How do I open a dialog box that opens centered on the current dialog box?

Try this,

var dlg = new dlgForecast

{

    Owner = this,

    WindowStartupLocation = WindowStartupLocation.CenterOwner

};

dlg.ShowDialog();

This does work:

public dlgForecast()

{

InitializeComponent();

WindowStartupLocation = WindowStartupLocation.CenterOwner;

Owner = Application.Current.MainWindow;

}//dlgForecast