how to create new window?

Question

var wnd=new LoginWindow();

and then?

Answer 1

For example, a simple window with a Button :

                Button btn1 = new Button()
                {
                    Content = "Click",
                    HorizontalAlignment = HorizontalAlignment.Center
                };
                Window wnd1 = new Window()
                {
                    Content = btn1
                };
                btn1.Click += (sender, e) =>
                {
                    Console.Beep(5000, 10);
                };               
                wnd1.Activate();

Answer 2

Or you can create a new window in Visual Studio, Right Click solution->Add->New Item...->Blank Window, and then refer it in code. It’s also recommended to read the document, Show multiple views for an app.

As a side note, it's necessary to modify the new window's IDL for WinUI3 c++.

// Copyright (c) Microsoft Corporation and Contributors.
// Licensed under the MIT License.

namespace App1
{
    [default_interface]
    runtimeclass BlankWindow : Microsoft.UI.Xaml.Window
    {
        BlankWindow();
        Int32 MyProperty;
    }
}