Hi

1. Actually if we need to implement the interface in managed code, we can
just import the interface into .net and then implement it as a managed
interface once it has been imported, because the managed code will run
based on .NET runtime.
e.g.
[
    object,
    uuid("59E6670E-8626-468e-88D4-2E4A86DB25EC"),
    dual,    helpstring("IMyInterface Interface"),
    pointer_default(unique)
]
__interface IMyInterface: IDispatch
{

    [id(1), helpstring("method TestString")] HRESULT TestString(LONG x,
[out,retval] LONG* y);
};

add a reference to the com tlb file and we can get the interop assembly
with the managed IMyInterface definition.

using namespace System;
using namespace System::Windows::Forms;
using namespace System::Runtime::InteropServices;
using namespace  Interop::ATLInterface;
namespace MCPP
{
//[InterfaceTypeAttribute(ComInterfaceType::InterfaceIsIUnknown)]
//public __gc __interface IMyInterface
//{
//    void HelloWorld();
//};
[ClassInterface(ClassInterfaceType::None)]
public __gc class MyClass : public IMyInterface
{
//public:
//    void HelloWorld()
//    {
//        Form* fm = new Form();
//        fm->Show();
//    }
public:
    int TestString(int x)
    {
        return x*x*x;
    }
};
}

2. I think this is C++ 's syntax, we can make a simple test.
class Shape  //which is interface in the C++ category
{
public:
  virtual void draw() = 0;
};

class Rectangle: public Shape
{
//public:
//   void draw();
};

The code below will not generate any compiler error both in VC6 and VC7.
C++ syntax allow the fact that the derived class did not implement virtual
method, we consider it as another vitual class. Because the MC++ will use
the cl.exe as the unmanaged C++, so they has the similar behavior.

namespace MCPP
{
    //[InterfaceTypeAttribute(ComInterfaceType::InterfaceIsIUnknown)]
    //public __gc __interface IMyInterface
    //{
    //    void HelloWorld();
    //};
    [ClassInterface(ClassInterfaceType::None)]
    public __gc class MyClass : public IMyInterface
    {
    //public:
    //    void HelloWorld()
    //    {
    //        Form* fm = new Form();
    //        fm->Show();
    //    }
    //public:
    //    int TestString(int x)
    //    {
    //        return x*x*x;
    //    }
    };
}
But if we plan to use the MyClass which has did not implement the
TestString, we will get compiler error.
    MCPP::MyClass* pCls = new MCPP::MyClass(); //the code line will generate
the compiler error as below.
error C2259: 'MCPP::MyClass' : cannot instantiate abstract class

Best regards,

Peter Huang
Microsoft Online Partner Support