C++ SiUtil.dll in C#

[carey_amanda]

Im doing a school project. have to do dllimport as the SiUtil.dll is written in C++.
But im not sure the coding below is correct as there is error. Please advise needed.

http://www.silabs.com/Support%20Documents/TechnicalDocs/an117.pdf

Thanks a million.

using System;
using System.Collections.Generic;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace ProgrammingInterface
{

static class Program
{
[DllImport("SiUtil.dll")]
public static extern int 
ConnectUSB( char sSerialNum, 
int nECprotocol, 
int nPowerTarget, 
int nDisableDialogboxes);

static void Main()
{
ConnectUSB( const char * sSerialNum="", int nECprotocol=0, int nPowerTarget=0, int nDisableDialogboxes=0);
}
}
}