C# 调用Dll 传递字符串指针参数 .

最近用c#写一个小软件，要调用VC写的dll,传递参数给dll,由于是非托管代码，所以调用起来有点麻烦，要把参数转换成非托管代码的指针参数，然后再传递给dll

using System.Runtime.InteropServices;   //操作Dll的类空间

 class CGicomIndex
    {
        [DllImport("Index_dll.dll")]
        unsafe private static extern Int32 create_index_file(IntPtr filename, IntPtr fieldname, UInt32 tongshu, IntPtr err);
        //VC原型　create_index_file( char*filename, char * fieldname, unsigned long tongshu, char *err);

        #region " bool CreateIndexFile( string m_strFileName, string m_strFieldName, UInt32 m_key, ref string strErr ) 对DBF文件按照指定的字段创建索引"
        /// <summary>
        /// 对DBF文件按照指定的字段创建索引
        /// </summary>
        /// <param name="m_strFileName">DBF文件路径</param>
        /// <param name="m_strFieldName">创建索引的字段名称</param>
        /// <param name="m_key">索引桶数(文件记录的最大数)</param>
        /// <param name="strErr">出错信息</param>
        /// <returns>成功 true</returns>
        public static bool CreateIndexFile( string m_strFileName, string m_strFieldName, UInt32 m_key, ref string strErr )
        {
            try
            {
                IntPtr ptrFileName, ptrFieldName, ptrErr;

                //根据数据的长度申请非托管空间
                ptrFileName = mallocIntptr(m_strFileName);
                ptrFieldName = mallocIntptr(m_strFieldName);
                ptrErr = mallocIntptr(300);

                //创建索引
                if (create_index_file(ptrFileName, ptrFieldName, m_key, ptrErr) != 1)
                {
                    strErr = Marshal.PtrToStringAnsi(ptrErr);
                    Marshal.FreeHGlobal(ptrFileName);
                    Marshal.FreeHGlobal(ptrFieldName);
                    Marshal.FreeHGlobal(ptrErr);
                    return false;
                }
                Marshal.FreeHGlobal(ptrFileName);
                Marshal.FreeHGlobal(ptrFieldName);
                Marshal.FreeHGlobal(ptrErr);
                return true;
            }
            catch (System.Exception ex)
            {
                strErr = ex.Message;
                return false;
            }
        }
        #endregion

        #region " IntPtr mallocIntptr( string strData ) 根据数据的长度申请非托管空间"
        /// <summary>
        /// 根据数据的长度申请非托管空间
        /// </summary>
        /// <param name="strData">要申请非托管空间的数据</param>
        /// <returns>指向非拖管空间的指针</returns>
        private static IntPtr mallocIntptr( string strData )
        {
            //先将字符串转化成字节方式
            Byte[] btData = System.Text.Encoding.Default.GetBytes(strData);

            //申请非拖管空间
            IntPtr m_ptr = Marshal.AllocHGlobal(btData.Length);

            //给非拖管空间清０
            Byte[] btZero = new Byte[btData .Length+ 1]; //一定要加1,否则后面是乱码，原因未找到
            Marshal.Copy(btZero, 0, m_ptr, btZero.Length);

            //给指针指向的空间赋值
            Marshal.Copy(btData, 0, m_ptr, btData.Length);

            return m_ptr;
        }

        /// <summary>
        /// 根据长度申请非托管空间
        /// </summary>
        /// <param name="strData">要申请非托管空间的大小</param>
        /// <returns>指向非拖管空间的指针</returns>
        private static IntPtr mallocIntptr( int length )
        {
            //申请非拖管空间
            IntPtr m_ptr = Marshal.AllocHGlobal(length);

            //给非拖管空间清０
            Byte[] btZero = new Byte[length + 1]; //一定要加1,否则后面是乱码，原因未找到
            Marshal.Copy(btZero, 0, m_ptr, btZero.Length);

            //给指针指向的空间赋值
            Marshal.Copy(btZero, 0, m_ptr, length);

            return m_ptr;
        }
        #endregion
    }