hide previous open forms in windows application

Question

Hello;

 

    I need to hide all forms ehich are open when I open particular  new form.

 

Ex: Suppose form A,B,C are open. And when I want to open form D, all other open forms (A,B,C) will be hide.

 

I tries this code when open new form (on form load event):

 

AlcoholDetails alc = new AlcoholDetails();

 AlcoholTestingRecord alTst = new AlcoholTestingRecord();

 alc .Hide();

 alTst .Hide();

 

But Its not working. 

 

 

   Thank you.