jstedfast/MailKit · error · IOException

Error deflating:

Error message

Error deflating: 

What it means

Write() throws IOException("Error deflating: " + zOut.msg) when JZlib's deflate(Z_FULL_FLUSH) returns a status other than Z_OK. This means zlib itself rejected the compression step, typically due to exhausted/inconsistent internal state rather than bad input bytes.

Solutions

1. Check zOut.msg in the exception message for the specific zlib error code/cause.
2. Discard the stream and create a new CompressedStream; a failed deflate state is not recoverable.
3. Ensure all writes are single-threaded or serialized - concurrent Write calls corrupt the zlib state machine.
4. Verify the underlying stream is writable and healthy before writing.

Example fix

// before
stream.Write(data, 0, data.Length); // IOException: Error deflating
// after
try {
    stream.Write(data, 0, data.Length);
} catch (IOException ex) when (ex.Message.Contains("Error deflating")) {
    stream.Dispose();
    stream = new CompressedStream(innerStream); // fresh deflate state
}

if (!stream.CanWrite)
    throw new InvalidOperationException("Cannot write: inner stream is not writable.");
// and ensure writes are serialized (no concurrent callers)

static bool IsSafeToWrite(CompressedStream s, object writeLock) =>
    s.CanWrite && !Monitor.IsEntered(writeLock) == false; // inside the lock => safe

try {
    stream.Write(data, 0, data.Length);
} catch (IOException ex) when (ex.Message.Contains("Error deflating")) {
    stream.Dispose();
    stream = new CompressedStream(innerStream); // recreate; state is unrecoverable
}

When it happens

Trigger: Calling CompressedStream.Write after the stream or zlib state is invalid (e.g. writing after disposal mid-state, corrupted zOut state, zlib-level failure such as Z_STREAM_ERROR from bad parameters).

Common situations: Concurrent unsynchronized writes corrupting the zlib state, writing to a stream whose underlying transport already failed mid-flush, resource exhaustion inside the deflate window.

Related errors

• Error inflating:
• ArgumentOutOfRangeException
• ObjectDisposedException
• NotSupportedException
• The IMAP server does not support the COMPRESS extension.

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/84f9a765a9f80660. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/CompressedStream.cs:325

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

			ValidateArguments (buffer, offset, count);

			if (count == 0)
				return;

			zOut.next_in = buffer;
			zOut.next_in_index = offset;
			zOut.avail_in = count;

			do {
				zOut.avail_out = zOut.next_out.Length;
				zOut.next_out_index = 0;

				if (zOut.deflate (JZlib.Z_FULL_FLUSH) != JZlib.Z_OK)
					throw new IOException ("Error deflating: " + zOut.msg);

				InnerStream.Write (zOut.next_out, 0, zOut.next_out.Length - zOut.avail_out);
			} while (zOut.avail_in > 0 || zOut.avail_out == 0);
		}

		/// <summary>
		/// Writes a sequence of bytes to the stream and advances the current
		/// position within this stream by the number of bytes written.
		/// </summary>
		/// <returns>A task that represents the asynchronous write operation.</returns>
		/// <param name="buffer">The buffer to write.</param>
		/// <param name="offset">The offset of the first byte to write.</param>
		/// <param name="count">The number of bytes to write.</param>
		/// <param name="cancellationToken">The cancellation token.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="buffer"/> is <see langword="null" />.
		/// </exception>
		/// <exception cref="System.ArgumentOutOfRangeException">

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