使用LINQ选择是否存在XML元素(Select if exist XML Element using LINQ)

 我有以下xml。 由于addr在这种情况下是空的，但大部分时间它都有像街道，城市，州等元素。 我如何确保它存在。 现在的原因LINQ查询引发错误“对象引用未设置为对象的实例”。 
 
  <author>
    <time value="20120620153233"/>
    <assignedAuthor>
      <id/>
      <addr/>
      <telecom value="tel:+1-781-271-3000"/>
      <assignedPerson>
        <name>Auto Generated</name>
      </assignedPerson>
    </assignedAuthor>
  </author>



 tbAuthor autor =
                    (from c in cdafile.Root.Elements(ns + "author")
                    select new tbAuthor
                    {                           
                        streetAddressLine = (string)c.Element(ns + "assignedAuthor").Element(ns + "addr").Element(ns + "streetAddressLine").Value,
                        city = (string)c.Element(ns + "assignedAuthor").Element(ns + "addr").Element(ns + "city").Value,
                        state = (string)c.Element(ns + "assignedAuthor").Element(ns + "addr").Element(ns + "state").Value,
                        postalCode = (string)c.Element(ns + "assignedAuthor").Element(ns + "addr").Element(ns + "postalCode").Value,
                        country = "US"
                    }).FirstOrDefault();

I have following xml . As addr is empty in this case but most time its has elements like street,city,state . How do I make sure it exist . Cause for now LINQ query throws an error "Object reference not set to an instance of an object."
 
  <author>
    <time value="20120620153233"/>
    <assignedAuthor>
      <id/>
      <addr/>
      <telecom value="tel:+1-781-271-3000"/>
      <assignedPerson>
        <name>Auto Generated</name>
      </assignedPerson>
    </assignedAuthor>
  </author>



 tbAuthor autor =
                    (from c in cdafile.Root.Elements(ns + "author")
                    select new tbAuthor
                    {                           
                        streetAddressLine = (string)c.Element(ns + "assignedAuthor").Element(ns + "addr").Element(ns + "streetAddressLine").Value,
                        city = (string)c.Element(ns + "assignedAuthor").Element(ns + "addr").Element(ns + "city").Value,
                        state = (string)c.Element(ns + "assignedAuthor").Element(ns + "addr").Element(ns + "state").Value,
                        postalCode = (string)c.Element(ns + "assignedAuthor").Element(ns + "addr").Element(ns + "postalCode").Value,
                        country = "US"
                    }).FirstOrDefault();

原文：https://stackoverflow.com/questions/17503614