When I run WPF MVVM application two instances of main window are displaying

Question

I develop WPF MVVM application in MS VS 2015 Professional (.NET Framework 4.6). When I run my WPF MVVM application then two instances of my application main window are displaying. Why does it have place? Below is markup of my application main window - MainWindow.xaml:

<Window x:Class="SuperMrgaChartDrawer.MainWindow"
       xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
       xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
       xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
       xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
       xmlns:local="clr-namespace:SuperMrgaChartDrawer"
       xmlns:vm="clr-namespace:SuperMrgaChartDrawer.ViewModel"
       mc:Ignorable="d"
       Title="MainWindow" Height="350" Width="525">
    <Grid>

    </Grid>
</Window>

Below is code of MainWindow.xaml.cs:

using System.Windows;

namespace SuperMrgaChartDrawer
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    }
}

And below is code of App.xaml.cs:

using System;
using System.Windows;
using SuperMrgaChartDrawer.ViewModel;

namespace SuperMrgaChartDrawer
{
    public partial class App : Application
    {
        /// <summary>
        /// My application OnStartup handler.
        /// </summary>
        /// <param name="e"></param>
        protected override void OnStartup(StartupEventArgs e)
        {
            // Call OnStartup in base class.
            base.OnStartup(e);
            // Create an instance of application main window.
            MainWindow window = new MainWindow();
            // Create an instance of View Model of main window.
            var viewModel = new MainWindowViewModel();
            // Define handler for "Main Window Request Close" event
            // and subscribe on it.
            EventHandler handler = null;
            handler = delegate
            {
                viewModel.RequestClose -= handler;
                window.Close();
            };
            viewModel.RequestClose += handler;
            // Set MainWindowViewModel as main window data context.
            window.DataContext = viewModel;
            // Display main window.
            window.Show();
        }
    }
}

What is the reason of this error? What should I do to eliminate this error. Please help. Your help will be highly appreciated.

Answer 1

Your App.xaml contains StartupUri="MainWindow.xaml". This attribute is added by WPF application template. Remove StartupUri, if you want to show window manually.

Besides, this is rather exotic way to add event handler:

EventHandler handler = null;
handler = delegate
{
    viewModel.RequestClose -= handler;
    window.Close();
};
viewModel.RequestClose += handler;

Since RequestClose is EventHandler, you can replace code from above (this is main window, so, there's no need to unsubscribe):

viewModel.RequestClose += (sender, args) => window.Close();