c# 检测代理服务器是否可用的问题


private bool CheckProxy(string Url, string ProxyAddr, string ProxyUser,string ProxyPassWord,string ProxyDomain)
        {
            try
            {
                WebProxy CurrentWebProxy = new WebProxy(ProxyAddr, true);
                if (ProxyPassWord.Trim().Length > 0 && ProxyUser.Trim().Length > 0)
                                    CurrentWebProxy.Credentials = new System.Net.NetworkCredential(ProxyUser, ProxyPassWord, ProxyDomain);                   
                                else
                                    CurrentWebProxy.Credentials = System.Net.CredentialCache.DefaultCredentials;                
                                WebRequest sr = WebRequest.Create(Url);
                sr.Proxy = CurrentWebProxy;
                sr.GetResponse();
                return true;
            }
            catch(Exception ee)
            {
                LogFile("代理服务器状态检测: 代理地址:"+ProxyAddr+"  用户名:"+ProxyUser+"  密码:"+ProxyPassWord+"  域:"+ProxyDomain+"  异常信息:" + ee.Message);
                return false;
            }
        }
以上代码.   
其中代理服务,只给定地址. 写一个命令行程序测试可以. 
但,写成Windows服务程序就不行. 不知为什么? 以上代码是否存在问题,还望大牛指点.谢谢c#