How to create a global handle in C++

Question

I am trying to use a C# class in a C++ project, but Visual Studio will only let me declare a handle (^) to a C# object in a local scope, just inside a function. The only way I got it working was declaring a global pointer to a handle:

SUTAdapter::Form1^ *ptForm1;

But if then I create an object inside a function and give its address to the global pointer:

SUTAdapter::Form1^ form1;
form1 = gcnew SUTAdapter::Form1();
ptForm1 = &form1;
(*ptForm1)->incCounter(0);

When the function exits and I try to use the incCounter function inside some other C++ function, the object seems gone (debugger says this == null). Is there a way to have a global handle of a C# code in C++? I guess global handles are forbidden for some reason I don't understand but I am out of ideas and I need this done. Thank you.

edit:

*ptForm1 = gcnew SUTAdapter::Form1();

Gives a null reference exception. Why?

Answer 1

I think - and that's without testing it - that the problem you're having is because you're storing a pointer to a local handle, which might confuse the garbage collector a little due to the fact that you're taking a pointer to an automatic object.

Have you tried to replace the above assignment with

*ptForm1 = gcnew SUTAdapter::Form1();

instead of the detour via the local object reference?

Answer 2

Will Visual Studio let you declare a static handle to a C# object in a local scope?

SUTAdapter::Form1^ theForm() {
   static SUTAdapter::Form1^ form1 = gcnew SUTAdapter::Form1();
   return form1;
}

Answer 3

Ok thanks for your answers but I finally solved it, I found it googling:

http://bytes.com/groups/net-vc/473036-how-define-global-com-object-vc-8-a

It seems that VS doesn't let global handles or static handles inside functions. I am puzzled about this because sometimes it is necessary to access managed objects globally.

The solution is to declare a "GlobalObjects" class with static handles inside it:

ref class GlobalObjects
{
public:
static SUTAdapter::Form1^ hndForm1;
};

This way I can access the C# form/class globally. The error code of the previoous error is C3145, I am still wondering why does not VS allow the declaration of global handles.

Answer 4

*ptForm1 = gcnew SUTAdapter::Form1();

Gives a null reference exception because you are dereferncing a null pointer - it is just like a pointer to any other type in that regard.

Answer 5

This restriction seems consistent with the other CLR-based languages. C# has no concept of a static, function-scoped variable for example. The lack of such functionality in C++/CLI suggests that is the way the CLR works - static objects must be defined at the class scope.

The CLR is object based, so this is just a case of that object-oriented nature influencing the design of languages that run on top of it.

From an OO view point, I like the C# way better anyway.