How to access ContentPage controls in master page and vice versa

void Page_Load()
{
   // Gets a reference to a TextBox control inside
   // a ContentPlaceHolder
   ContentPlaceHolder mpContentPlaceHolder;
   TextBox mpTextBox;
   mpContentPlaceHolder =
     (ContentPlaceHolder)Master.FindControl("ContentPlaceHolder1");
   if(mpContentPlaceHolder != null)
   {
       mpTextBox =
           (TextBox) mpContentPlaceHolder.FindControl("TextBox1");
       if(mpTextBox != null)
       {
           mpTextBox.Text = "TextBox found!";
       }
   }
   
   // Gets a reference to a Label control that not in
   // a ContentPlaceHolder
   Label mpLabel = (Label) Master.FindControl("masterPageLabel");
   if(mpLabel != null)
   {
       Label1.Text = "Master page label = " + mpLabel.Text;
   }
}