Check the form is loaded

Question

Hello! I know that if I want to check the Form2 is loaded, so I can use the following coding, but how to define Form2 form2 = null at Form1, where to define this and how? At Form1 you must define:Form2 form2=null and at the btn_Click() if(form2==null) //First Time Open { form2=new Form2(); form2.Show(); } else form2.Activate(); Please give me reply asap. Thanks!