More calling DLL questions -- this time types

I have a dll that has a constructor that takes a C++ type string.
example definition in header file
        Foo(const string &filepath);

        [DllImport("FooDLL.dll", CallingConvention = CallingConvention.Cdecl)]
        public static extern IntPtr Foo(ref System.String filepath);

How do I call this in C#?  below gets me an "EntryPointNotFoundException", which indicates to me that the calling signature doesnt match....

        public IntPtr ip;
        string str = "junk";

        ip = SignumIU(ref str);

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You should strictly refrain from using C++-style exports in your DLL, for the simple reason being that C++ symbols are 'mangled' in order for the linker being able to process different overloads with the same name (i.e. 'Foo(int)'). See

So, one way to work around this would be to create a C-stly layer that wraps the C++ code and can be called using the usual P/Invoke mechanisms, e.g.

// header.h
extern "C" // ensure so name mangling will take place
unsigned long CreateFoo(const char* p);
extern "C" // ensure so name mangling will take place
unsigned long CallFooMethod(const unsigned long pul, int n);

// code.cpp
unsigned long CreateFoo(const char* p) {

  Foo* pf = new Foo(p);

  return (unsigned long) pf;

unsigned long CallSomeFooMethod(const unsigned long pul, int n) {

  Foo* pf = (Foo*) pul;

  return pf->SomeFooMethod(n);

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