title: WPF Validation Not Firing on First LostFocus of the TextBox
body: <p>I am trying to validate the WPF form against an object. The validation fires when I type something in the textbox lose focus come back to the textbox and then erase whatever I have written. But if I just load the WPF application and tab off the textbox without writing and erasing anything from the textbox, then it is not fired. </p>
  
  <p>Here is the Customer.cs class: </p>
  
  <pre><code>public class Customer : IDataErrorInfo
      {
          public string FirstName { get; set; }
          public string LastName { get; set; }
  
          public string Error
          {
              get { throw new NotImplementedException(); }
          }
          public string this[string columnName]
          {
              get
              {
                  string result = null;
  
                  if (columnName.Equals("FirstName"))
                  {
                      if (String.IsNullOrEmpty(FirstName))
                      {
                          result = "FirstName cannot be null or empty"; 
                      }
                  }
                  else if (columnName.Equals("LastName"))
                  {
                      if (String.IsNullOrEmpty(LastName))
                      {
                          result = "LastName cannot be null or empty"; 
                      }
                  }
                  return result;
              }
          }
      }
  </code></pre>
  
  <p>And here is the WPF code: </p>
  
  <pre><code>&lt;TextBlock Grid.Row="1" Margin="10" Grid.Column="0"&gt;LastName&lt;/TextBlock&gt;
  &lt;TextBox Style="{StaticResource textBoxStyle}" Name="txtLastName" Margin="10"
           VerticalAlignment="Top" Grid.Row="1" Grid.Column="1"&gt;
      &lt;Binding Source="{StaticResource CustomerKey}" Path="LastName"
               ValidatesOnExceptions="True" ValidatesOnDataErrors="True"
               UpdateSourceTrigger="LostFocus"/&gt;         
  &lt;/TextBox&gt;
  </code></pre>