Question

I get an error trying to send a string from one window to another in my wpf application:

Unable to cast object of type ‘WpfApplication4.LoginWindow’ to type ‘WpfApplication4.MainWindow’.

In my login window the error is on this line:

((MainWindow)Application.Current.MainWindow).StudentID = UserAuthenticationID;

In my main window I have this to test:

    public string StudentId { get; set; }
    void MainWindow_Loaded(object sender, RoutedEventArgs e)
    {
        label1.Content = StudentID;
    }

What am I doing wrong?

EDIT:

No answer has made sense so far, which will be due to my obscure question, I created a wpf application and I have two windows MainWindow.xaml and LoginWindow.xaml.

I want to pass a string (student id) from the login window to the main window after authentication.

I thought the above method was how to do it, as I read it: ((MainWindow)Application.Current.MainWindow).StudentID says where I am intending the string UserAuthenticationID to be sent to?

Then in the MainWindow.xaml I get the string UserAuthenticationID and set it, I then assign a labels content to this public string?

Answer 1

Presumably you create your login window from your main window. If you need to reference your main window from your login window then pass a main window reference to your login window when you construct it. Eg

void MainWindow_Loaded(object sender, RoutedEventArgs e) {
    LoginWindow login = new LoginWindow(this);
    login.ShowDialog();
}

class LoginWindow : Window {
    MainWindow app_window;
    public LoginWindow(MainWindow app_window) {
        Owner = app_window;
        this->app_window = app_window;
    }
}