How to: Hold Object Reference in Unmanaged Memory

You can use gcroot.h, which wraps GCHandle, to hold a CLR object reference in unmanaged memory. Alternatively, you can use GCHandle directly.

Examples

// hold_object_reference.cpp
// compile with: /clr
#include "gcroot.h"
using namespace System;

#pragma managed
class StringWrapper {

private:
   gcroot<String ^ > x;

public:
   StringWrapper() {
      String ^ str = gcnew String("ManagedString");
      x = str;
   }

   void PrintString() {
      String ^ targetStr = x;
      Console::WriteLine("StringWrapper::x == {0}", targetStr);
   }
};
#pragma unmanaged
int main() {
   StringWrapper s;
   s.PrintString();
}

StringWrapper::x == ManagedString

GCHandle gives you a means to hold a managed object reference in unmanaged memory. You use the Alloc method to create an opaque handle to a managed object and Free to release it. Also, the Target method allows you to obtain the object reference back from the handle in managed code.

// hold_object_reference_2.cpp
// compile with: /clr
using namespace System;
using namespace System::Runtime::InteropServices;

#pragma managed
class StringWrapper {
   IntPtr m_handle;
public:
   StringWrapper() {
      String ^ str = gcnew String("ManagedString");
      m_handle = static_cast<IntPtr>(GCHandle::Alloc(str));
   }
   ~StringWrapper() {
      static_cast<GCHandle>(m_handle).Free();
   }

   void PrintString() {
      String ^ targetStr = safe_cast< String ^ >(static_cast<GCHandle>(m_handle).Target);
      Console::WriteLine("StringWrapper::m_handle == {0}", targetStr);
   }
};

#pragma unmanaged
int main() {
   StringWrapper s;
   s.PrintString();
}

StringWrapper::m_handle == ManagedString

See also

Using C++ Interop (Implicit PInvoke)