如何在Windows 8中从KnowFolders检索文件？

Question

我想创建用于在PictureFolder中获取文件列表的方法。 我创建此方法：

public Task<List<string>> GetImages()
{
var task = new Task<List<string>>(() =>
{
var files = new List<string>(5);
StorageFolder picturesFolder = KnownFolders.PicturesLibrary;
IReadOnlyList<IStorageItem> itemsList = picturesFolder.GetItemsAsync().GetResults();
foreach(var item in itemsList)
{
if(item is StorageFile)
{
files.Add(item.Name);
}
}
return files;

});
return task;
}

和我用于测试此方法的代码：

 var pic = new PictureManager();
pic.GetImages().RunSynchronously();
List<string> images = pic.GetImages().Result;
bool hasValue = images.Count > 0;
Debug.WriteLine(string.Format("Has value {0}", hasValue));

我得到这个例外

System.InvalidOperationException was unhandled by user code
  HResult=-2147483634
  Message=A method was called at an unexpected time. (Exception from HRESULT: 0x8000000E)
  Source=Windows.Foundation
  StackTrace:
       at Windows.Foundation.IAsyncOperation`1.GetResults()
       at Praktyka.Models.PictureManager.<GetImages>b__0() in d:\Documents\Visual Studio 2012\Projects\Praktyka\Praktyka\Models\PictureManager.cs:line 63
       at System.Threading.Tasks.Task`1.InnerInvoke()
       at System.Threading.Tasks.Task.Execute()
  InnerException: 

在这一行（方法GetImages）

 IReadOnlyList<IStorageItem> itemsList = picturesFolder.GetItemsAsync().GetResults();

如何解决这个问题？

Answer 1

RunSynchronously方法在当前线程上启动任务。 但是，从GetItemsAsync返回的任务已经启动，因此您会收到异常。

您需要重构代码以利用async / await关键字：

public async Task<List<string>> GetImages()
{
 var files = new List<string>();
 StorageFolder picturesFolder = KnownFolders.PicturesLibrary;
 IReadOnlyList<IStorageItem> itemsList = await picturesFolder.GetItemsAsync();
 foreach(var item in itemsList)
 {
  if(item is StorageFile)
  {
   files.Add(item.Name);
  }
 }
return files;
}

同样，调用GetImages的方法应利用await：

var pic = new PictureManager();
List<string> images = await pic.GetImages();;
bool hasValue = images.Count > 0;
Debug.WriteLine(string.Format("Has value {0}", hasValue));

Answer 2

我相信您应该从Completed处理程序中调用GetResults，而不是立即从GetItemsAsync（）的结果中调用。