using (WebClient client = new WebClient())
{
    WebRequest.DefaultWebProxy.Credentials = CredentialCache.DefaultCredentials;

    try
    {
        string webPageStr = client.DownloadString(URL);
        Console.WriteLine("OK");
    }
    catch (Exception ex)
    {
        Console.WriteLine("FAIL");
        Console.WriteLine(ex.Message);
    }
}

但是,此代码可以在.NET 4.0中顺利运行,因为此行足以传递代理身份验证,而不适用于.NET 3.5.

WebRequest.DefaultWebProxy.Credentials = CredentialCache.DefaultCredentials;

因此,我尝试了许多其他方法来解决这个问题,但没有一个工作：

1)用.替换CredentialCache.DefaultCredentials行

WebRequest.DefaultWebProxy.Credentials = new NetworkCredential(user,password,domain);

2)创建新的代理对象

IWebProxy proxy = new WebProxy(proxyUrl,port);
proxy.Credentials = new NetworkCredential(user,pass,domain);
client.Proxy = proxy;
client.Credentials = new NetworkCredential(user,domain);

3)添加此行

client.UseDefaultCredentials = true;

4)使用HttpWebRequest而不是WebClient并重复上面的每个过程.这是示例代码.

HttpWebRequest webRequest = WebRequest.Create(URL) as HttpWebRequest;
webRequest.Proxy = WebRequest.DefaultWebProxy;
webRequest.Credentials = new NetworkCredential(user,domain);
webRequest.Proxy.Credentials = new NetworkCredential(user,domain);

try
{
    webRequest.GetResponse();
    Console.WriteLine("OK");
}
catch (Exception ex)
{
    Console.WriteLine("FAIL");
    Console.WriteLine(ex.Message);
}

因为我必须使用.NET 3.5,所以我觉得自己走到了尽头.我不知道这两个.NET版本之间必须有区别.非常感谢你提前.