Form's lost focus in C#

Question

This may be a simple C# question but I need a solution.

I have two forms, form1 and form2, with form1 having a

Answer 1

Use the Deactivate event handler

Answer 2

If I understand your question, I think you actually want to trap deactivation. Button handler inside your main form:

private void button1_Click(object sender, EventArgs e)
{
    Form childForm = new Form();
    childForm.Deactivate += delegate
    {
        childForm.Close();
    };

    childForm.Show();
}