What causes "offset" in jstedfast/MailKit?

Clamp the computed offset with Math.Max(0, offset).

jstedfast/MailKit · error · ArgumentOutOfRangeException

offset

Error message

offset

What it means

TryQueueGetStreamCommand (UID variant) throws ArgumentOutOfRangeException("offset") when offset < 0. The offset into the message stream must be non-negative to build a valid IMAP FETCH BODY[]<offset.size> section.

Solutions

Clamp the computed offset with Math.Max(0, offset).
Validate range arithmetic before calling the API.
Use Math.Clamp or explicit checks when chunking message streams.

Example fix

// before
var offset = summary.Length - chunkSize;
var stream = folder.GetStream(uid, offset, chunkSize);
// after
var offset = Math.Max(0, summary.Length - chunkSize);
var stream = folder.GetStream(uid, offset, chunkSize);

Defensive patterns

Strategy: validation

Validate before calling

offset = Math.Max(0, offset);
if (offset < 0) throw new InvalidOperationException("Offset computation failed.");

Try / catch

try { stream = folder.GetStream(uid, offset, count); }
catch (ArgumentOutOfRangeException ex) when (ex.ParamName == "offset") { /* fix range math */ }

Prevention

Clamp all computed offsets with Math.Max(0, ...).
Use checked arithmetic to catch underflow early.
Unit-test chunk-boundary math for sizes smaller than the chunk.

When it happens

Trigger: Calling GetStream/GetStreamAsync (UID overload) with a negative offset (ImapFolderFetch.cs:4187), e.g. from integer underflow when computing a partial-range start.

Common situations: Computing offset as `total - chunkSize` when total < chunkSize; subtracting header size from a smaller value; uninitialized counter variables.

Related errors

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/4fc9b000d4075d73. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/Net/Imap/ImapFolderFetch.cs:4187

		/// </exception>
		public override Task<MimeEntity> GetBodyPartAsync (int index, BodyPart part, CancellationToken cancellationToken = default, ITransferProgress? progress = null)
		{
			if (index < 0 || index >= Count)
				throw new ArgumentOutOfRangeException (nameof (index));

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

			return GetBodyPartAsync (index, part.PartSpecifier, cancellationToken, progress);
		}

		bool TryQueueGetStreamCommand (UniqueId uid, int offset, int count, CancellationToken cancellationToken, ITransferProgress? progress, [NotNullWhen (true)] out ImapCommand? ic, [NotNullWhen (true)] out FetchStreamContext? ctx)
		{
			if (!uid.IsValid)
				throw new ArgumentException ("The uid is invalid.", nameof (uid));

			if (offset < 0)
				throw new ArgumentOutOfRangeException (nameof (offset));

			if (count < 0)
				throw new ArgumentOutOfRangeException (nameof (count));

			CheckState (true, false);

			if (count == 0) {
				ctx = null;
				ic = null;
				return false;
			}

			ic = new ImapCommand (Engine, cancellationToken, this, "UID FETCH %u (BODY.PEEK[]<%d.%d>)\r\n", uid.Id, offset, count);
			ic.RegisterUntaggedHandler ("FETCH", FetchStreamHandler);
			ic.UserData = ctx = new FetchStreamContext (progress);

			Engine.QueueCommand (ic);

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