async await ，一步一步试用 GIS

 public   class downClass1
    {
     public  IStorageFile pResultStorageFile;
        public    async Task<IStorageFile> down()
        {      
            string CountriesFile = "fruit.xml";
           
 StorageFolder InstallationFolder =Windows.ApplicationModel.Package.Current.InstalledLocation;        
          StorageFile file = await InstallationFolder.CreateFileAsync(CountriesFile);
            BackgroundDownloader ppp = new BackgroundDownloader();
          DownloadOperation pDownloadOperation=  ppp.CreateDownload(new Uri("https://files.cnblogs.com/gisbeginner/fruit.xml"), file);
  await pDownloadOperation.StartAsync();
          pResultStorageFile = pDownloadOperation.ResultFile;
          return pResultStorageFile;         
            
        }
      
    }
}/// 

istorageFile类型可以返回给Task〈istorageFile〉，调用这个方法的时候IstorageFile pDownLoadFile = pdownClass1.down() as IStorageFile; 这样是不行的

public IStorageFile pDownLoadFile;

 pDownLoadFile =await  pdownClass1.down()，这样就可以了

http://msdn.microsoft.com/en-us/library/hh191443(v=vs.110).aspx#Y749