delphi - FURTHER CLARIFICATION: How to correctly write Try..Finally..Except statements?

Question

RE: How to correctly write Try..Finally..Except statements?

I'm still confused by the OP's original question. Specifically, the last line of the procedure (outside of the try..finally..end) that reads "Screen.Cursor:=crDefault".

My understanding is that any exceptions raised inside a try..except|finally..end block WILL execute the code after the "end" of the "try".

procedure TForm1.Button1Click(Sender: TObject);
var
  Obj: TSomeObject;
begin
  Screen.Cursor := crHourGlass;

  Obj := TSomeObject.Create;
  try
    // do something
  finally
    Obj.Free;
  end;
  Screen.Cursor := crDefault;
end;

In the above example, I don't see any reason why "Screen.Cursor:=crDefault" would not be executed. Please correct me if I'm wrong.

As a further example, I've compiled this little bit of code to help illustrate. When the code is ran, THREE (3) ShowMessage() dialogs will be presented. The first "Exception Raised" and the second "finally" and the third "at end".

procedure TForm1.Button1Click(Sender: TObject);
begin
   try
      try
         showMessage(format('%s', [12]));
      except
         showMessage('Exception raised');
      end;
   finally
      showMessage('finally');
   end;
   showMessage('at end');
end;

So, I'm confused on why his "Screen.Cursor:=crDefault" isn't being ran, in it's original form and code. Can someone please elaborate?

Answer 1

The code you posted seems to work fine, because you're able to handle all possibilities. Try changing it somewhat though, so that there's an exception raised your code doesn't handle:

procedure TForm1.Button1Click(Sender: TObject);
begin
  try
    try
      raise Exception.Create('42');
    except
      on E: EDivByZero do
        ShowMessage('DivByZero');
    end;
  finally
    ShowMessage('Finally');
  end;
  ShowMessage('Got here');
end;

Run this, and you'll see Finally, then the exception for 42, but no Got here message. This is because the exception took you out of the current block, the stack is unwound, and the code from the end of the finally to the end of the procedure never executes.

Move the final ShowMessage call from where it is to inside the finally and run again.

procedure TForm1.Button1Click(Sender: TObject);
begin
  try
    try
      raise Exception.Create('42');
    except
      on E: EDivByZero do
        ShowMessage('DivByZero');
    end;
  finally
    ShowMessage('Finally');
    ShowMessage('Got here');
  end;
  ShowMessage('Will never get here');
end;

You'll now see both calls to ShowMessage in the finally block, one after the other, but not the one after the finally block's end;. Code inside the finally block is guaranteed to execute, while code beyond it may or may not.

To make it even more clear, the presence of the try..except block can be removed:

procedure TForm1.Button1Click(Sender: TObject);
begin
  try
    raise Exception.Create('42');
  finally
    ShowMessage('Finally');
    ShowMessage('Got here');
  end;
  ShowMessage('Will never get here');
end;

The entire purpose of the try..finally block is to ensure that code inside the finally section will execute before the procedure ends.