jstedfast/MailKit · error · NotSupportedException

The stream does not support seeking.

Error message

The stream does not support seeking.

What it means

ProgressStream.Seek is deliberately not implemented: the wrapper exists to report read progress on forward-only streams (e.g. network/HTTP streams), which cannot reposition. It throws NotSupportedException with the message "The stream does not support seeking." whenever Seek is called.

Solutions

1. Avoid seeking; read the stream sequentially from start to end
2. If random access is needed, buffer the source into a MemoryStream first and wrap that in ProgressStream
3. Check CanSeek before calling Seek and use an alternative code path

Example fix

// before
progressStream.Seek(0, SeekOrigin.Begin);
// after
if (progressStream.CanSeek)
    progressStream.Seek(0, SeekOrigin.Begin);
else {
    using var buffered = new MemoryStream();
    progressStream.CopyTo(buffered);
    buffered.Seek(0, SeekOrigin.Begin);
}

bool CanSeekSafely(Stream s) => s.CanSeek; // check before calling Seek/Position

try { stream.Seek(0, SeekOrigin.Begin); }
catch (NotSupportedException) { /* non-seekable: buffer or re-open instead */ }

When it happens

Trigger: Calling stream.Seek(...) or stream.Position = ... on a ProgressStream; setting Stream.Position (which internally calls Seek); code paths like TestSeek or reading Position that require seekability.

Common situations: Passing a ProgressStream to APIs that assume a seekable Stream (media players, ZipArchive with default mode, serializers that rewind); copying code that worked with FileStream/ MemoryStream to a network-backed stream.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• NotSupportedException
• null
• The stream does not support resizing.
• The folder does not support partial searches.
• The folder does not support partial sorts.

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/6b8a641c1280284d. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/ProgressStream.cs:159

		public override void Write (byte[] buffer, int offset, int count)
		{
			Source.Write (buffer, offset, count);

			if (count > 0)
				Update (count);
		}

		public override async Task WriteAsync (byte[] buffer, int offset, int count, CancellationToken cancellationToken)
		{
			await Source.WriteAsync (buffer, offset, count, cancellationToken).ConfigureAwait (false);

			if (count > 0)
				Update (count);
		}

		public override long Seek (long offset, SeekOrigin origin)
		{
			throw new NotSupportedException ("The stream does not support seeking.");
		}

		public void Flush (CancellationToken cancellationToken)
		{
			if (cancellable != null)
				cancellable.Flush (cancellationToken);
			else
				Source.Flush ();
		}

		public override void Flush ()
		{
			Source.Flush ();
		}

		public override Task FlushAsync (CancellationToken cancellationToken)
		{
			return Source.FlushAsync (cancellationToken);

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