FileStream fstPreview = new FileStream(stPath, FileMode.Open, FileAccess.Read, FileShare.Read);
            StreamReader srdPreview = new StreamReader(ss);
            String temp = string.Empty;
            while (srdPreview.Peek() > -1)
            {
                String input = srdPreview.ReadLine();
                temp += input;

            }
            srdPreview.Close();
            return temp ;

-------------------------------string转stream-------------------------------

           Stream   s   =   new   MemoryStream(ASCIIEncoding.Default.GetBytes("Test   String"));