Sending a value from an interface to a previous interface

Question

I have three simple interfaces Windows form. I want to send a value from the third interface it appears in the second interface .Where the second interface remains open, which is the main, and the third sub-interface is closed . The windows are opened in order from 1 to 3. Where I want the following: that the label appears on the second interface and a value is added to it.

I tried the following code and nothing works؟؟.

MAIN FORM2 = new MAIN();

           FORM2.LAB_A.Visible = true;
           FORM2.LAB_A.Text = DATATABLA.Rows.Count.ToString();

Answer 1

Try replacing the first line:

 MAIN form2 = (MAIN)Application.OpenForms["MAIN"];

It assumes that there is a single form called "MAIN".