I am not sure why you are unable to make it work.

Just follow the steps
1. Add a new C# windows application project
2. Add couple of buttons and one text box to the forum
3. Add the following code to the event handlers of the buttons
Code:
private void button1_Click(object sender, EventArgs e)
{
    textBox1.Text = "Updated in the clicked event";
}

private void button2_Click(object sender, EventArgs e)
{
    x x1 = new x(test);
    x1.BeginInvoke(null, null);
}

public delegate void x();

public void test()
{
    for (int i = 0; i <= 1000000000; i++)
    {
    }

}
It should do the job for you.