title: Force IDataErrorInfo validation
body: <p>I have two controls on some panel: textbox and combobox:</p>
  
  <pre><code>&lt;TextBox Text="{Binding ShapeName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" /&gt;
  
  &lt;ComboBox SelectedItem="{Binding ActiveStageViewModel, Mode=OneWayToSource, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" /&gt;
  </code></pre>
  
  <p>I need to force idataerrorinfo validation for textbox, when I select something on combobox. How to do that?</p>
  
  <p>The code from viewmodel:</p>
  
  <pre><code>string IDataErrorInfo.this[string propertyName]
      {
        get
        {
          var error = string.Empty;
  
          if (propertyName == Expression.GetPropertyName(() =&gt; ActiveStageViewModel))
          {
            // TODO: Add functionality to force ShapeName property validation
            return error;
          }
  
          if (propertyName == Expression.GetPropertyName(() =&gt; ShapeName))
          {
            error = ValidateShapeName();
          }
  
          TooltipMessage = error;
          return error;
        }
      }
  </code></pre>