When monitoring C2D messages in VS Code, D2C messages fail to send

[drwill-ms]

1. Obtain an MxChip and run the Get Started tutorial at http://aka.ms/mxchip.
2. Confirm from the device UI that messages are being sent (e.g. 4/4).
3. In VS Code, right-click on your device and select "Start Monitoring C2D Message".
4. Examine the device screen to see that message attempts increase, but successful message counts do not change (e.g. 22/29).

Result D2C messages fail to send while monitoring C2D messages. The user has no expectation that D2C will stop working and it either should not break D2C or, if it cannot be avoided, a warning should be shown about the functionality loss while monitoring.

Grammar note: the command text should probably be "Start Monitoring C2D Messages" (plural) because it does not stop monitoring after just 1 message.

Also tracked at Bug 4876947: When monitoring C2D messages in VS Code, D2C messages fail to send.

[formulahendry]

Some updates for this issue:

• We could also reproduce this issue with the same repro steps
• If we use the C SDK to send D2C message, we would also reproduce this issue
• Per the docs (https://docs.microsoft.com/en-us/azure/iot-hub/iot-hub-mqtt-support#sending-device-to-cloud-messages): IoT Hub only supports one active MQTT connection per device. Any new MQTT connection on behalf of the same device ID causes IoT Hub to drop the existing connection.

[formulahendry]

Update status:

• Get confirm from SDK side https://github.com/Azure/azure-iot-sdk-c/issues/1149#issuecomment-526263052: the device disconnecting is expected behavior if you have two devices trying to connect with the same connection string. Since D2C messages and monitoring C2D messages are using the same device connection string
• We have renamed monitoring C2D messages to receiving C2D messages to have a closer description to this command.

[formulahendry]

Hi @drwill-ms , do you have any more comment or suggestion on this issue?