Multi threading in wpf using c# (with background worker)

Question

Hi,

I have written a code to save the image which is generated by the application. The size of image is around 32-35 MB. While saving the image to a bmb file, it is taking long time around 3-5 secs. For the purpose, I have used background worker but when running background worker, it shows an error like..."cant access the object as it is created on different thread".

Following is the code:

 private void btnSaveDesign_Click(object sender, RoutedEventArgs e)
    {
        Microsoft.Win32.SaveFileDialog sfd = new Microsoft.Win32.SaveFileDialog();
        sfd.Title = "Save design as...";
        sfd.Filter = "BMP|*.bmp";
        if (sfd.ShowDialog() == true)
        {
            ww = new winWait();
            ww.Show();
            System.ComponentModel.BackgroundWorker bw = new System.ComponentModel.BackgroundWorker();
            bw.DoWork += new System.ComponentModel.DoWorkEventHandler(bw_DoWork);
            bw.RunWorkerCompleted += new System.ComponentModel.RunWorkerCompletedEventHandler(bw_RunWorkerCompleted);
            fName = sfd.FileName;
            cache = new CachedBitmap((BitmapSource)imgOut.Source, BitmapCreateOptions.None, BitmapCacheOption.OnLoad);

            bw.RunWorkerAsync();


        }  
    }

    void bw_RunWorkerCompleted(object sender, System.ComponentModel.RunWorkerCompletedEventArgs e)
    {
        ww.Close();
    }

    void bw_DoWork(object sender, System.ComponentModel.DoWorkEventArgs e)
    {
        BmpBitmapEncoder encoder = new BmpBitmapEncoder();
        encoder.Frames.Add(BitmapFrame.Create(cache)); //here... it says cant access...
        using (FileStream file = File.OpenWrite(fName))
        {
            encoder.Save(file);
        }
    }

I have declared "cache" as a global object. (The similar trick was worked when I was programming in WinForms with Vb.Net)

ww is the wait window that I want to be displayed while the precess is being executed.

Any idea how to do this? Is there any other simple method for multi threading in Wpf??

Thanks

Answer 1

I think you have to pass cache as a parameter to the new thread:

bw.RunWorkerAsync(cache);

and get it from the DoWork method:

var cache=(CacheType) e.Argument;

Answer 2

When WPF objects are created they are assigned to a Dispatcher object. This disallows any threads other than the creating thread to access the object. This can be circumvented by freezing the object by calling the freeze method. You would need to call Freeze on your bitmapsource object. Once you have frozen your object it becomes uneditable

Answer 3

ur probelm is because you are accessing a object which is not created by background worker thread normally this would happen if you access a UI control which is created in Main thread and accessed from different thread. try below code

 Dispatcher.Invoke
                    (
                      new Action(
                          delegate()
                          {
                               BmpBitmapEncoder encoder = new BmpBitmapEncoder();    
                               encoder.Frames.Add(BitmapFrame.Create(cache)); 
    using (FileStream file = File.OpenWrite(fName))    
    {    
        encoder.Save(file);    
    }  
                          }
                          )
                    );