Complete received C2D message within semaphore - Device Client sample (…

…#3198)

* Update MessageReceiveSample.cs

* Update MessageReceiveSample.cs

* added cancellation token to semaphore.

* cr changes - added try/finally blocks

iothub/device/samples/getting started/MessageReceiveSample/MessageReceiveSample.cs

@@ -22,6 +22,8 @@ public class MessageReceiveSample
         private readonly TimeSpan? _maxRunTime;
         private readonly DeviceClient _deviceClient;
         private readonly TransportType _transportType;
+        private SemaphoreSlim _processMessageSemaphore;
+        private CancellationTokenSource _cts;
 
         public MessageReceiveSample(DeviceClient deviceClient, TransportType transportType, TimeSpan? maxRunTime)
         {
@@ -32,21 +34,25 @@ public MessageReceiveSample(DeviceClient deviceClient, TransportType transportTy
 
         public async Task RunSampleAsync()
         {
-            using var cts = _maxRunTime.HasValue
+            _cts = _maxRunTime.HasValue
                 ? new CancellationTokenSource(_maxRunTime.Value)
                 : new CancellationTokenSource();
+
+            // Semaphore to ensure sequential execution of received C2D messages.
+            _processMessageSemaphore = new SemaphoreSlim(1, 1);
+
             Console.CancelKeyPress += (sender, eventArgs) =>
             {
                 eventArgs.Cancel = true;
-                cts.Cancel();
+                _cts.Cancel();
                 Console.WriteLine("Sample execution cancellation requested; will exit.");
             };
             Console.WriteLine($"{DateTime.Now}> Press Control+C at any time to quit the sample.");
 
             // First receive C2D messages using the polling ReceiveAsync().
             Console.WriteLine($"\n{DateTime.Now}> Device waiting for C2D messages from the hub...");
             Console.WriteLine($"{DateTime.Now}> Use the Azure Portal IoT hub blade or Azure IoT Explorer to send a message to this device.");
-            await ReceiveC2dMessagesPollingAndCompleteAsync(cts.Token);
+            await ReceiveC2dMessagesPollingAndCompleteAsync(_cts.Token);
 
             if (_transportType != TransportType.Http1)
             {
@@ -61,15 +67,19 @@ public async Task RunSampleAsync()
 
                 try
                 {
-                    await Task.Delay(-1, cts.Token);
+                    await Task.Delay(-1, _cts.Token);
                 }
                 catch (TaskCanceledException)
                 {
                     // Done running.
                 }
-
-                // Now unsubscibe from receiving the callback.
-                await _deviceClient.SetReceiveMessageHandlerAsync(null, null);
+                finally
+                {
+                    // Now unsubscibe from receiving the callback.
+                    await _deviceClient.SetReceiveMessageHandlerAsync(null, null);
+                    _processMessageSemaphore.Dispose();
+                    _cts.Dispose();
+                }
             }
         }
 
@@ -104,13 +114,21 @@ private async Task ReceiveC2dMessagesPollingAndCompleteAsync(CancellationToken c
 
         private async Task OnC2dMessageReceivedAsync(Message receivedMessage, object _)
         {
-            Console.WriteLine($"{DateTime.Now}> C2D message callback - message received with Id={receivedMessage.MessageId}.");
-            PrintMessage(receivedMessage);
-
-            await _deviceClient.CompleteAsync(receivedMessage);
-            Console.WriteLine($"{DateTime.Now}> Completed C2D message with Id={receivedMessage.MessageId}.");
+            try
+            {
+                // Use a semaphore to ensure C2D messages are processed in order - a requirement of IoT hub.
+                await _processMessageSemaphore.WaitAsync(_cts.Token).ConfigureAwait(false);
+                Console.WriteLine($"{DateTime.Now}> C2D message callback - message received with Id={receivedMessage.MessageId}.");
+                PrintMessage(receivedMessage);
 
-            receivedMessage.Dispose();
+                await _deviceClient.CompleteAsync(receivedMessage);
+                Console.WriteLine($"{DateTime.Now}> Completed C2D message with Id={receivedMessage.MessageId}.");
+            }
+            finally
+            {
+                receivedMessage.Dispose();
+                _processMessageSemaphore.Release();
+            }
         }
 
         private static void PrintMessage(Message receivedMessage)
@@ -123,8 +141,8 @@ private static void PrintMessage(Message receivedMessage)
             {
                 formattedMessage.AppendLine($"\tProperty: key={prop.Key}, value={prop.Value}");
             }
-            // System properties can be accessed using their respective accessors, e.g. DeliveryCount.
-            formattedMessage.AppendLine($"\tDelivery count: {receivedMessage.DeliveryCount}");
+            // System properties can be accessed using their respective accessors, e.g. ContentType.
+            formattedMessage.AppendLine($"\tContent type: {receivedMessage.ContentType}");
 
             Console.WriteLine($"{DateTime.Now}> {formattedMessage}");
         }