public void SetDefinitionQuery( 
   string defQuery
)

{
  //Note: Call within QueuedTask.Run()
  //Remove Groups
  // The fields in the datasource used for the report
  var listFields = new List<string> {
       "STATE_NAME"
      };
  report.RemoveGroups(listFields);

  //Add Group
  report.AddGroup("STATE_NAME", true, true, "");

  //Modify the Definition Query
  var defQuery = "STATE_NAME LIKE 'C%'";
  report.SetDefinitionQuery(defQuery);
}