetStream(summary.UniqueId, part);

if (bodyPart is null)
    throw new InvalidOperationException("No body part resolved; fetch the body structure before calling GetStream.");

bool HasUsablePart([NotNullWhen(false)] BodyPart? part) => part != null;

try {
    var stream = folder.GetStream(uid, bodyPart);
} catch (ArgumentNullException ex) when (ex.ParamName == "part") {
    var structure = folder.GetBodyPart(uid, "");
    stream = folder.GetStream(uid, structure.PartSpecifier);
}

When it happens

Trigger: Calling MailFolder.GetStream(uid, null) — typically because a BodyPart lookup returned null or a variable was never assigned.

Common situations: Searching a BodyPart tree (e.g. BodyParts.OfType or FirstOrDefault) for a text/html part that does not exist and passing the null result straight to GetStream; refactoring where the part argument was dropped.

Related errors

• ArgumentNullException
• ArgumentOutOfRangeException
• One or more of the messages is null.
• The number of messages and the number of flags must be…
• The uid is invalid.

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/b3d30b44943df70e. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/MailFolder.cs:5976

		/// <exception cref="System.OperationCanceledException">
		/// The operation was canceled via the cancellation token.
		/// </exception>
		/// <exception cref="System.IO.IOException">
		/// An I/O error occurred.
		/// </exception>
		/// <exception cref="ProtocolException">
		/// The server's response contained unexpected tokens.
		/// </exception>
		/// <exception cref="CommandException">
		/// The command failed.
		/// </exception>
		public virtual Stream GetStream (UniqueId uid, BodyPart part, CancellationToken cancellationToken = default, ITransferProgress? progress = null)
		{
			if (!uid.IsValid)
				throw new ArgumentException ("The uid is invalid.", nameof (uid));

			if (part == null)
				throw new ArgumentNullException (nameof (part));

			return GetStream (uid, part.PartSpecifier, cancellationToken, progress);
		}

		/// <summary>
		/// Asynchronously get a body part as a stream.
		/// </summary>
		/// <remarks>
		/// Asynchronously gets a body part as a stream.
		/// </remarks>
		/// <example>
		/// <code language="c#" source="Examples\ImapBodyPartExamples.cs" region="GetBodyPartStreamsByUniqueId"/>
		/// </example>
		/// <returns>The body part stream.</returns>
		/// <param name="uid">The UID of the message.</param>
		/// <param name="part">The desired body part.</param>
		/// <param name="cancellationToken">The cancellation token.</param>
		/// <param name="progress">The progress reporting mechanism.</param>

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