C#

private void button1_Click(object sender, EventArgs e) {
    Thread th = new Thread(run);
    th.Start();
    run();
}
public void run() {
    for (int i = 0; i < 2; i++) {
        UpdateTextBox("I is now " + i);
        Thread.Sleep(100);
    }
}

private delegate void DUpdateTextBox(string text);
private void UpdateTextBox(string text) {
    if (textBox1.InvokeRequired) {
        var d = new DUpdateTextBox(UpdateTextBox);
        textBox1.Invoke(d, text);
    } else {
        textBox1.Text = text;
    }
}

den2k88 wrote:

Thread is the name of the class, not of the instance.

Bastard Programmer from Hell
If you can't read my code, try converting it here[^]

// as already mentioned in the other replies there are issues when trying to access a textbox or any
// other ui object (in forms). So the invoke is needed to be called, but let me give you a quick tip 
// here: Threads are pretty performance related. Microsoft does give you the ability of using tasks. 
// They are easy to use and pretty similar to the threading system. So try to use your code like this 
// for example:

// (you can decrease the code you've written to just a few lines)
// just navigate to your custom form type constructor:
public FormType() {
     button.Cick += (o, e) => {
          Task.Factory.StartNew(() => {
               for (var i = 0; i < 2; i++) {
                    // if you are always using the invoke method it will still work. There's no need to use
                    // the 'InvokeRequired' property before 
                    textBox.Invoke((Action)delegate { 
                         textBox.Text = string.Format("Variable i currently set to: {0}", i); 
                    });
                    // you are still inside a thread
                    Thread.Sleep(100);
               }
          });
     }
}

// Hope that this gives you some kind of help. Looks like you're just starting with threads and
// background working stuff. If so try to use learn the task thing instead ;) Greetz