CSHARP-1628: fixed issue with unobserved task exception.

Author: craiggwilson

src/MongoDB.Driver.Core/Core/Connections/BinaryConnection.cs

@@ -50,6 +50,7 @@ internal class BinaryConnection : IConnection
         private readonly Dropbox _dropbox = new Dropbox();
         private DateTime _lastUsedAtUtc;
         private DateTime _openedAtUtc;
+        private Exception _openException;
         private readonly object _openLock = new object();
         private Task _openTask;
         private readonly SemaphoreSlim _receiveLock;
@@ -224,7 +225,7 @@ public void Open(CancellationToken cancellationToken)
             }
 
             if (connecting)
-            { 
+            {
                 try
                 {
                     OpenHelper(cancellationToken);
@@ -233,6 +234,12 @@ public void Open(CancellationToken cancellationToken)
                 catch (Exception ex)
                 {
                     taskCompletionSource.TrySetException(ex);
+
+                    // this line is here to ensure we treat this exception
+                    // as observed and prevent the TaskScheduler.UnobservedException
+                    // event from getting raised.
+                    _openException = taskCompletionSource.Task.Exception;
+
                     throw;
                 }
             }
@@ -740,7 +747,7 @@ public void FailedReceivingMessage(Exception exception)
                 }
 
                 var handler = _connection._failedReceivingMessageEventHandler;
-                if (handler!= null)
+                if (handler != null)
                 {
                     handler(new ConnectionReceivingMessageFailedEvent(_connection.ConnectionId, _responseTo, exception, EventContext.OperationId));
                 }