Windows application Form Focus in C#.NET

 Form1 form1 = null;
 Form2 form2 = null;
 .........

       private void button1_Click(object sender, EventArgs e)
        {
          if (form1 == null)
            {
                form1 = new Form1();
                form1.FormClosed += SetFormToNull; //Raise event
                form1.Show(this); // SHow form
            }
            form1.Focus();
        }

        private void SetFormToNull(object sender, EventArgs e) 
        {
            form1 = null; //set null when closed
        }


Hope this will help you

Cheers
Pankaj