access to active MDIChild

Hi everyone. I have a problem. I am trying to create a texteditor with multiple documents. I have main form with buttons,etc..... and when I press button i create MDIChild forms with RichTextBox.: private void button_NEW_Click(object sender, System.EventArgs e) { clsDocumentForm doc = new clsDocumentForm(); doc.MdiParent = this; } But if I create more of them and I want to do something with RichTextBox in one which is currently active,I don't know how. This code below throw error,that I can't convert Form to my clsDocumentForm. private void TextEditor_MdiChildActivate(object sender, EventArgs e) { clsDocumentForm activeChild = this.ActiveMdiChild; activeChild.RichTextBox1.Text = "something"; } It is propably easy ,but I just don't know. Thank you very much.