尝试读取或写入受保护的内存，内存或版本的内置版本没有问题

Question

我正在尝试在C＃中访问第三方.dll（非托管）。 我的方法如下所示

static class NativeMethods
{    
    [DllImport("kernel32.dll")]
    public static extern IntPtr LoadLibrary(string dllToLoad);

    [DllImport("kernel32.dll")]
    public static extern IntPtr GetProcAddress(IntPtr hModule, string procedureName);

    [DllImport("kernel32.dll")]
    public static extern bool FreeLibrary(IntPtr hModule);

}
class VTCCAN
{
       [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
       private delegate int CAN_Transmission(ref can_msg message);
        static readonly string dllfile = System.IO.Path.GetDirectoryName(System.Reflection.Assembly.GetExecutingAssembly().Location) + @"\VTC1010_CAN_Bus.dll";
        public int SendCAN(ref can_msg msg)
        {
            IntPtr pDll = NativeMethods.LoadLibrary(dllfile);
            if (pDll == IntPtr.Zero)
            {
                MessageBox.Show("Loading Failed");
            }
            IntPtr pAddressOfFunctionToCall = NativeMethods.GetProcAddress(pDll, "CAN_Transmission");
            CAN_Transmission sendCAN = (CAN_Transmission)Marshal.GetDelegateForFunctionPointer(pAddressOfFunctionToCall, typeof(CAN_Transmission));
            int result = sendCAN(ref msg);
            bool iresult = NativeMethods.FreeLibrary(pDll);
            return result;
        }
    }
}

访问“SendCAN”功能时出错 错误是     “附加信息：尝试读取或写入受保护的内存。这通常表明其他内存已损坏。”我看了一些建议，但它不适用于我的代码。这里有什么不对吗？请建议。我无法找到答案。 （64位Windows 7操作系统）

Abe提供的相关结构

[System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential, CharSet = System.Runtime.InteropServices.CharSet.Ansi)]
public struct can_msg
{
    /// unsigned short
    public short ide;

    /// unsigned int
    public int id;

    /// unsigned short
    public short dlc;

    /// unsigned char[100]
    [System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.ByValTStr, SizeConst = NativeConstants.CAN_MSG_DATA_LEN)]
    public string data;

    /// unsigned short
    public short rtr;
}