Simply MapInfo Professional cannot call a managed C# DLL, so you create a managed C++ DLL which then calls your managed C# DLL. At this stage I have only got this partially working. But this is what I have got so far.

// This is the main DLL file.
#include "MI_Bridge.h"
#include 
#include "stdafx.h"

extern "C" {
#include 
}

#using 
#using "MI_CS_Sample.DLL"

using namespace System;
using namespace MI_CS_Sample;

extern "C"
{
	__declspec(dllexport) void __cdecl bridge_csForm(void)
	{
		// Local Variables
		MI_CS_Sample::csMapInfo __gc *DotNetObject;

		// Create new for Object
		DotNetObject = new MI_CS_Sample::csMapInfo;
		DotNetObject;
	}
}

// MI_Bridge.h
// MapInfo C# .NET DLL Bridge

#pragma once