Close the windows form if its instance is running

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C#

Windows

Close the windows form if its instance is running.

I have three forms
1) Normal Windows Forms (Named Main Screen)
2) MDI form (Named POS)
3) MDI form (Named Stock)

When I click Button on [Main Screen]

I am opening a MDIPOS FORM from [Main Screen] by button click event. The code is:

MDIPOS pos=new MDIPOS();
pos.MdiParent = this;
pos.Activate();           
pos.Show();

Getting Problem in Closing the form.
When I close the MDI Form or Main Screen Form
one its instance remain running.
I used :

frmMain.Dispose();  //or
frmMain.Close();

Then also it instance remain running.
If I use only one form, it closes perfectly.

How to close the instance of another form.
because, to access a form, we need to create a new object of that form and that that new object is destroyed , but what about the previous one which was opened.

I hope I explain the problem nicely.

Appreciate your best reply

Thanking You