What is the best way to download image from url in xamarin iOS?

I need to download Open street map tile for every panning and pinch zooming. I have used WebClient for downloading image from Uri. But I am getting "An HttpWebRequest was added to the connection group queue because the connection limit was reached and soon,I am getting System.Net.WebException in Xamarin iOS. But I am doing the same in Xamarin.Android, where images are downloaded comparatively better than the Xamarin iOS.

What I have tried:

WebClient webClient = new WebClient();
                byte[] imageBytes = null;
                Uri uri = new Uri("http://tile.openstreetmap.org/" + Scale.ToString() + "/" + i.ToString() + "/" + j.ToString() + ".png");
                imageBytes = await webClient.DownloadDataTaskAsync(uri);
                webClient.Dispose();
                webClient= null;