CS0266无法隐式转换类型＆＃39; IAsyncOperation <string>＆＃39; to＆＃39; IAsyncOperationWithProgress <string，int =“”>＆＃39;

Question

这个让我疯了......我对C＃语法还不够熟悉，但我已经到了那里。

代码：

    /// <summary>
    ///  OLD: public static IAsyncOperationWithProgress<string, int> GetStringAsyncWithProgress(this HttpClient client, HttpRequestMessage request, CancellationToken pCancelToken, TextBox textBoxURL)
    ///  NEW: public static IAsyncOperationWithProgress<string, int> GetStringAsyncWithProgress(HttpClient client, HttpRequestMessage request, CancellationToken pCancelToken, TextBox textBoxURL)
    /// </summary>
    /// <param name="client"></param>
    /// <param name="request"></param>
    /// <param name="pCancelToken"></param>
    /// <param name="textBoxURL"></param>
    /// <returns></returns>
    public static IAsyncOperationWithProgress<string, int> GetStringAsyncWithProgress(HttpClient client, HttpRequestMessage request, CancellationToken pCancelToken, TextBox textBoxURL)
    {
        int read = 0;
        int offset = 0;
        string result = string.Empty;

        // OLD: byte[] responseBuffer = new byte[9000]; // Was 500, corrected to 9000 (Jumbo Frame Network Packet)
        /// NEW: Source: https://msdn.microsoft.com/en-us/library/hh598387(v=vs.110).aspx
        IBuffer buffer = System.Runtime.InteropServices.WindowsRuntime.WindowsRuntimeBuffer.Create(9000);

        /// var operation = client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, cancelToken);

        Uri uri = new Uri(textBoxURL.Text);
        var operation = client.GetAsync(uri, HttpCompletionOption.ResponseHeadersRead);

        /// CancellationToken cancelToken;
        return AsyncInfo.Run( (asyncInfo) =>
           Task.Run(async () =>
           {
               //CancellationToken cancelToken;
               //if (pCancelToken != CancellationToken.None)
               //{
               //    cancelToken = pCancelToken;
               //}

               using (var responseStream = await operation.GetResults().Content.ReadAsInputStreamAsync())
               {
                   do
                   {
                       //if (cancelToken.IsCancellationRequested)
                       //{
                       //    cancelToken.ThrowIfCancellationRequested();
                       //}

                       var iAsyncOperationWithProgress = await responseStream.ReadAsync(buffer, 0, InputStreamOptions.ReadAhead);
                       /// result += Encoding.UTF8.GetString(responseBuffer, 0, read);
                       offset += read;

                       Debug.WriteLine(string.Format("\t{0:o}\tExtension Read Bytes: {1}", DateTime.Now, offset));

                       // OLD: progress.Report(offset);
                       /// NEW: iAsyncOperationWithProgress.Progress();
                   } while (read != 0);
               }

               return result;
           } ) );
    }

失败了：

return AsyncInfo.Run( (asyncInfo) =>
   Task.Run(async () =>
   { ... }

我仍然欢迎任何想法，想法和评论，因为我仍然在学习WinRT和UWP开发。