How should I re-raise a Delphi exception after logging it?

Question

Do you know a way to trap, log, and re-raise exception in Delphi code? A simple example:

procedure TForm3.Button1Click(Sender: TObject);
begin
  try
    rais         


        

Answer 1

You could acquire the exception object before calling your handler and keep the handler itself one liner. However, you still have a lot of "Try/Except/Do/End" burden.

Procedure MyExceptionHandler(AException: Exception);
Begin
  Log(AException); // assuming it accepts an exception
  ShowMessage(AException.Message);
  raise AException; // the ref count will be leveled if you always raise it
End;

Procedure TForm3.Button1Click(Sender: TObject);
Begin
  Try
    Foo;
  Except On E:Exception Do
    MyExceptionHandler(Exception(AcquireExceptionObject));
  End;
End;

However, if what you only want to do is to get rid of repetitive error handling code in event handlers, you could try this:

Procedure TForm3.ShowException(AProc : TProc);
Begin
  Try
    AProc;
  Except On E:Exception Do Begin
    Log(E);
    ShowMessage(E.Message);
  End; End;
End;

Reducing your event handler code to this:

Procedure TForm3.Button1Click(Sender: TObject);
Begin
  ShowException(Procedure Begin // anon method
    Foo; // if this call raises an exception, it will be handled by ShowException's handler
  End);
End;

You can also make it work for functions, using parametrized functions:

Function TForm3.ShowException(AFunc : TFunc) : T;
Begin
  Try
    Result := AFunc;
  Except On E:Exception Do Begin
    Log(E);
    ShowMessage(E.Message);
  End; End;
End;

And making ShowException return a value (acting as a passthru):

Procedure TForm3.Button1Click(Sender: TObject);
Var
  V : Integer;
Begin
  V := ShowException(Function : Integer Begin // anon method
    Result := Foo; // if this call raises an exception, it will be handled by ShowException's handler
  End);
End;

Or even making the anon procedure touch directly the outer scope variable(s):

Procedure TForm3.Button1Click(Sender: TObject);
Var
  V : Integer;
Begin
  ShowException(Procedure Begin // anon method
    V := Foo; // if this call raises an exception, it will be handled by ShowException's handler
  End);
End;

There are some limitations on the interaction of variables from inside the body of the anonymous function and the ones defined in the outer scope, but for simple cases like these, you will be more than fine.