How to use XPath with XElement or LINQ?

To use XPath with LINQ to XML add a using declaration for System.Xml.XPath, this will bring the extension methods of System.Xml.XPath.Extensions into scope.

In your example:

var value = (string)xml.XPathEvaluate("/response/data/hash");

Others have entirely reasonably suggested how to use "native" LINQ to XML queries to do what you want.

However, in the interests of providing lots of alternatives, consider XPathSelectElement, XPathSelectElements and XPathEvaluate to evaluate XPath expressions against an XNode (they're all extension methods on XNode). You can also use CreateNavigator to create an XPathNavigator for an XNode.

Personally I'm a big fan of using the LINQ to XML API directly, as I'm a big LINQ fan, but if you're more comfortable with XPath, the above may help you.