What do parentheses in binding paths mean?
The () parentheses refer to Attached Properties.
Quoting the MSDN library (I'm quoting MSDN here because I couldn't have written it down better):
This syntax is generally used for one of the following cases:
- The path is specified in XAML that is in a style or template that does not have a specified
TargetType. A qualified usage is generally not valid for cases other than this, because in non-style, non-template cases, the property exists on an instance, not a type.- The property is an attached property.
- You are binding to a static property.
For use as storyboard target, the property specified as
propertyNamemust be aDependencyProperty.
(Validation.Errors) references the attached property Errors in the Validation class. Since the binding has a RelativeSource = Self, it's gonna look for the value of that attached property with respect to the TextBox itself.