WPF Validation Not Firing on First LostFocus of the TextBox

Question

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.

Here is the Customer.cs class:

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;
            }
        }
    }

And here is the WPF code:

<TextBlock Grid.Row="1" Margin="10" Grid.Column="0">LastName</TextBlock>
<TextBox Style="{StaticResource textBoxStyle}" Name="txtLastName" Margin="10"
         VerticalAlignment="Top" Grid.Row="1" Grid.Column="1">
    <Binding Source="{StaticResource CustomerKey}" Path="LastName"
             ValidatesOnExceptions="True" ValidatesOnDataErrors="True"
             UpdateSourceTrigger="LostFocus"/>         
</TextBox>

Answer 1

If you're not adverse to putting a bit of logic in your code behind, you can handle the actual LostFocus event with something like this:

.xaml

<TextBox LostFocus="TextBox_LostFocus" ....

---

.xaml.cs

private void TextBox_LostFocus(object sender, RoutedEventArgs e)
{
     ((Control)sender).GetBindingExpression(TextBox.TextProperty).UpdateSource();
}

Answer 2

Unfortunately this is by design. WPF validation only fires if the value in the control has changed.

Unbelievable, but true. So far, WPF validation is the big proverbial pain - it's terrible.

One of the things you can do, however, is get the binding expression from the control's property and manually invoke the validations. It sucks, but it works.