C# 请求后WebClient中的标题消失

我对WebClient类中的Headers集合有一个非常奇怪的问题

以下是我的例子：

WebClient client = new WebClient();
        client.Headers.Add("User-Agent", "Mozilla/5.0 (Windows NT 6.2; WOW64; rv:23.0) Gecko/20100101 Firefox/23.0");
        client.Headers.Add("Accept", "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8");
        client.Headers.Add("Accept-Language", "pl,en-us;q=0.7,en;q=0.3");

        Console.WriteLine("Before request:");
        foreach (string key in client.Headers)
        {
            Console.WriteLine(key + ": " + client.Headers[key]);
        }

        client.DownloadString("http://www.google.com");

        Console.WriteLine();
        Console.WriteLine("After request:");

        foreach (string key in client.Headers)
        {
            Console.WriteLine(key + ": " + client.Headers[key]);
        }

        Console.ReadLine();

我运行此简单程序的结果：

Before request:
User-Agent: Mozilla/5.0 (Windows NT 6.2; WOW64; rv:23.0) Gecko/20100101 Firefox/23.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: pl,en-us;q=0.7,en;q=0.3

After request:
Accept-Language: pl,en-us;q=0.7,en;q=0.3

---

为什么我的标题会消失？

因为发送了标题，webclient完成了它的工作。如果您想为下一个请求添加相同的头，您应该再次添加它们