Why does Thread.Sleep() freeze the Form?

Question

I try to experiment with Thread.Sleep(). I created basic Windows Forms application with one button.

    private void button1_Click(object sender, Ev         


        

Answer 1

To keep the UI active, you need for the main UI thread to service its message pump. It can only do that when it is not handling UI events. In your case the function

private void button1_Click(object sender, EventArgs e)
{
    Thread thread1 = new Thread(DoStuff);
    thread1.Start();

    for (int i = 0; i < 100000; i++)
    {
        Thread.Sleep(500);
        button1.Text +=".";
    }
}

does not return for around 100000*500 milliseconds. While this event handler is executing, the UI thread is busy. It is executing this event handler. As such it is not able to service the message pump. Hence your application's UI freezes.