Many people don't pay attention to the fact that controls couldn't be directly invoked within a thread context other than the one within which they have been created. And then their applications fall down because a run time error will be raised when firing the application. It is technically called none valid cross thread operation. It generates an InvalidOperationException.

As explanation to this error, I can say that each thread has its proper boundary exactly like a state that has a boundary. And it is not legal to travel from a county to another one without achieving some administrative tasks. It is exactly the same fact from an analogical point of view.

Therefore, it is not possible for a control that has been created within a given thread to be directly used within another thread event when one try to avoid the problem by using a Backgroundworker he/she won't resolve the problem if he/she doesn't have a complete idea about dealing with controls and or objects within a multithreaded contexts.

There are many used techniques in order to resolve the problem and I will represent one of those techniques.
But first let's see what will happen when an invalid cross threaded operation is designed and tested:

First, create a new windows application project and add a progress bar, a button and a label into the form1 so that it appears as under:

Add the namespace

using System.Threading;

Then try this code

public partial class Form1 : Form
 {

Thread myThread;
public Form1()
{
InitializeComponent();

myThread = new Thread(new ThreadStart(ModifyControlsState));
}

public void ModifyControlsState()
{
progressBar1.Minimum = 0;
progressBar1.Maximum = 100;
for (int i = 0; i <= 100; i = i + 1)
{
progressBar1.Increment(i);
label1.Text = progressBar1.Value.ToString() + "%omplete"; Thread.Sleep(10);
}
DialogResult dlg = MessageBox.Show("Do you want to close this window",
"Do you want to exit this window",
MessageBoxButtons.OKCancel);
if (dlg == DialogResult.OK) Application.Exit(); }

private void button1_Click(object sender, EventArgs e)
{
myThread.Start();
}

If you click the button1 you will receive this error:



Figure1

Now the solution is performed through the following steps:

  1. Add a void delegate class with the same characteristics conforming to the thread method signature

    private delegate void MyDelegate();

    MyDelegate myDelegate;
  2. Add a method with the same signature as the thread method except the name Of Corse

    public void Perform()
    {
    //TO DO: Cut the thread method implementation and paste it here

    }

  3. Create a new delegate instance that points to the Perform method

    private void Form1_Load(object sender, EventArgs e)
    {
    myDelegate = new MyDelegate(Perform);

    }

  4. Cut and paste the thread method implementation within the Perform method zone as follow

    public void Perform()
    {
    //TO DO: Cut the thread method implementation and paste it here
     progressBar1.Minimum = 0;
    progressBar1.Maximum = 100;
    for (int i = 0; i <= 100; i = i + 1)
    {
    progressBar1.Increment(i);
    label1.Text = progressBar1.Value.ToString() + "%omplete";
    Thread.Sleep(10);
    }
    DialogResult dlg = MessageBox.Show("Do you want to close this window",
    "Do you want to exit this window",
    MessageBoxButtons.OKCancel);
    if (dlg == DialogResult.OK) Application.Exit();

    }

  5. And finally, you add this code within the thread method zone

    public void ModifyControlsState()
    {
    Form1 form1 = this;
    form1.Invoke(myDelegate);
    }



    Figure 2

It works great, that's it.
Good Dotneting!!!