Regarding the issue of calling Dll to pass string pointer parameters

using ;   //A class space for operating Dll  
 class CGicomIndex  
    {  
        [DllImport("Index_dll.dll")]  
        unsafe private static extern Int32 create_index_file(IntPtr filename, IntPtr fieldname, UInt32 tongshu, IntPtr err);  
        //VC prototype 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 ) Create indexes for DBF files according to the specified fields"        /// <summary>   
        /// Create indexes for DBF files according to specified fields        /// </summary>   
        /// <param name="m_strFileName">DBF file path</param>        /// <param name="m_strFieldName">Create indexed field name</param>        /// <param name="m_key">Number of index buckets (maximum number of file records)</param>        /// <param name="strErr">Error message</param>        /// <returns>Successful true</returns>        public static bool CreateIndexFile( string m_strFileName, string m_strFieldName, UInt32 m_key, ref string strErr )  
        {  
            try  
            {  
                IntPtr ptrFileName, ptrFieldName, ptrErr;  
  
                //Application for unmanaged space based on the length of the data                ptrFileName = mallocIntptr(m_strFileName);  
                ptrFieldName = mallocIntptr(m_strFieldName);  
                ptrErr = mallocIntptr(300);  
  
                //Create an index                if (create_index_file(ptrFileName, ptrFieldName, m_key, ptrErr) != 1)  
                {  
                    strErr = (ptrErr);  
                    (ptrFileName);  
                    (ptrFieldName);  
                    (ptrErr);  
                    return false;  
                }  
                (ptrFileName);  
                (ptrFieldName);  
                (ptrErr);  
                return true;  
            }  
            catch ( ex)  
            {  
                strErr = ;  
                return false;  
            }  
        }  
        #endregion  
 
        #region " IntPtr mallocIntptr( string strData ) applies for unmanaged space based on the length of the data"        /// &lt;summary&gt;   
        /// Apply for unmanaged space based on the length of the data        /// &lt;/summary&gt;   
        /// <param name="strData">Data to apply for unmanaged space</param>        /// <returns> Pointer to non-drag space</returns>        private static IntPtr mallocIntptr( string strData )  
        {  
            //Convert the string into bytes first            Byte[] btData = (strData);  
  
            //Apply for non-drag space            IntPtr m_ptr = ();  
  
            //Cleave unmanaged space 0            Byte[] btZero = new Byte[btData .Length+ 1]; //Be sure to add 1, otherwise there will be garbled code afterwards, and the reason is not found            (btZero, 0, m_ptr, );  
  
            // Assign value to the space pointed to by the pointer            (btData, 0, m_ptr, );  
  
            return m_ptr;  
        }  
  
        /// &lt;summary&gt;   
        ///Application for unmanaged space based on length        /// &lt;/summary&gt;   
        /// <param name="strData">The size of the unmanaged space to be applied for</param>        /// <returns> Pointer to non-drag space</returns>        private static IntPtr mallocIntptr( int length )  
        {  
            //Apply for non-drag space            IntPtr m_ptr = (length);  
  
            //Cleave unmanaged space 0            Byte[] btZero = new Byte[length + 1]; //Be sure to add 1, otherwise there will be garbled code afterwards, and the reason is not found            (btZero, 0, m_ptr, );  
  
            // Assign value to the space pointed to by the pointer            (btZero, 0, m_ptr, length);  
  
            return m_ptr;  
        }  
        #endregion   
    }