How do you validate a clr property after it has been updated instead of before when implementing IDataErrorInfo?

Question

I have implemented IDataErrorInfo in one of my classes to validate a property of that class. The property is bound to a control on my wpf user control. The validataion works fine, except there is one vital flaw.

It seems to be calling the IDataErrorInfo member public string this[string columnName] before the property is updated, so when I check the value of the property being changed it contains the previous value not the one just entered. Therefore, the validation is always one step behind. This means that I can't check for incorrect values that have just been entered.

Is there any way of forcing this validation to be called after the property has been updated and not before. I have tried changing the UpdateSourceTrigger to both LostFocus and PropertyChanged but they still report the previous value, just at different times.

Thanks.

Answer 1

The behavior that you are referring to can be reproduced when property change notifications are sent before the property is assigned.

public string FirstName
{
    get { return _firstName; }
    set
    {
        if (this.PropertyChanged != null)
        {
            this.PropertyChanged
                (this, new PropertyChangedEventArgs("FirstName"));
        }
        _firstName = value;
    }
}

May be calling the change notification after you have actually set the value may do the trick. And just for the record I don't call property changed notification like this!