using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace MWS.Mrac.test {
  public class Customer_B {
    private string _name;
    public Customer_B( string p1 ) {
      _name = p1;
      }
    public string CustName {
      get {
        return _name;
        }
      set {
        _name = value;
        if( String.IsNullOrEmpty( value ) ) {
          throw new ApplicationException( "Customer name is mandatory." );
          }
        }
      }
    }
  }

<Window x:Class="MWS.Mrac.Win_Test_ValidationException"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Win_Test_ValidationException" Height="300" Width="300">
  <Grid>

    <TextBox  Name="text_ex3"
              Height="36" HorizontalAlignment="Left" Margin="23,114,0,0" VerticalAlignment="Top" Width="189" >
      <TextBox.Text>
        <Binding Path="CustName"  UpdateSourceTrigger="PropertyChanged" >
          <Binding.ValidationRules>
            <ExceptionValidationRule />
          </Binding.ValidationRules>
        </Binding>
      </TextBox.Text>
    </TextBox>
    
    <Button Content="Button" Height="23" HorizontalAlignment="Left" Margin="38,205,0,0" Name="button1" VerticalAlignment="Top" Width="75" />

      </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
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;

using MWS.Mrac.test;

namespace MWS.Mrac {
  public partial class Win_Test_ValidationException: Window {
    public Win_Test_ValidationException( ) {
      InitializeComponent( );
      Customer_B c2 = new Customer_B( "bbBBBBBBBBBBBBB" );
      this.DataContext = c2;
      }
    }