title: Using WPF Validation rules and the disabling of a 'Save' button
body: <p>I have a page where a few textboxes cannot be empty before clicking a Save button.  </p>
  
  <pre><code>&lt;TextBox...
  
                  &lt;TextBox.Text&gt;
                      &lt;Binding Path ="LastName" UpdateSourceTrigger="PropertyChanged"&gt;
  
                          &lt;Binding.ValidationRules&gt;
                              &lt;local:StringRequiredValidationRule /&gt;
                          &lt;/Binding.ValidationRules&gt;                              
                      &lt;/Binding&gt;
                  &lt;/TextBox.Text&gt;
  </code></pre>
  
  <p>My rule works, I have a red border around my textbox until I enter a value.  I now want to add this validation rule to my other text boxes.</p>
  
  <p>How do I disable the Save button until the page has no validation errors?  I'm not sure what to check.</p>