How to Open Window Form in C#

First -Create a MDIParent form( this is master form which hold all child form)
Second -Create a windows form -for instance- PayrollSetupForm -
Then  open a MenuToolStrip by double clicking

Note-:
MdiParent - Master / Parent Form

Trick 1-To Open a Page

private void payrollSetupFormToolStripMenuItem_Click(object sender, EventArgs e)
        {
            PayrollSetupForm obj = new PayrollSetupForm();
            obj.MdiParent = this;
            obj.Show();
        }

Trick 2-To Open a Page

first create a method

 void OpenChildWindow(string FormName, Object NewFrm)

        {

            FormCollection AllForms = Application.OpenForms;

            foreach (Form frm in AllForms)

            {

                if (frm.Name == FormName)

                {

                    frm.Activate();

                    return;

                }

            }

            ((Form)NewFrm).MdiParent = this;

            ((Form)NewFrm).Show();

        }

then double click on a  menu Tool Strip

private void payrollSetupFormToolStripMenuItem_Click(object sender, EventArgs e)
        {

                FrmLedger1 ObjDef = new FrmLedger1();

            OpenChildWindow("FrmLedger", ObjDef);

       }