Using an unmanaged dll in c#.net

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C++

Hi everyone, I am just new in using unmanaged dll and now I'm stuck with a dll created in c++ program that I need to reference in c#.

The dll has a LaserLib.h file, inside it is the following code

// The following ifdef block is the standard way of creating macros which make exporting 
// from a DLL simpler. All files within this DLL are compiled with the LASERLIB_EXPORTS
// symbol defined on the command line. this symbol should not be defined on any project
// that uses this DLL. This way any other project whose source files include this file see 
// LASERLIB_API functions as being imported from a DLL, wheras this DLL sees symbols
// defined with this macro as being exported.
#ifdef LASERLIB_EXPORTS
#define LASERLIB_API __declspec(dllexport)
#else
#define LASERLIB_API __declspec(dllimport)
#endif

extern "C"
{
LASERLIB_API BOOL Laser_ScanInit(LPCWSTR lpDevName);
LASERLIB_API BOOL Laser_StartScan();
LASERLIB_API BOOL Laser_StopScan();
LASERLIB_API unsigned char Laser_ReadScanCode(char code[], DWORD timeout);
LASERLIB_API void Laser_ScanDeinit();
}
code

I'm trying to call the LaserLib.dll in c# project

using System.Runtime.InteropServices;
namespace SmartDeviceProject1
{
    public partial class Form1 : Form
    {
        private void Form1_Load(object sender, EventArgs e)
        {
            textBox1.Text = test.Laser_ScanInit("COM2:").ToString();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            test.Laser_StartScan();
            //textBox1.Text = test.Laser_ReadScanCode("256", Convert.ToUInt32(0));
        }

    }

    public static class test
    {
        [DllImport("LaserLib.dll")]

        public static extern bool Laser_ScanInit(string com2);
        public static extern bool Laser_StartScan();
        //public static extern string Laser_ReadScanCode(string code, uint timeOut);
    }
}

There is no error in loading the SmartDeviceProject1 program, however when I click the button1, an exception will prompt "InvalidProgramException". I hope someone can point me if I'm in the right track. Thanks in advance.