How to Check whether node exist or not

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Below is my code . I am getting response of xml in responseElement and corredponding values.I want to know how can i check whether firstName/contactid node exist or not in following expression\[code\]var responseElement = XElement.Parse(putContactResponse);ObjInvoice.Contact_Id = responseElement.XPathSelectElement("//Contact/ContactID").Value;ObjInvoice.FirstName = responseElement.XPathSelectElement("//Contact/FirstName").Value;\[/code\]