C#

int lastUsedRow = worksheet.Cells.SpecialCells(XlCellType.xlCellTypeLastCell, Type.Missing).Row;

MyOtherForm myOtherForm = new MyOtherForm();
myOtherForm.Show(); // I think this is the right method to call to make it show up

#define WIN32_LEAN_AND_MEAN
#include <windows.h>
BOOL WINAPI DllMain(
    HINSTANCE hinstDLL,  // handle to DLL module
    DWORD fdwReason,     // reason for calling function
    LPVOID lpReserved )  // reserved
{
    // Perform actions based on the reason for calling.
    switch( fdwReason ) 
    { 
        case DLL_PROCESS_ATTACH:
         // Initialize once for each new process.
         // Return FALSE to fail DLL load.
            break;

        case DLL_THREAD_ATTACH:
         // Do thread-specific initialization.
            break;

        case DLL_THREAD_DETACH:
         // Do thread-specific cleanup.
            break;

        case DLL_PROCESS_DETACH:
         // Perform any necessary cleanup.
            break;
    }
    return TRUE;  // Successful DLL_PROCESS_ATTACH.
}

extern "C" __declspec(dllexport)
double Add(double a, double b)
{
  return a + b;
}

extern "C" __declspec(dllexport)
double Subtract(double a, double b)
{
  return a-b;
}

extern "C" __declspec(dllexport)
double Multiply(double a, double b)
{
  return a*b;
}

extern "C" __declspec(dllexport)
double Divide(double a, double b)
{
  if ( b != 0 )
    return a/b;
  return -999999999999999999999.0; //don't crash
}

using System;
using System.Runtime.InteropServices;
namespace HelloWorld
{
    class Hello 
    {
        [DllImport("SimpleDll.dll", EntryPoint="Add", CallingConvention=CallingConvention.Cdecl)]
        public static extern Double Add(Double a,Double b);

        [DllImport("SimpleDll.dll", EntryPoint="Subtract", CallingConvention=CallingConvention.Cdecl)]
        public static extern Double Subtract(Double a,Double b);

        [DllImport("SimpleDll.dll", EntryPoint="Multiply", CallingConvention=CallingConvention.Cdecl)]
        public static extern Double Multiply(Double a,Double b);

        [DllImport("SimpleDll.dll", EntryPoint="Divide", CallingConvention=CallingConvention.Cdecl)]
        public static extern Double Divide(Double a,Double b);

        static void Main() 
        {
            System.Console.WriteLine("Add:       {0}", Add(3,4));
            System.Console.WriteLine("Subtract:  {0}", Subtract(7,4));
            System.Console.WriteLine("Multiply:  {0}", Multiply(3,4));
            System.Console.WriteLine("Divide:    {0}", Divide(12,4));
        }
    }
}