DownloadStringAsync.cs

private Task<string> DownloadStringAsync(Uri uri)
{
    var taskSource = new TaskCompletionSource<string>();

    var webClient = new WebClient();
    webClient.DownloadStringCompleted += (sender, args) =>
    {
        try
        {
            if (args.Error != null)
            {
                taskSource.SetException(args.Error);
            }
            else if (args.Cancelled)
            {
                taskSource.SetCanceled();
            }
            else
            {
                taskSource.SetResult(args.Result);
            }
        }
        finally
        {
            webClient.Dispose();
        }
    };
    webClient.DownloadStringAsync(uri);

    return taskSource.Task;
}