title: LINQ List gives null exception in where Clause
body: <pre><code>var q = from p in query
  where 
     ((criterias.birthday == p.BirthDay|| criterias.birthday == null))                                
  &amp;&amp; ((criterias.marriageDate == null || criterias.marriageDate == p.MarriageDate))                                
  &amp;&amp; ((criterias.gender == p.Gender) || (criterias.gender == null))
  &amp;&amp; ((criterias.nationalities.Contains(p.Nationality)) || (criterias.nationalities == null))
  </code></pre>
  
  <p>criterias isa class where i store my search criterias. nationalities is a string list. the problem occurs when i have no items in string. the query throws null reference exception. the query doesnt accept null value in nationalities. how can i fix this? </p>