Thread Crossing in Windows Forms Application

Question

Hi,

I have used two threads in my application(WindowsForms). The one thread for getting ThumbnailImages of clients and another thread for getting fullsize Images of Clients...its working but not properly... When i click thumbnail image button its giving thumbnail image properly after that i click that fullsize image button,It came as fullsize image and thumbnail images too... then return back to thumbnail image button it came as thumbnail images and Fullsize images also....

The two threads are crossing Thats why i didnt get proper output....

How can i Solve this Problem? Tell me the solution Of this...

Here is my Code....

{
    System.Windows.Forms.Control.CheckForIllegalCrossThreadCalls = false;
    th = new Thread(new ThreadStart(startlooping));
    th.Start();
}

void StartLooping()
{
    localconnection();

    for (int i = 0; i < Num_Picbox; i++)
    {
        clsImage pcimg = frmDisplay.Serviceobj.ConnectToPcAndGetImage(listBox1.Items[i].ToString(), imgid);

        Image img = objConvertByteToStream.byteArrayToImage(pcimg.pcimage);

        if (listBox1.Items[i].ToString() == pcimg.IPadd && imgid == 0)
        {
            shapes[i].Image = img;
        }

        pcimg = null;
    }
}

private void PictureBox_Click(object sender, EventArgs e)
{
    pb= sender as PictureBox;

    System.Windows.Forms.Control.CheckForIllegalCrossThreadCalls = false;

    thread = new Thread(new ThreadStart(GetImage));

    thread.Start();
}

void GetImage()
{
    pictureBox1.Visible = true;
    btnBack.Visible = true;

    pictureBox1.SizeMode = PictureBoxSizeMode.StretchImage;

    label1.Visible = true;
    pictureBox1.Height = this.flowLayoutPanel1.Height;
    pictureBox1.Width = this.flowLayoutPanel1.Width;

    btnGetConnectedPcs.Location = new Point(168, 616);

    btnGetImageFromSelectedPc.Location = new Point(352, 616);

    for (; ; )
    {
        count++;
        int imageid = 1;
        localconnection();

        string strIpaddress = listBox1.Items[Convert.ToInt32(pb.Name)].ToString();
        .........................
        .........................
    }
}

Answer 1

Do not use CheckForIllegalCrossThreads and use Invoke on the controls you're using.

Answer 2

You may want to take a quick look at Control.InvokeRequired(). That is likely to shed some light and give you a clear path to success.

Good luck.

Answer 3

I think you should use the BackgroundWorker, instead. Don't ever use CheckForIllegalCrossThreadCalls; it's there for a reason.

It deals with "marshalling" (getting calls on one thread to switch to another thread for UI updating purposes).

Here's a good starter: http://bigballofmud.wordpress.com/2009/03/28/thread-marshalling-part-2-using-backgroundworker/

Also, you need to control the UI when the other threads are running. Typically by disabling buttons/menus etc to prevent the user asking for more things like one is already running.

At the very least you should be using Control.Invoke. When your thread has finished downloading images, it should call a method to display them. That method should then check this.InvokeRequired, and if it is true, should call itself via this.Invoke or this.BeginInvoke. This will effectively switch back to the main UI thread and avoid the errors you were probably seeing.

Plz to send me the codez?

Okay, this time you can have a bit more, but you also need to master the skill of taking some directions then finding your own way. If we help you with actual code for this problem you'll just come back when you get stuck next time. We'd prefer to set you going like a wind-up toy than steer you the whole way like a remote-controlled car :)

BackgroundWorker

I'm going to refer you to the BackgroundWorker article on MSDN for this.

The BackgroundWorker is a component you drop on your form from the toolbox. It's not a visual control like a textbox, instead it's more like a Timer or Tooltip.

Using it in it's simplest form means subscribing to the DoWork and RunWorkerCompleted events, as described in the MSDN article. When you want some downloading to happen, you need to do the following:

• Disable your UI (to prevent subsequent button presses).
• Set some sort of progress/working animation or message.
• Call BackgroundWorker.RunWorkerAsync, passing in an object containing parameters that tell it what to do (i.e. download large or small images).

The DoWork event in your case will be raised, and your method will be running in a different thread to your UI, so you are not allowed to update the UI at all in this method. Write your DoWork method to download the images and set the DoWorkEventArgs.Result property to that collection of images once the download is complete.

When the DoWork method completes, the BackgroundWorker will fire the RunWorkerCompleted event, and the method you subscribed to it will be called. This code will now automatically be running on the UI thread again, so you should do the following:

• Report on any errors in the RunWorkerCompletedEventArgs.Error property (essentially this is any untrapped errors that occurred in the DoWork method. Deal with them here so you can use the UI to inform the user of the problem).
• Create your controls to display the images contained in the RunWorkerCompletedEventArgs.
• Hide any progress indicators.
• Re-enable your UI (preferably using a finally block so that any exceptions don't result in your UI being permanently and irretrievably locked).