facing problem in validating wpf control

hi

my xaml code is

<Label x:Name="lblDeviceName" Content="Device Type Name" Grid.Column="0" Grid.Row="0"/><TextBox x:Name="txtDevicename"   Grid.Column="1" Grid.Row="0"><TextBox.Text><Binding Path="DeviceName" ValidatesOnDataErrors="True" UpdateSourceTrigger="PropertyChanged"><Binding.ValidationRules><ExceptionValidationRule /></Binding.ValidationRules></Binding></TextBox.Text></TextBox><Label x:Name="lblGTIN"  Content="GTIN" Grid.Column="3" Grid.Row="0"/><TextBox x:Name="txtGTIN" Grid.Column="4" Grid.Row="0"/><Label x:Name="lblBasicCode"  Content="Basic Code" Grid.Column="0" Grid.Row="1"/><TextBox x:Name="txtBasicCode" Grid.Column="1" Grid.Row="1"/>

after that i have one model class where the code is

   public string DeviceName
        {
            get { return _deviceName; }

            set
           { 
                _deviceName = value;
                    if (String.IsNullOrEmpty(_deviceName))
                    {
                        MessageBox.Show("field compulsary");
                        //throw new ApplicationException("Customer name is mandatory.");

                    }
                
      
            }
        }

and in my xaml. cs file i have bind the model class

the problem is that , i have to first enter in that textbox and when i am deleting the text it throws error msg , i want that as soon as the user clink on the second textbox it should display error that first textbox is empty.