wpf - 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