System.IO.FileStream.Dispose C# (CSharp) Method

        protected override void Dispose(bool disposing)
        {
            // Nothing will be done differently based on whether we are 
            // disposing vs. finalizing.  This is taking advantage of the
            // weak ordering between normal finalizable objects & critical
            // finalizable objects, which I included in the SafeHandle 
            // design for Win32FileStream, which would often "just work" when 
            // finalized.
            try
            {
                if (_fileHandle != null && !_fileHandle.IsClosed)
                {
                    // Flush data to disk iff we were writing.  After 
                    // thinking about this, we also don't need to flush
                    // our read position, regardless of whether the handle
                    // was exposed to the user.  They probably would NOT 
                    // want us to do this.
                    if (_writePos > 0)
                    {
                        FlushWriteBuffer(!disposing);
                    }
                }
            }
            finally
            {
                if (_fileHandle != null && !_fileHandle.IsClosed)
                {
                    if (_fileHandle.ThreadPoolBinding != null)
                        _fileHandle.ThreadPoolBinding.Dispose();

                    _fileHandle.Dispose();
                }

                if (_preallocatedOverlapped != null)
                    _preallocatedOverlapped.Dispose();

                _canSeek = false;

                // Don't set the buffer to null, to avoid a NullReferenceException
                // when users have a race condition in their code (i.e. they call
                // Close when calling another method on Stream like Read).
                //_buffer = null;
                base.Dispose(disposing);
            }
        }

 /// 获取token,如果存在且没过期，则直接取token
 /// <summary>
 /// 获取token,如果存在且没过期，则直接取token
 /// </summary>
 /// <returns></returns>
 public static string GetExistAccessToken()
 {
     // 读取XML文件中的数据
     string filepath = System.Web.HttpContext.Current.Server.MapPath("/XMLToken.xml");
     FileStream fs = new FileStream(filepath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
     StreamReader str = new StreamReader(fs, System.Text.Encoding.UTF8);
     XmlDocument xml = new XmlDocument();
     xml.Load(str);
     str.Close();
     str.Dispose();
     fs.Close();
     fs.Dispose();
     string Token = xml.SelectSingleNode("xml").SelectSingleNode("AccessToken").InnerText;
     DateTime AccessTokenExpires = Convert.ToDateTime(xml.SelectSingleNode("xml").SelectSingleNode("AccessExpires").InnerText);
     //如果token过期，则重新获取token
     if (DateTime.Now >= AccessTokenExpires)
     {
         AccessToken mode = Getaccess();
         //将token存到xml文件中，全局缓存
         xml.SelectSingleNode("xml").SelectSingleNode("AccessToken").InnerText = mode.access_token;
         DateTime _AccessTokenExpires = DateTime.Now.AddSeconds(mode.expires_in);
         xml.SelectSingleNode("xml").SelectSingleNode("AccessExpires").InnerText = _AccessTokenExpires.ToString();
         xml.Save(filepath);
         Token = mode.access_token;
     }
     return Token;
 }