c# 判断远程文件是否存在

// 1:
public   static   bool  IsExist( string  uri)
        {
            HttpWebRequest req  =   null ;
            HttpWebResponse res  =   null ;
             try
            {
                req  =  (HttpWebRequest)WebRequest.Create(uri);
                req.Method  =   " HEAD " ;
                req.Timeout  =   100 ;
                res  =  (HttpWebResponse)req.GetResponse();
                 return  (res.StatusCode  ==  HttpStatusCode.OK);
            }
             catch
            {
                 return   false ;
            }
             finally
            {
                 if  (res  !=   null )
                {
                    res.Close();
                    res  =   null ;
                }
                 if  (req  !=   null ) 
                {
                    req.Abort();
                    req  =   null ;
                }
            }
        } 

// 2:

private   bool  UrlExistsUsingXmlHttp( string  url)
{
   // 注意：此方法需要引用Msxml2.dll
  MSXML2.XMLHTTP _xmlhttp  =   new  MSXML2.XMLHTTPClass();
  _xmlhttp.open( " HEAD " , url,  false ,  null ,  null );
  _xmlhttp.send( "" );
   return  (_xmlhttp.status  ==   200 );
}

// 3:
private   bool  UrlExistsUsingSockets( string  url)
{
   if  (url.StartsWith( " http:// " )) url  =  url.Remove( 0 ,  " http:// " .Length);
   try
  {
    System.Net.IPHostEntry ipHost  = System.Net.Dns.GetHostEntry(url); //  System.Net.Dns.Resolve(url);
     return   true ;
  }
   catch  (System.Net.Sockets.SocketException se)
  {
    System.Diagnostics.Trace.Write(se.Message);
     return   false ;
  }
}