How to communicate between forms?

Form2.cs:

        private Form1 mainForm = null;

        public Form2(Form callingForm) //put the calling form as arg
        {
                InitializeComponent();
                mainForm = callingForm as Form1;
        }
       

Form1.cs

        private Form2 subForm = null;

        subForm = new Form2(this);
        subForm.show();