I seem to have a problem with TwoWay DataBinding - my application has a window with a bunch of textboxes that allow to edit values of the properties they are bound to. Everything works well except for textboxes that also have a validation rule defined, in which case no text is displayed in the textbox when the window opens (binding back-to-source still works fine for those). If I remove Validation rule, everything's back to normal. I searched for an answer to this for a few hours now, but somehow did not even find anyone else complaining of the same issue. I am completely new to WPF, and I am sure it is just a silly mistake I have somewhere in my code... I will greatly appreciate any feedback... <TextBox Margin="40,2,20,0" Grid.Column="0" Grid.Row="1" Background="#99FFFFFF" > <Binding Path="LastName" Mode="TwoWay" ValidatesOnDataErrors="true" UpdateSourceTrigger="LostFocus" > <Binding.ValidationRules> <validation:StringNameValidationRule /> </Binding.ValidationRules> </Binding> </TextBox>