Desde el evento Loaded de la ventana llamamos al siguiente código

privatevoid ShowInMonitor(int screen)

{

this.WindowStartupLocation = System.Windows.WindowStartupLocation.Manual;

this.Left = System.Windows.Forms.Screen.AllScreens[screen].Bounds.Left;

this.Top = System.Windows.Forms.Screen.AllScreens[screen].Bounds.Top;

this.Width = System.Windows.Forms.Screen.AllScreens[screen].Bounds.Width;

this.Height = System.Windows.Forms.Screen.AllScreens[screen].Bounds.Height;

this.WindowState = WindowState.Maximized;

this.Topmost = true;

WindowState = System.Windows. WindowState.Maximized;

this.Show();

}

Supongamos que tenemos

 <ContentControl Name="cntNuevoProducto" Margin="10,10,20,10"
                        ContentTemplate="{StaticResource itemVentaDT}"></ContentControl>

Queremos acceder a los controles definidos en la DataTemplate "itemVentaDT"

 DependencyObject oooo=VisualTreeHelper.GetChild(cntNuevoProducto,0);
 ContentPresenter cp=(ContentPresenter)oooo;
 TextBox n = (TextBox)cntNuevoProducto.ContentTemplate.FindName("txtboxDescripItem",cp );