Direct Messages fail with Status: 501 Payload: Null - azure-iot-hub

Question

I have a dotnet core project running version 1.37.2 of Microsoft.Azure.Devices.Client on top of Linux Raspbian (Buster 5.4.83) on a RaspBerry Pi4B 2GB. The project sends telemetry every 5 mins to Azure IoT Hub. The project also registers a number of Direct Methods to be called using 'SetMethodHandler' as follows:
await _iotHubClient.SetMethodHandlerAsync("MethodToCall", MethodToCallHandler, null);
I have a number of devices out in the real world, connected to the internet over wireless LAN and standard home internet connections.
I have noticed that some devices I can call DirectMethods on relatively reliably some I have to often try three of four times before I get a successful response and others I am never able to call successfully. When the DirectMethod calls fail I always receive {"status:501", "payload":null}.
Any help appreciated.

Answer 1

Do you have the same code\version deployed on both devices that work and don't work? 501 usually stands for "Not Implemented." In azure-iot-sdk-csharp you can read the following for 501 Error Code:
"Equivalent to HTTP status code 501, not implemented server error. It
is used when a method call from the service specifies a method name
not registered with the client for callback."
Ref: https://github.com/Azure/azure-iot-sdk-csharp/blob/master/iothub/device/src/MethodResponseStatusCode.cs#L21
some I have to often try three of four times before I get a successful response and others I am never able to call successfully.
Before you can call the methods successfully on those devices do you ever receivie a 501 error or you receive those as described here (404 or 504)? If never seeing 501 errors on "working devices" then you need to make sure you have same versions of the code deployed on "non-working devices".