OpenReadAsync anonymous method not called when using WaitHandler - using Silverlight 4

Question

i have a question about OpenReadAsync using anonymous method.

It seems like that when i use the anonymous method for OpenReadAsync it doesn't run in a separate thread.

I try to read an image from an url and insert it in a excel. (not important)

If i run this code i get every output from Debug.WriteLine (in wrong order though)

Output is:

• Starting OpenReadAsync
• Next line WaitAll() method
• WaitAll passed
• Begin of OpenReadCompleted
• Next line .Set() method
• End of OpenReadCompleted

My goal would be:

• Starting OpenReadAsync
• Next line WaitAll() method
• Begin of OpenReadCompleted
• Next line .Set() method
• End of OpenReadCompleted
• WaitAll passed

The question now is, why does the OpenReadAsync Method never get called when i set a breakpoint on the line

Debug.WriteLine("-> Next line WaitAll() method");

The OpenReadAsync method call is passed at this point but no sign from the other thread.

Output with breakpoint set is only:

• Starting OpenReadAsync

Hope i was able to clarify my question/problem and apologise for my english.

Any help highly appreciated.

This is the code:

    // open file dailog for selecting export file
        sDialog.Filter = "Excel Files(*.xls)|*.xls";

        if (sDialog.ShowDialog() == true)
        {
            // create a workbook object
            Workbook workbook = new Workbook();
            //Create a worksheet object 
            Worksheet worksheet1 = new Worksheet("Org-Export");

            // create a spreadsheet picture object
            Lite.ExcelLibrary.SpreadSheet.Picture pic = new Lite.ExcelLibrary.SpreadSheet.Picture();

            foreach (var item in grdOrg.Items)
            {
                Organization organization = item as Organization;

                WaitHandle[] waitHandles = new WaitHandle[] { new AutoResetEvent(false) };

                WebClient webClient = new WebClient();

                Debug.WriteLine("-> Starting OpenReadAsync");

                webClient.OpenReadAsync(new Uri(organization.Picture, UriKind.RelativeOrAbsolute));

                webClient.OpenReadCompleted += (s, args) =>
                {
                    Debug.WriteLine("-> Begin of OpenReadCompleted");

                    try
                    {
                        Stream stream = args.Result;

                        int length = (int)stream.Length;
                        byte[] buffer = new byte[length];
                        int count;
                        int sum = 0;

                        while ((count = stream.Read(buffer, sum, length - sum)) > 0)
                            sum += count;

                        //ImageTranslator.TranslateImageToBytes translate an image control to byte array
                        // that will be used by excel picture object to plot picture in excel file.
                        pic.Image = new Lite.ExcelLibrary.SpreadSheet.Image(buffer, 0xF01E);
                        //set picture size
                        pic.TopLeftCorner = new CellAnchor(1, 1, 10, 10);
                        pic.BottomRightCorner = new CellAnchor(8, 5, 10, 10);
                        // add picture to spreadsheet
                        worksheet1.AddPicture(pic);

                        Debug.WriteLine("-> Next line .Set() method");
                        ((AutoResetEvent)waitHandles[0]).Set();

                        Debug.WriteLine("-> End of OpenReadCompleted");
                    }
                    catch (Exception ex)
                    { }
                };

                Debug.WriteLine("-> Next line WaitAll() method");
                //WaitHandle.WaitAll(waitHandles);
                Debug.WriteLine("-> WaitAll passed");
            }

            /// add worksheet to workbook
            workbook.Worksheets.Add(worksheet1);

            // get the stream of selected file
            Stream sFile = sDialog.OpenFile();

            // save excel file 
            workbook.Save(sFile);
        }

Answer 1

I found the reason by myself.

The problem was that i blocked the main thread, therefore every thread stopped working on the line WaitHandle.WaitAll(waitHandles); (same if you set a breakpoint)

Solution:

Use a backgroundworker for running the code (code from main thread) and everything works fine.

EDIT: watch this -> http://blogs.planetsoftware.com.au/paul/archive/2010/12/05/waiting-for-a-task-donrsquot-block-the-main-ui-thread.aspx