Archive for July 6th, 2005
Managed Assemblies, IJW, and .h Files
I solved an intersting problem yesterday, although I still cannot fully explain the solution. My project uses a mixed mode DLL to load some native binaries. I constructed the DLL as a managed assembly so that in my managed code, I can use the following:
#using "mydll.dll"
This statement–It Just Works, or IJW–imports all types. Now I had defined my exception hierarchy in a separate header common to the whole project (not just the mixed dll). The exe and dll thus had the same #include statement. I could both compile and link with this setup. When the DLL threw an exception, though, I encountered a problem. Consider this snippet:
//Exception.h
__gc public MyException : public Exception
{
MyException(String *s) : Exception(s) {}
};
//mydll.dll, built with /LD /clr
#include "Exception.h"
myFunc() {
throw new MyException(S"BLAH");
}
//main.cpp, built with /clr
#include "Exception.h"
try
{
myFunc();
}
catch(MyException *e)
{
//This handler does not catch MyException!
}
catch(Exception *e)
{
Type *t = e->GetType();
Console::WriteLine(t->ToString());
}
The second exception handler illuminated the most elusive aspect of this issue. Although the first handler could not catch the exception, the second catch block showed a MyException type. I have not found a sufficient explanation for this behavior, but it must stem from the interaction of native code and MSIL in the mixed DLL.
