CSHARP-5725: C# driver stopped working with Unity since v3.2.0

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

@@ -203,31 +203,39 @@ private void Connect(Socket socket, EndPoint endPoint, CancellationToken cancell
 
         private async Task ConnectAsync(Socket socket, EndPoint endPoint, CancellationToken cancellationToken)
         {
-            var timeoutTask = Task.Delay(_settings.ConnectTimeout, cancellationToken);
-            var connectTask = socket.ConnectAsync(endPoint);
+            Task connectTask;
 
-            await Task.WhenAny(connectTask, timeoutTask).ConfigureAwait(false);
-
-            if (!connectTask.IsCompleted)
+#if !NET472
+            connectTask = socket.ConnectAsync(endPoint);
+#else
+            if (endPoint is DnsEndPoint dnsEndPoint)
             {
-                try
-                {
-                    socket.Dispose();
-                    // should await on the read task to avoid UnobservedTaskException
-                    await connectTask.ConfigureAwait(false);
-                } catch { }
-
-                cancellationToken.ThrowIfCancellationRequested();
-                throw new TimeoutException($"Timed out connecting to {endPoint}. Timeout was {_settings.ConnectTimeout}.");
+                // mono doesn't support DnsEndPoint in its BeginConnect method.
+                connectTask = Task.Factory.FromAsync(socket.BeginConnect(dnsEndPoint.Host, dnsEndPoint.Port, null, null), socket.EndConnect);
             }
-
+            else
+            {
+                connectTask = Task.Factory.FromAsync(socket.BeginConnect(endPoint, null, null), socket.EndConnect);
+            }
+#endif
             try
             {
-                await connectTask.ConfigureAwait(false);
+                await connectTask.WaitAsync(_settings.ConnectTimeout, cancellationToken).ConfigureAwait(false);
             }
-            catch
+            catch (Exception ex)
             {
-                try { socket.Dispose(); } catch { }
+                try
+                {
+                    socket.Dispose();
+                    connectTask.IgnoreExceptions();
+                }
+                catch { }
+
+                if (ex is TimeoutException)
+                {
+                    throw new TimeoutException($"Timed out connecting to {endPoint}. Timeout was {_settings.ConnectTimeout}.");
+                }
+
                 throw;
             }
         }