//引用System.Net
private bool RemoteFileExists(string fileUrl)
{
    try
    {
        HttpWebRequest httpWebRequest = (HttpWebRequest)WebRequest.CreateDefault(new Uri(fileUrl));
        httpWebRequest.Method = "HEAD";
        httpWebRequest.Timeout = 1000;
        return (((HttpWebResponse)httpWebRequest.GetResponse()).StatusCode == HttpStatusCode.OK);
    }
    catch
    {
        return false;
    }
}