title: Disable a button if either of the two TextBoxes are empty
body: <p>I have two text boxes and one button. If either of the boxes is empty or white space, the button should be disabled.</p>
  
  <pre><code>&lt;TextBox Name="firstNameTxtBox" /&gt;
  &lt;TextBox Name="lastNameTxtBox" /&gt;
  
  &lt;Button Content="Save" Command="{Binding Save}"  Style="{StaticResource saveButtonEnabler}" /&gt;
  </code></pre>
  
  <p>And the <code>saveButtonEnabler</code> resource:</p>
  
  <pre><code>&lt;UserControl.Resources&gt;
      &lt;converters:IsButtonEnabledConverter x:Key="isButtonEnabledConverter" /&gt;
          &lt;Style x:Key="saveButtonEnabler" TargetType="Button"&gt;
              &lt;Setter Property="IsEnabled" Value="False"/&gt;
              &lt;Style.Triggers&gt;
                  &lt;MultiDataTrigger&gt;
                      &lt;MultiDataTrigger.Conditions&gt;
                          &lt;Condition Binding="{Binding ElementName=firstNameTxtBox, Path=Text.Length, Converter={StaticResource isButtonEnabledConverter}}" Value="???" /&gt;
                          &lt;Condition Binding="{Binding ElementName=lastNameTxtBox, Path=Text.Length, Converter={StaticResource isButtonEnabledConverter}}" Value="???" /&gt;
                      &lt;/MultiDataTrigger.Conditions&gt;
                      &lt;Setter Property="IsEnabled" Value="True"/&gt;
                  &lt;/MultiDataTrigger&gt;
              &lt;/Style.Triggers&gt;
          &lt;/Style&gt;
  &lt;/UserControl.Resources&gt;
  </code></pre>
  
  <p>Converter gets called appropriately, but I have not idea how to set the result to the <code>Value</code> of the <code>Condition</code>.</p>
  
  <pre><code>public class IsButtonEnabledConverter : IValueConverter
  {
      public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
      {
          return (int)value == 0 ? false : true;
      }
  
      public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
      {
          throw new NotImplementedException();
      }
  }
  </code></pre>