title: C# Dictionary ContainsValue look up by object attribute
body: <p>I have  <code>Dictionary&lt;long, Location&gt; PostalCodes</code> .</p>
  
  <p>While i m adding new elements to this Dictionary I want to make a lookup to this dictionary and if the <code>Location.PostalCode</code> is not in this dictionary, I want to insert it. Otherwise I want to skip it. So , need to know if the PostalCode is already in the Collection. Cant use it as the key.</p>
  
  <p>How to do this?</p>
  
  <p>Thanks.</p>