Response for http://stackoverflow.com/questions/23842009/open-new-window-after-splash-window-in-wpf/23843416#23843416

App.xaml

<Application x:Class="WpfApplication2.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Application.Resources>
         
    </Application.Resources>
</Application>

App.xaml.cs

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Threading.Tasks;
using System.Windows;

namespace WpfApplication2
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application
    {

    async protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);

        // Create and show SplashWindow
        SplashWindow splashWindow = new SplashWindow();
        splashWindow.Show();

        // Let's wait 3 seconds
        await Task.Delay(3000);

        // Create and show the SelectionWindow
        SelectionWindow selectionWindow = new SelectionWindow();
        selectionWindow.Show();

        // Hide the SplashWindow we previosly created
        splashWindow.Close();
    }
}
}

MainWindow.xaml

<Window x:Class="WpfApplication2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel>
        <Label>This is the main window :)</Label>
    </StackPanel>
</Window>

MainWindow.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Xml.Linq;

namespace WpfApplication2
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    }
}

SelectionWindow.xaml

<Window x:Class="WpfApplication2.SelectionWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="SelectionWindow" Height="300" Width="300">
    <StackPanel>
        <Button Click="Button_Click">Click to open MainWindow</Button>
    </StackPanel>
</Window>

SelectionWindow.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;

namespace WpfApplication2
{
    /// <summary>
    /// Interaction logic for SelectionWindow.xaml
    /// </summary>
    public partial class SelectionWindow : Window
    {
        public SelectionWindow()
        {
            InitializeComponent();
        }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        // Create and show MainWindow
        var mainWindow = new MainWindow();
        mainWindow.Show();

        // Closes SelectionWindow
        this.Close();
    }
    }
}

SplashWindow.xaml

<Window x:Class="WpfApplication2.SplashWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="SplashWindow" Height="300" Width="300">
    <StackPanel>
        <Label FontSize="50">Splash Screen</Label>
    </StackPanel>
</Window>

SplashWindow.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;

namespace WpfApplication2
{
    /// <summary>
    /// Interaction logic for SplashWindow.xaml
    /// </summary>
    public partial class SplashWindow : Window
    {
        public SplashWindow()
        {
            InitializeComponent();
        }
    }
}

without_async_await.cs

        protected override void OnStartup(StartupEventArgs e)
        {
            base.OnStartup(e);

            // Create and show SplashWindow
            SplashWindow splashWindow = new SplashWindow();
            splashWindow.Show();


            var thread = new System.Threading.Thread(p =>
            {
                System.Threading.Thread.Sleep(3000);

                Dispatcher.Invoke(() =>
                {
                    // Create and show the SelectionWindow
                    SelectionWindow selectionWindow = new SelectionWindow();
                    selectionWindow.Show();

                    // Hide the SplashWindow we previosly created
                    splashWindow.Close();
                });
            });

            thread.Start();
        }
    }