title: Use Linq and Lambda to include default
body: <p>I have two lists: <code>relatedContact</code> and <code>fileContactIds</code>. 
  I am using the query to loop the relatedContacts list and return value of userClientCode (which I will make the image name) only if it is also in the <code>fileContactIds</code> list. How can I put in a value (eg: "Default") if the value isn't in the second list ?   </p>
  
  <pre><code>var result = relatedContact.SelectMany(rc =&gt; rc.contacts.Select(pc =&gt; new RelatedContactsDescription
                  {
                      imageUrl = mappedRelatedContactsPath + pc.userclientcode + ".jpg",
                      userclientcode = pc.userclientcode,
                      description = rc.clienttaxonomy,
                      fullname = pc.fullname,
                      email = pc.contactdetails != null &amp;&amp; pc.contactdetails.Count &gt;= 1 ? pc.contactdetails[0].contactdata : "",
                      address = pc.contactdetails != null &amp;&amp; pc.contactdetails.Count &gt;= 2 ? pc.contactdetails[1].contactdata : "",
                      phoneNumber = pc.contactdetails != null &amp;&amp; pc.contactdetails.Count &gt;= 3 ? pc.contactdetails[2].contactdata : "",
                      populated = string.IsNullOrEmpty(pc.userclientcode) ||
                                  string.IsNullOrEmpty(pc.fullname) ||
                                  string.IsNullOrEmpty(pc.contactdetails[0].contactdata) ? false : true,
                  }))
                  .Where(el =&gt; fileContactIds.Contains(el.userclientcode)).ToList();
  </code></pre>
  
  <p>I have read: <a href="https://stackoverflow.com/questions/11344019/linq-and-lambda-expression">Lambdas and Linq</a> and <a href="https://stackoverflow.com/questions/2767709/c-sharp-joins-where-with-linq-and-lambda">Joins</a> <a href="https://stackoverflow.com/questions/4923103/how-to-do-this-in-lambda-expression-or-linq">First or Default</a> etc but all I can see is how to get matching data from both not also how to put a "fall-back" or default value if they don't match.</p>
  
  <p>Thanks in advance</p>