What do parentheses in binding paths mean?

Question

Recently i've read 'Databinding overview' article at MSDN and there is such sample code:

<TextBox.ToolTip>
  <Binding RelativeSource="{RelativeSource Self}" Path="(Validation.Errors)[0].ErrorContent"/>
</TextBox.ToolTip>

I know that {} means markup extensions but what mean () parentheses here? It would be nice someone share link to explanation such syntax. Thanks!

Path="(Validation.Errors)[0].ErrorContent"

Answer 1

The () parentheses refer to Attached Properties.

Binding to an Attached Property

Answer 2

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 propertyName must be a DependencyProperty.

Answer 3

(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.