title: DataColumn.Expression Property. Convert current column's bool value to string
body: <p>I'm trying to add expression,which changes boolean value of current column to string value, for DataColumn:</p>
  
  <pre><code>col = new DataColumn("isDirectionIn", typeof(string),
                                       "IIF(isDirectionIn = true, 'in', 'out')");
  </code></pre>
  
  <p>But I always get an ArgumentException: <em>Cannot set Expression property due to circular reference in the expression.</em></p>
  
  <p>What should I do to avoid this exception? Thank in advance.</p>