I have a 3rd party dll written in VC6.0 that I want to use within a C# application. When I call one of the exported functions I get a can't find entry point exception.

The dll is called diodrv.dll and there are 4 exported functions. Dumpbin and Depends both report this:

File Type: DLL

Section contains the following exports for diodrv.dll

ordinal hint RVA name

1 0 00001110 ?CloseDio@@YGXXZ
2 1 00001040 ?CreateDio@@YGKXZ
3 2 00001080 ?ReadDioReg@@YGKK@Z
4 3 000010D0 ?WriteDioReg@@YGXKK@Z

I have the source code and VC6 project for the dll. The function definitions are like this:
#define DIODRV_API __declspec(dllexport)

DIODRV_API DWORD __stdcall CreateDio(void);
My C# for importing the functions is this:

static extern Int32 CreateDio();
What's the best way to get access to these functions?