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;
}
}