WPF ValidationRules does not fire on load

Question

I have a ValidationRules on a textbox;

<TextBox Margin="5,5,5,0" Name="myTextBox" >
<Binding Path="myID" NotifyOnValidationError="True" ValidatesOnDataErrors="True" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged"  >
    <Binding.ValidationRules>
        <local:ValueCannotBlankValidator ValidatesOnTargetUpdated="True"  />
    </Binding.ValidationRules>
</Binding>

Now this works if the user changes the value in the textbox. The problems that it doesn't fire on load. Figured it would be a simple fix of changing UpdateSourceTrigger="PropertyChanged" to UpdateSourceTrigger="LostFocus" but that causes the ValidationRules not to fire. Thanks for the help.

Answer 1

If you set UpdateSourceTrigger="LostFocus" , the validation happens when input focus is set to another control, on the other hand, UpdateSourceTrigger="PropertyChanged" fires every time the text is changed, acts much like a TextBox's TextChanged event.

ValidatesOnTargetUpdated="True" ensures that the text is validated on load, your XAML code is correct. If you set a breakpoint in ValueCannotBlankValidator.Validate method you would probably find it is actually fired on load.

I doubt your validator returns a valid result at the first validation, at that moment the Text property of the TextBox is null , if you compare null against string.Empty ("") , you get an incorrect result.

public override ValidationResult Validate(object value, CultureInfo cultureInfo)
{
    ValidationResult trueResult = new ValidationResult(true, "not blank");
    string str = value as string; //value is null on load, don't compare it against ""
    if (string.IsNullOrEmpty(str))
        return new ValidationResult(false, "blank");
    else
        return trueResult;
}