C# Error handling issue

Hi,

I have write Assembly using C# stored procedure (web services), it's working fine.
Only issue with Error Handling.
some errors i can catch (if Order Exisits) but most of Errors i can't catch. but when i was testing in the
unit test i can see it's working preoprly.But when after comipled and create assembly
most of Errors Can't track.
Any idea appriciated.

//property
        public static string ReturnMsg { get; set; }

//Error Catching part
catch (Exception ex)
            {
                //pipe.Send(ex.ToString());
                //pipe.Send(strValue + "\n" + strcount);
                //Get stack trace for the exception with source file information
                var st = new StackTrace(ex, true);
                // Get the top stack frame
                var frame = st.GetFrame(0);
                // Get the line number from the stack frame
                var line = frame.GetFileLineNumber();

                strcount = "Line Number " + line.ToString() + "-->" + ex.ToString();//ex.Message.ToString() +
                //pipe.Send(strValue + "\n" + strcount);
                ReturnMsg = ex.Message.ToString();
               //throw new Exception("Error: " + ex.Message);
            }

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Note: i have check many examples. stackflows.But all are not working. I knew how to search and google find answer.
i would appriciate if any one show me or direct me any idea what's im doing wrong