Set form as Parent throw exception "Top-level control cannot be added to a control"

I want to access variables of a form from another form. On clicking a button inside my Main form, I want to set my Main form as Parent, then bring up another form (child form) wherein I will access variables of the Main form. My click handler is as follow: ``` private void btnSystem_Click(object sender, EventArgs e) { Form_EnterPassword EP = new Form_EnterPassword(); EP.Parent = this; //error: Top-level control cannot be added to a control EP.ShowDialog(); } ``` It compiles fine without any error. However, when I run the Main form and click on the System button, it throws me an exception. I do something similar in another code (not mine) with the same button click, and encounter no error (just with setting Main form as Parent). What am I doing wrong? Is there something in my Main code that cause this?