Windows Phone 8 - 尝试从WP8中的文件夹中打开文件时未经授权的AcessException

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中的文件中。当我尝试再次打开文件进行第二次写入时，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;   
        }

SaveToStorage 具有以下代码

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;
            }
        }

它第一次保存没有错误，第二次，抛出异常。知道为什么会这样吗？

Answer 1

我想通了！这显然是因为我在尝试打开它之前没有包含文件存在检查。

我已经像这样重新编写了代码

       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;