MDI question

Question

I want to access a rich text box in the Active MDI Child Form from the parent MDI form and may be any numbers of MDI Childs instance but I want to access the a public rich text box on the Active one , I try to use the ActiveMdiChild method but I can’t, And I can access it if I know the instance child form name only like: Form2 f=new Form2(); f.MdiParent=this; f.richTextBox1.Text="John George"; What can I do? John George.