C++ .NET DLL base class problem ???

Posted on 2004-09-06
Last Modified: 2008-01-09
Hi guys ...

I'm using visual C++ .net 2003 and am having linker problems when building the following managed class library (DLL) project :

using namespace System;
namespace Common
   public __gc class Base

   public __gc class Derived : public Base

It compiles successfully, but I get the following linker error :
TestFile error LNK2001: unresolved external symbol "void __cdecl __CxxCallUnwindDtor(void (__thiscall*)(void *),void *)" (?__CxxCallUnwindDtor@@$$J0YAXP6EXPAX@Z0@Z)

The problem can be solved by commenting out the base destructor. However this is not a real solution ... as the base destructor will have some necessity in my project.

Anybody have any ideas why this happens ... and any possible workarounds would be much appreciated ?

Other postings of the same question include :

Question by:steve_hsk
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LVL 19

Accepted Solution

drichards earned 250 total points
ID: 11992763
Here's a solution someone posted:

>>I have also encountered this problem. In order to compile
>> succesfully you have to link msvcrt.lib with ur class lib.

I did not have any issues compiling and linking a class library with your code.  I checked and I already was linking with msvcrtd.lib (it was a debug build) for other reasons.  I removed msvcrtd.lib from my settings and got your problem along with the others which originally caused me to link with msvcrt.
LVL 19

Expert Comment

ID: 11992795
Just in case you need instructions:  go to the project properties Linker Input page and add msvcrtd.lib in debug configuration "Additional Dependencies".  Add msvcrt.lib to release configuration settings.  Make sure this matches the C\C++ Code Generation "Runtime Library" setting.   msvcrt(d) goes with "Multi-threaded Debug Dll (/MDd)".  If you choose other settings, I believe you will have to change the lib.  Here's a link to the docs for the runtime library compiler switch:

Author Comment

ID: 11995875
Thanks for this solution ... it works as stated :-)

If you could fill me in on 2 things I'd greatly appreciate it :-
(1) I'm curious as to why msvcrt.lib wasn't included as part of my project settings by default, and what this library is used for (MS VC++ Run time or MS v? Common Run Time ? What do MS say about this lib ?
(2) Which search engine and website did you find the solution ?

Many thanks,
LVL 19

Expert Comment

ID: 11996960
(1) I can't say for sure since I haven't found it documented anywhere, but I would guess it's because the runtime is not needed with a pure managed project and so it is not included by default.  Only when you use runtime functions (printf, etc.) do you need the runtime.    It appears that adding a destructor introduces unmanaged code and you need the runtime to handle the destructor code.  You do not need the derived class to have the problem with the destructor.  I tried removing "Derived" from the project and still it had linker errors without msvcrt.

(2) I used Google and searched for 'CxxCallUnwindDtor'.  Here's the ultimate link with the solution - Google found an index page for the message group that had this link.

Author Comment

ID: 11997498
Good enough :-)

Many thanks !

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