private void Button_Click(object sender, RoutedEventArgs e)
        {
            if (Combobox2.SelectedItem != null)
            {
                Window2 tb = new Window2(Combobox1.Text + "'  и таблицей:  '" + Combobox2.Text);
                tb.Show();
               
            }
            else
            {
                MessageBox.Show("Выберите таблицу");
            }
        }

     public Window2(string s)
        {
           InitializeComponent();

            Lable1.Content = "Вы работаете с базой:  '" + s + "'";
        }

<Window x:Class="WpfApplication1.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:local="clr-namespace:WpfApplication1"
        Title="Window1" Height="300" Width="300">
    <Window.DataContext>
        <local:MainWindow/>
    </Window.DataContext>
    <Grid>
        <Label Content="{Binding Path=ss}" HorizontalAlignment="Left" Margin="58,33,0,0" VerticalAlignment="Top" />
        <Label Content="{Binding Path=sss}" HorizontalAlignment="Left" Margin="58,86,0,0" VerticalAlignment="Top"/>

    </Grid>
</Window>

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;

namespace WpfApplication1
{
    /// <summary>
    /// Логика взаимодействия для MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public string ss { get; set; }
        public string sss { get; set; }
        public MainWindow()
        {
            InitializeComponent();
            ss = text1.Text;
            sss = text2.Text;
        }
    }
}

   public partial class Window1 : Window
    {
        public string ss;
        public Window1()
        {
          
            InitializeComponent();
            combo1();
            Combobox2.IsEnabled = false;
            ss = Combobox2.Text;
        }
     }

 public partial class Window2 : Window
    {
       // public string dd;
        public Window2()
        {
            Window1 w1 = new Window1();
            InitializeComponent();
            
             Lable1.Content = w1.ss;
            
        }
     
        
    }

   public void Button_Click(object sender, RoutedEventArgs e)
        {

            NavigationService.Navigate(new Uri("/Window2.xaml?UserString=" + Combobox1.Text, UriKind.Relative));
        }

   public  void Button_Click(object sender, RoutedEventArgs e)
        {
            NavigationService nav = NavigationService.GetNavigationService(this);
            nav.Navigate(new Uri("/Window2.xaml?UserString=" + Combobox1.Text, UriKind.Relative));
        }