这并不能保证功能正常，但至少可以检查到URL连接：

var url = "http://url.to.che.ck/serviceEndpoint.svc";

try
{
    var myRequest = (HttpWebRequest)WebRequest.Create(url);

    var response = (HttpWebResponse)myRequest.GetResponse();

    if (response.StatusCode == HttpStatusCode.OK)
    {
        //  it's at least in some way responsive
        //  but may be internally broken
        //  as you could find out if you called one of the methods for real
        Debug.Write(string.Format("{0} Available", url));
    }
    else
    {
        //  well, at least it returned...
        Debug.Write(string.Format("{0} Returned, but with status: {1}", 
            url, response.StatusDescription));
    }
}
catch (Exception ex)
{
    //  not available at all, for some reason
    Debug.Write(string.Format("{0} unavailable: {1}", url, ex.Message));
}

这种方法对我很有效。
我用Socket来检查进程是否可以连接。如果你尝试检查连接1-3次，HttpWebRequest就可以工作，但是如果你有一个24小时运行的进程，并且不时需要检查Web服务器的可用性，那么它就不再工作了，因为它会抛出TimeOut异常。

Socket socket 
   = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);

var result = socket.BeginConnect("xxx.com", 80, null, null);

// test the connection for 3 seconds
bool success = result.AsyncWaitHandle.WaitOne(3000,false); 

var resturnVal = socket.Connected;
if (socket.Connected)
    socket.Disconnect(true);
                
socket.Dispose();
                
return resturnVal;