Why i can't capture exceptions from Dlls?

A very common topic in newsgroups is Dll Exception handling. If your dll raises an exception, the exception handler in the host app should not be able to recognize the class of the exception it trapped. That's because the host application doesn't recognizes the exception class raised by the dll.

The recommended aproach is to capture any exception within the Dll and create functions that return values of success or failure, then the host application can evaluate such results and raise an exception.

This is an example produces different results when compiled with Delphi or FreePascal, you shouldn't do this:

library failedlib;

uses
 SysUtils;


function divide(a, b: integer): Boolean; cdecl;
begin
 if b = 0 then
   raise Exception.Create('Can''t divide by zero');
 Result := True;

end;

exports
 divide;

end.

program sample;

{$APPTYPE CONSOLE}

type
 TDivide = function (a, b: integer): Boolean; cdecl;


var
 FuncDivide: TDivide;
 lHandle:    Cardinal;

begin
lHandle := LoadLibrary('failedlib.dll');
if lHanlde <> 0 then

begin
  FuncDivide = GetProcAddress(lHandle, 'divide');
  if pointer(@FuncDivide) <> nil then
  begin

    try
      if FuncDivide(10, 0) then
        writeln('Success!');
    except

      writeln('Failure!');
    end;
  end;
  UnloadLibrary(lHandle);
end;
end.

The correct method is this:

library correctlib;

uses
 SysUtils;


function divide(a, b: integer): Boolean; cdecl;
var
 c: Integer;
begin
 (* Catch any exception before return *)
 try

   c := a div b;
   Result := True;
 except
   Result := False;
 end;
end;

exports
 divide;


end.

program sample;

{$APPTYPE CONSOLE}

type
 TDivide = function (a, b: integer): Boolean; cdecl;


var
 FuncDivide: TDivide;
 lHandle:    Cardinal;

begin
lHandle := LoadLibrary('correctlib.dll');
if lHanlde <> 0 then

begin
  FuncDivide = GetProcAddress(lHandle, 'divide');
  if pointer(@FuncDivide) <> nil then
  begin

    if FuncDivide(10, 0) then
      writeln('Success!')
    else
      (* The exception is raised in the host app, not in the DLL *)

      raise 'Failure!';
  end;
  UnloadLibrary(lHandle);
end;
end.