C DLL with C# exception error (Access Violation)

I have a C DLL that I am call from C# C Code

 // The real DLL entry point
 //int __export CALLBACK Do_7plus(char *cmd_line)
__declspec(dllexport) int  Do_7plus(char *cmd_line)

C# Code

using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;     // DLL support

namespace Packet
{

public partial class _7PlusFrm : Form
{
[DllImport("7plus.dll")]
public static extern void Do_7plus(string args);
    public _7PlusFrm()
    {
        InitializeComponent();
    }

    private void button_ok_Click(object sender, EventArgs e)
    {
        Do_7plus("c:\\temp\\7plus.zip -SAVE \"c:\temp\\\" -SB 5000");
    }
}
}

I get a exception error (Access Violation) How do I fix it?