jstedfast/MailKit · error · ArgumentNullException

uri

Error message

uri

What it means

BodyPartCollection.IndexOf(Uri) throws ArgumentNullException when the uri parameter is null. The library requires a non-null Uri to search for the body part whose Content-Id or Location matches, so it fails fast instead of silently returning a bogus index. This follows the standard .NET convention for mandatory arguments.

Solutions

1. Ensure the Uri passed to IndexOf is non-null; check for null before calling.
2. If the value may be a string, parse it with Uri.TryCreate and only call IndexOf when parsing succeeded.
3. If null legitimately means 'no reference', skip the IndexOf call and handle the absence explicitly instead.

Example fix

// before
var index = bodyParts.IndexOf(cidUri);
// after
var index = cidUri is null ? -1 : bodyParts.IndexOf(cidUri);

if (uri is null)
    return -1; // or handle absence explicitly
var index = bodyParts.IndexOf(uri);

bool HasCidUri(Uri? uri) => uri is not null && uri.IsAbsoluteUri && uri.Scheme.Equals("cid", StringComparison.OrdinalIgnoreCase);

try {
    index = bodyParts.IndexOf(uri);
} catch (ArgumentNullException ex) {
    // ex.ParamName == "uri"; treat as 'no reference to search for'
    index = -1;
}

When it happens

Trigger: Calling BodyPartCollection.IndexOf(Uri) with a null Uri reference, e.g. passing an unset field or the result of a lookup that failed to produce a Uri (indexOf(null)).

Common situations: Developers resolve a cid: URI from message text or a config value, and the parsing step returns null (unparseable cid, missing Content-Id); they then pass the null Uri straight into IndexOf without checking.

Related errors

• array
• visitor
• bodyParts
• visitor
• visitor

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/1773e1a10021d006. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/BodyPartCollection.cs:210

		/// </summary>
		/// <remarks>
		/// <para>Finds the index of the body part matching the specified URI, if it exists.</para>
		/// <para>If the URI scheme is <c>"cid"</c>, then matching is performed based on the Content-Id header
		/// values, otherwise the Content-Location headers are used. If the provided URI is absolute and a child
		/// part's Content-Location is relative, then then the child part's Content-Location URI will be combined
		/// with the value of its Content-Base header, if available, otherwise it will be combined with the
		/// multipart/related part's Content-Base header in order to produce an absolute URI that can be
		/// compared with the provided absolute URI.</para>
		/// </remarks>
		/// <returns>The index of the part matching the specified URI if found; otherwise <c>-1</c>.</returns>
		/// <param name="uri">The URI of the body part.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="uri"/> is <see langword="null" />.
		/// </exception>
		public int IndexOf (Uri uri)
		{
			if (uri == null)
				throw new ArgumentNullException (nameof (uri));

			bool cid = uri.IsAbsoluteUri && uri.Scheme.Equals ("cid", StringComparison.OrdinalIgnoreCase);

			for (int index = 0; index < Count; index++) {
				if (this[index] is not BodyPartBasic bodyPart)
					continue;

				if (uri.IsAbsoluteUri) {
					if (cid) {
						if (!string.IsNullOrEmpty (bodyPart.ContentId)) {
							// Note: we might have a Content-Id in the form "<id@domain.com>", so attempt to decode it
							var id = MimeUtils.EnumerateReferences (bodyPart.ContentId!).FirstOrDefault () ?? bodyPart.ContentId;

							if (id == uri.AbsolutePath)
								return index;
						}
					} else if (bodyPart.ContentLocation != null) {
						if (!bodyPart.ContentLocation.IsAbsoluteUri)

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