关于C# 调用Dll 传递字符串指针参数的问题

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   
    }