title: Custom field type derived from SPFieldCalculated
body: <p>I am trying to create a custom field type which will display values based on another field in the same list. (basically, display some images based on the value of the source column)</p>
  
  <p>Since this column derives it's data entirely on the basis of another column, i need to make sure the column values are not stored in the database. </p>
  
  <p>So, i think i need to create a custom field type which derives from SPFieldCalculated class.</p>
  
  <p>But, when i try to do so,  I get an error message 
  "The type 'Microsoft.SharePoint.SPFieldCalculated' has no constructors defined"</p>
  
  <p>How do i create a custom field tpye deriving from SPFieldCalculated? 
  If this is not possible, can somebody at least tell me how do i go about satisfying my requirement?</p>