How to display Form on the center of Secondaryscreen

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C#3.5

I am developing a windows desktop application dual monitor where i need to display my form sometimes on Primary Screen and sometimes on secondary,
Which works fine but when i display it on my secondary Screen i want it to be displayed on centre of my screen which is not working here is my code:

if (Screen.AllScreens.Length > 1)
 myForm.Location = Screen.AllScreens[1].WorkingArea.Location;
 myForm.StartPosition = FormStartPosition.Manual; // because i wrote manual it is displayed on Top left of my secondaryScreen which is ok
 myForm.show();

but i want to diplay it on centre so i wrote

  myForm.StartPosition = FormStartPosition.CentreScreen;
//it is not working again a form is displayed on Centre of PrimaryScreen..

any idead why?????