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

<Window x:Class="WpfTestApplication.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">
    <Grid>
        <Button Width="75" Height="24" Click="ButtonBase_OnClick"/>
    </Grid>
</Window>

<Window x:Class="WpfTestApplication.TestWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="TestWindow" Height="300" Width="300"/>

private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
{
    var wnd = new TestWindow();
    //wnd.Close();
    //wnd.ShowDialog();
}