从 C# VS2013 WPF 通过多线程将图像添加到地图的错误

Question

I would like to add an image to a map based on OpenStreetMap from C# VS2013 WPF desktop application.

But, the time of generating the image may be long so I need to use multithreading to do this. In main thread, I created a new thread to draw and add the image to the WPF desktop application.

In single thread, it works well. But, in multithreading, no image can be added to the map.

The code is as follows:

 using System.ComponentModel;
 private BackgroundWorker worker_addImage;
 public MainWindow()
    {
        InitializeComponent();
        worker_addImage = new BackgroundWorker();
        worker_addImage.DoWork += (s, a) =>
        {
            MessageBox.Show("the worker started \r\n");
            DrawImage();
        };
        _worker_addImage.ProgressChanged += _worker_ProgressChanged;
        _worker_addImage.RunWorkerCompleted += (s, a) =>
        {
            MessageBox.Show("image added \r\n");
            AddImage();
        };
    }

    private void ShowImage_Clicked(object sender, RoutedEventArgs e)
    {
        MessageBox.Show("Show Image \r\n");
        if (worker_addImage.IsBusy == false)
            worker_addImage.RunWorkerAsync();
    }
    // why this progress function is not executed ? 
    private static void _worker_ProgressChanged(object sender, ProgressChangedEventArgs e)
    {
        MessageBox.Show(e.ProgressPercentage.ToString());
    }

UPDATE

This is the code that add image to the map.

    private void AddImage()
    {
        Uri uri = new Uri(resultImageString, UriKind.Absolute);
        System.Windows.Media.ImageSource imgSource = new BitmapImage(uri);
        // the image generated by DrawImage() is saved in this URI. I can 
        // open and see it. But, it cannot be added to the map. But, if I
        // used single thread, no problems.
        mapImage.Source = imgSource;  
    }

In DrawImage(). I saved the image in a local folder as a png file. But, in AddImage(), I was told that the png file cannot be accessed because another thread or process was using it. Why ?

Is that due to that RunWorkerCompleted handler is invoked in the UI thread but the png file was being used by the "worker_addImage" thread ?

      using (MemoryStream memory = new MemoryStream())
      {
               using (FileStream fs = new FileStream(res, FileMode.Create, FileAccess.ReadWrite))
                {
                    myImage.Save(memory, ImageFormat.Png);
                    byte[] bytes = memory.ToArray();
                    fs.Write(bytes, 0, bytes.Length);
                }
      }

Answer 1

You are missing RunWorkerAsync() on worker_addImage. Also message box should be marshalled as your process is not in the main UI thread.