private void Page_Load(object sender, System.EventArgs e)
{

 string url1 = "http://dotnet.aspx.cc/";
 string url2 = "http://dotnet.aspx.cc/Images/logo.gif";
 Response.Write("<li>方法1：");
 Response.Write(url1 + " 存在：" + UrlExistsUsingHttpWebRequest(url1).ToString());
 Response.Write("<li>方法2：");
 Response.Write(url1 + " 存在：" + UrlExistsUsingSockets(url1).ToString());   
 Response.Write("<li>方法3：");
 Response.Write(url1 + " 存在：" + UrlExistsUsingXmlHttp(url1).ToString());

 Response.Write("<li>方法1：");
 Response.Write(url2 + " 存在：" + UrlExistsUsingHttpWebRequest(url2).ToString());
 Response.Write("<li>方法3：");
 Response.Write(url2 + " 存在：" + UrlExistsUsingXmlHttp(url2).ToString());
}
private bool UrlExistsUsingHttpWebRequest(string url)
{
 try
 {
  System.Net.HttpWebRequest myRequest =(System.Net.HttpWebRequest)System.Net.WebRequest.Create(url);
  myRequest.Method = "HEAD";
  myRequest.Timeout = 100;
  System.Net.HttpWebResponse res = (System.Net.HttpWebResponse)myRequest.GetResponse();
  return (res.StatusCode == System.Net.HttpStatusCode.OK);
 }
 catch(System.Net.WebException we)
 {
  System.Diagnostics.Trace.Write(we.Message);
  return false;
 }
}
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 );
}

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