C++/CLI Mixed Mode DLL Creation

Well, no, it doesn't get to be mix-mode until you tell the C++/CLI compiler that your legacy DLL was written in unmanaged code. Which should have been noticeable, you should have gotten linker errors from the unmanaged DLL exports. You need to use #pragma managed:

#pragma managed(push, off)
#include "oldskool.h"
#pragma comment(lib, "oldskool.lib")
#pragma managed(pop)

using namespace System;

public ref class Wrapper {
private:
    COldSkool* pUnmanaged;
public:
    Wrapper() { pUnmanaged = new COldSkool; }
    ~Wrapper() { delete pUnmanaged; pUnmanaged = 0; }
    !Wrapper() { delete pUnmanaged; }
    void sampleMethod() { 
        if (!pUnmanaged) throw gcnew ObjectDisposedException("Wrapper");
        pUnmanaged->sampleMethod(); 
    }
};

A good option to prevent /clr from affecting your existing code is to compile all the existing code into a native static library and then include that static library at the link step of your C++/CLI dll.

Begin a new C++/CLI project and then move your native classes to it.

Instead of Turning on the "Common Language RunTime Support" at the Project Level, it's possible to enable it on a file-by-file basis only by looking at the Properties of the File, and going to C/C++ | General | Common Language Support.

This might make it easier to have your native and C++/CLI code in the same project, rather than creating a separate C++/CLI DLL just containing the wrapper, or having to use lots of managed/unmanaged pragmas.

So just do that on the C++/CLI .NET wrapper class you want to write.