UnauthorizedAcessException尝试从WP8中的文件夹打开文件

Question

在我的应用程序中，我将位图转换为PNG并将其存储在文件夹内的文件中。当我试图再次打开文件写第二次时，我得到一个UnauthorisedAccessException说"Access is denied"。 单击保存按钮时，将调用以下函数。

private async void SaveClicked(object sender, RoutedEventArgs e) 
     { 
      WriteableBitmap wb = new WriteableBitmap(InkCanvas2, null); 
      Image image = new Image(); 
      image.Height = 150; 
      image.Width = 450; 
      image.Source = wb; 
      await SaveToStorage(wb, image); 
      TransparentLayer.Visibility = System.Windows.Visibility.Collapsed; 

     } 

的SaveToStorage具有下面的代码

private async Task SaveToStorage(WriteableBitmap i, Image im) 
     { 
      try 
      {  
        var dataFolder = await local.CreateFolderAsync("Page", CreationCollisionOption.OpenIfExists);      
        using (var testpng = await dataFolder.OpenStreamForWriteAsync("testpng.png", CreationCollisionOption.ReplaceExisting)) 
// HITS EXCEPTION AND GOES TO CATCH BLOCK 
        {  
         i.WritePNG(testpng);  
         testpng.Flush(); 
         testpng.Close(); 
        }      
      } 
      catch(Exception e) 
      { 
      string txt = e.Message; 
      } 
     } 

它节省了第一次，没有错误，第二时间，则抛出异常。任何想法为什么发生这种情况

Answer 1

我想通了！很明显，因为在尝试打开它之前我没有包含文件存在检查。

我已经重新写成这样

 IStorageFolder dataFolder = await local.CreateFolderAsync("Page", CreationCollisionOption.OpenIfExists); 
      StorageFile Ink_File = null; 
      //Using try catch to check if a file exists or not as there is no inbuilt function yet 
      try 
      { 
       Ink_File = await dataFolder.GetFileAsync("testpng.png"); 
      } 
      catch (FileNotFoundException) 
      { 
       return false; 
      } 

      try 
      { 
       if (Ink_File != null) 
       { 

        using (var testpng = await Ink_File.OpenStreamForWriteAsync()) 
        { 

         i.WritePNG(testpng); 

         testpng.Flush(); 
         testpng.Close(); 
         return true; 
        } 
       } 

      } 
      catch(Exception e) 
      { 
       string txt = e.Message; 
       return false; 
      } 
      return false; 

代码