title: CRM ConditionExpression to compare two attributes
body: <p>I'm using Microsoft CRM SDK to retrieve entities. I'm writing <code>QueryExpression</code> with <code>FilterExpression</code>. </p>
  
  <p>How to write a condition, that takes two attributes in consideration? I want to retrieve entities, for which attribute "Export Date" is less than "Modified On". (ie. all that have been modified since last export).</p>
  
  <pre><code>QueryExpression query = new QueryExpression();
  FilterExpression filter = new FilterExpression();
  filter.FilterOperator = LogicalOperator.Or;
  
  ConditionExpression condition = new ConditionExpression();
  // ...
  //how to write this condition? 
  
  filter.AddCondition(condition);
  </code></pre>