dotnet/yarp · error · Exception

Expected to receive '{textToSend}', but got '{text}'.

Error message

Expected to receive '{textToSend}', but got '{text}'.

What it means

Asserted in WebSocketsScenario after decoding the received frame: the echoed UTF-8 text must equal what was sent ("Hello {i}"). A mismatch means bytes were corrupted, truncated, reordered, or a different message was returned.

Source

Thrown at testassets/TestClient/Scenarios/WebSocketsScenario.cs:54

                WebSocketMessageType.Text,
                endOfMessage: true,
                cancellation);

            var message = await client.ReceiveAsync(buffer, cancellation);
            if (message.MessageType != WebSocketMessageType.Text)
            {
                throw new Exception($"Expected to receive a text message, got '{message.MessageType}' instead.");
            }

            if (!message.EndOfMessage)
            {
                throw new Exception("Expected to receive EndOfMessage = true.");
            }

            var text = Encoding.UTF8.GetString(buffer.AsSpan(0, message.Count));
            if (text != textToSend)
            {
                throw new Exception($"Expected to receive '{textToSend}', but got '{text}'.");
            }

            Console.Write(".");
        }

        Console.WriteLine();
        Console.WriteLine($"Completed 256 text messages in {stopwatch.ElapsedMilliseconds} ms.");

        Console.WriteLine("Sending binary messages...");
        stopwatch.Restart();
        for (var i = 0; i < 256; i++)
        {
            var textToSend = $"Hello {i}";
            var numBytes = Encoding.UTF8.GetBytes(textToSend, buffer.AsSpan());
            await client.SendAsync(new ArraySegment<byte>(buffer, 0, numBytes),
                WebSocketMessageType.Binary,
                endOfMessage: true,
                cancellation);

View on GitHub (pinned to bd11867bee)

Solutions

  1. Use the matching test backend that echoes the exact bytes sent.
  2. Ensure the receive buffer is sized for the message and that message.Count bounds the decode (the scenario does this correctly; custom variants must too).
  3. Remove payload-transforming intermediaries.
  4. Compare sent vs received bytes (not just decoded text) to localize corruption.
Defensive patterns

Strategy: try-catch

Try / catch

try { await WebSocketsScenario.RunAsync(client, args, cancellation); }
catch (Exception ex) when (ex.Message.Contains("Expected to receive") && ex.Message.Contains("but got")) {
    Console.Error.WriteLine("WebSocket echo payload mismatch; backend returned different bytes.");
    return 1;
}

Prevention

When it happens

Trigger: Running WebSocketsScenario against a WebSocket endpoint whose echo does not return the exact sent payload, or where framing/count caused the decode to cover the wrong bytes.

Common situations: Backend echoing the wrong buffer or a previous message; truncation due to a too-small receive buffer; encoding mismatch; an intermediary mutating payload; the receive buffer not being cleared between iterations.

Related errors

AI-assisted analysis of dotnet/yarp@bd11867bee (2026-08-13). Data as JSON: /api/errors/8d237ade63694c94. Report an issue: GitHub.