这对我来说很好:

This works fine for me:

public static bool UrlIsValid(string url)
{
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
    request.Proxy = null;
    try
    {
        HttpWebResponse response = (HttpWebResponse)request.GetResponse();
        return true;
    }
    catch //If exception thrown then couldn't get response from address
    {
        return false;
    }
}
private void Page_Load(object sender, EventArgs e)
{
    Response.BufferOutput = true;

    string url = "http://www.google.com";
    if (UrlIsValid(url))
    {
        Response.Redirect(url);
    }
    else
    {
        Response.Redirect("http://www.yahoo.com");
    }
}

您可以像这样检查URL(我将更改URL检查功能的正文):

You can check the URL like this (I will change the body of your URL-check function):

public static bool UrlIsValid(string url)
{
   HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
   request.Proxy = null;
   try
   {
      HttpWebResponse response = (HttpWebResponse)request.GetResponse();
      return true;
   }
   catch //If exception thrown then couldn't get response from address
   {
      return false;
   }
}