title: Data validation issue in MetroWindow
body: <p>I'm really new to Wpf and I'm trying out various controls. I took this ViewModel from Metro Mahapps Demo and left only (assuming I didn't miss anything) the code which handles the data validation (number greater than 10) : </p>
  
  <pre><code> public class Modelis : INotifyPropertyChanged, IDataErrorInfo
  {
      int? _integerGreater10Property;
  
      public Modelis(IDialogCoordinator dialogCoordinator)
      {    
      }
      public int? IntegerGreater10Property
      {
          get { return this._integerGreater10Property; }
          set
          {
              if (Equals(value, _integerGreater10Property))
              {
                  return;
              }
  
              _integerGreater10Property = value;
              RaisePropertyChanged("IntegerGreater10Property");
          }
      }
      public event PropertyChangedEventHandler PropertyChanged;
  
      /// &lt;summary&gt;
      /// Raises the PropertyChanged event if needed.
      /// &lt;/summary&gt;
      /// &lt;param name="propertyName"&gt;The name of the property that changed.&lt;/param&gt;
      protected virtual void RaisePropertyChanged(string propertyName)
      {
          if (PropertyChanged != null)
          {
              PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
          }
      }
  
      public string this[string columnName]
      {
          get
          {
              if (columnName == "IntegerGreater10Property" &amp;&amp; this.IntegerGreater10Property &lt; 10)
              {
                  return "Number is not greater than 10";
              }
              return null;
          }
      }
  
      public string Error { get { return string.Empty; } }
  }
  </code></pre>
  
  <p>Then, I referenced it in my Window (also taken from the demo) :</p>
  
  <pre><code>         xmlns:Registracija="clr-namespace:Registracijos_sistema"
           mc:Ignorable="d"
           d:DesignHeight="600"
           d:DesignWidth="800"
           d:DataContext="{d:DesignInstance Registracija:Modelis}"
  </code></pre>
  
  <p>But when I add a textbox (again from the demo) which notifies if number is not greater than 10, it doesn't seem to work:</p>
  
  <pre><code>        &lt;TextBox   
          Controls:TextBoxHelper.Watermark="Number smaller than 10"
          Text="{Binding IntegerGreater10Property, 
          ValidatesOnDataErrors=True, 
          UpdateSourceTrigger=PropertyChanged, 
          NotifyOnValidationError=True}"  
          Margin="104,197,47,104" /&gt;
  </code></pre>
  
  <p>What am I doing wrong?</p>