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

This does work:

public dlgForecast()

{

InitializeComponent();

WindowStartupLocation = WindowStartupLocation.CenterOwner;

Owner = Application.Current.MainWindow;

}//dlgForecast

Try this,

var dlg = new dlgForecast

{

    Owner = this,

    WindowStartupLocation = WindowStartupLocation.CenterOwner

};

dlg.ShowDialog();

One caution about the constructor version: Owner = Application.Current.MainWindow centers the new dialog on the application's main window, which isn't always the current dialog. If one dialog opens another, the new dialog won't center on the window that opened it.

The first attempt didn't work because WindowStartupLocation was set on the calling window instead of dlg, and dlg had no owner. Without an owner, WPF positions the window as if the setting were Manual.

To center on the current dialog, set both properties on the new window from the calling code:

var dlg = new dlgForecast();
dlg.Owner = this;
dlg.WindowStartupLocation = WindowStartupLocation.CenterOwner;
dlg.ShowDialog();

Set them before calling ShowDialog(). The owner must be a window that has already been shown, so this works when the calling dialog is open.