FIX: __declspec(dllimport) Classes Are Not Caught Correctly

The information in this article applies to:

• The Microsoft C/C++ Compiler (CL.EXE), included with Microsoft Visual C++, 32-bit Edition, version 2.0 on the following platforms: x86 and MIPS

SYMPTOMS

C++ Exception Handling (EH) can be used to throw and catch objects of a specified type. If a class declared with the __declspec(dllimport) keyword is thrown, it will not be recognized by the catch handler for that type.

If a handler for unspecified types, catch (...), is present, the __declspec(dllimport) object will be caught in that handler. Otherwise, it will not be handled, and an exception will terminate the application or cause a general protection (GP) fault.

RESOLUTION

To work around this problem, derive a class from the __declspec(dllimport) class, and throw or catch an object of the derived class.

If you are throwing the derived object in one .DLL or .EXE, and catching it in another, you must do the same derivation in each module.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem was fixed in Microsoft Visual C++, 32-bit Edition, version 4.0.

MORE INFORMATION

The following sample application demonstrates the problem with catching __declspec(dllimport) classes and the workaround of using a derived class.

Here's the output:

   In the EXE.
   In the DLL.
   In the EXE again.
   Throwing ExceptionI...
   DLL: ExceptionI was not caught as correct type.
   Throwing ExceptionII...
   EXE: Class ExceptionII caught: code = 0.

Sample Code

/* dllsamp.cpp : Defines the implementation routines for the DLL

*/

#include <iostream.h>

// ExceptionI is declared with _declspec(dllimport), and will not be
// caught correctly

   int code;
 public:
   ExceptionI();
   ExceptionI(int i);
   ~ExceptionI() {};
   int GetCode(void);

ExceptionI::ExceptionI (void) { code = 0; }

ExceptionI::ExceptionI (int i) { code = i; }

int ExceptionI::GetCode(void)

// ExceptionII is derived from ExceptionI, and will be caught correctly

__declspec(dllexport) void dllfunc(void (*exefunc)(void)) {

 try
   {
   cout << "In the DLL." <<endl;
   exefunc();
   }
 catch (ExceptionI x)
   {
   cout << "DLL: Class ExceptionI caught: code = " << x.GetCode() <<

   cout << "Throwing ExceptionII..." << endl;
   throw ExceptionII();
   }
 catch (...)
   {
   cout << "DLL: ExceptionI was not caught as correct type." << endl;
   cout << "Throwing ExceptionII..." << endl;
   throw ExceptionII();
   }

/* exesamp.cpp : Defines the implementation routines for the EXE

*/

#include <iostream.h>

class __declspec(dllimport) ExceptionI {

   int code;
 public:
   ExceptionI();
   ExceptionI(int i);
   ~ExceptionI() {};
   int GetCode(void);

// ExceptionII needs to be defined in both modules to be caught correctly

void exefunc(void);

void main(void)

 try
   {
   cout << "In the EXE." << endl;
   dllfunc(exefunc);
   }
 catch (ExceptionII x)
   {
   cout << "EXE: Class ExceptionII caught: code = " << x.GetCode() <<

   }
 catch (...)
   {
   cout << "EXE: ExceptionII was not caught as correct type." << endl;
   }

void exefunc(void)

 try
   {
    cout << "In the EXE again." << endl;
    cout << "Throwing ExceptionI..." << endl;
    throw ExceptionI(-1);
   }
 catch (ExceptionI x)
   {
   cout << "EXE: Class ExceptionI caught: code = " << x.GetCode() <<

   cout << "Throwing ExceptionI..." << endl;
   throw ExceptionI(-1);
   }