jstedfast/MailKit · error · ArgumentNullException

contentType

Error message

contentType

What it means

The protected BodyPart(ContentType, string) constructor throws ArgumentNullException when the contentType parameter is null. MailKit requires every BODYSTRUCTURE part to carry a parsed MIME ContentType, so a null value would leave the part in an invalid state. The parameter name is reported via nameof, which is why the message is exactly "contentType".

Solutions

1. Pass a valid parsed ContentType, e.g. new ContentType("text", "plain"), instead of null
2. If the source MIME part had no Content-Type header, default to new ContentType("text","plain") per RFC 2045
3. Guard the value before invoking the constructor and throw a more descriptive error

Example fix

// before
var part = new CustomBodyPart(contentType, "1"); // contentType was null
// after
contentType ??= new ContentType("text", "plain");
var part = new CustomBodyPart(contentType, "1");

if (contentType == null) throw new ArgumentException("A parsed ContentType is required; use new ContentType(\"text\", \"plain\") when the MIME header is missing.", nameof(contentType));

static bool HasContentType(ContentType? ct) => ct is not null;

try { var part = new CustomBodyPart(contentType, specifier); }
catch (ArgumentNullException ex) when (ex.ParamName == "contentType") { /* supply default ContentType and retry */ }

When it happens

Trigger: Calling the protected constructor from a derived BodyPart subclass and passing a null ContentType — e.g. new MyBodyPart(null, "1").

Common situations: Custom subclassing of BodyPart when the MIME Content-Type header was missing or failed to parse and the caller passes null instead of a default like text/plain; refactoring code that previously accepted a nullable content type.

Related errors

• partSpecifier
• visitor
• part
• Annotation entry paths must not include a part-specifier.
• Invalid annotation entry path.

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/9f493099e6cd345e. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/BodyPart.cs:79

		}

		/// <summary>
		/// Initializes a new instance of the <see cref="MailKit.BodyPart"/> class.
		/// </summary>
		/// <remarks>
		/// Creates a new <see cref="MailKit.BodyPart"/>.
		/// </remarks>
		/// <param name="contentType">The content type.</param>
		/// <param name="partSpecifier">The part specifier.</param>
		/// <exception cref="ArgumentNullException">
		/// <para><paramref name="contentType"/> is <see langword="null" />.</para>
		/// <para>-or-</para>
		/// <para><paramref name="partSpecifier"/> is <see langword="null" />.</para>
		/// </exception>
		protected BodyPart (ContentType contentType, string partSpecifier)
		{
			if (contentType == null)
				throw new ArgumentNullException (nameof (contentType));

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

			ContentType = contentType;
			PartSpecifier = partSpecifier;
		}

		/// <summary>
		/// Gets the Content-Type of the body part.
		/// </summary>
		/// <remarks>
		/// Gets the Content-Type of the body part.
		/// </remarks>
		/// <value>The content type.</value>
		public ContentType ContentType {
			get; set;
		}

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