Hi Michael,
I haven't included the command object set up but I have given it an OP param
& tested it by not using a statement complete event & it returns a value fine
after ExecuteNonQuery. It's only when I try to retrieve it in the event
handler via the event object (as below) that I get the error.

Thanks very much for helping out here

Ant

try
           {
               connection.Open();

               // execute the SP
               commandBackup.ExecuteNonQuery();

           }
           catch (Exception ex)
           { // exception handling here }

           finally
           {  connection.Close();  }

   void commandBackup_StatementCompleted(object sender,
StatementCompletedEventArgs e)
   {
       // retrieving the command object fromn the sender param of the event
       SqlCommand commandResult = ((SqlCommand)sender);
       
       // !! GET ERROR HERE when trying to access the param value

       // retrieve the value of the output parm
       string resultString =
commandResult.Parameters["@ResultString"].Value.ToString();

       textboxCompleted.Text = resultString;
   }