你如何登录网页并在C#中检索其内容？

Install-Package System.Net.Http

using System;
using System.Net.Http;
using System.Threading.Tasks;

namespace WebLoginExample
{
    class Program
    {
        static async Task Main(string[] args)
        {
            // 创建一个HttpClient实例
            using HttpClient client = new HttpClient();

            // 设置登录表单的数据
            var formContent = new FormUrlEncodedContent(new[]
            {
                new KeyValuePair<string, string>("username", "your_username"),
                new KeyValuePair<string, string>("password", "your_password")
            });

            // 发送登录请求
            HttpResponseMessage loginResponse = await client.PostAsync("https://example.com/login", formContent);

            // 确保登录请求成功
            if (loginResponse.IsSuccessStatusCode)
            {
                // 获取登录后的Cookie
                string cookie = loginResponse.Headers.GetValues("Set-Cookie").ToString();

                // 创建一个新的HttpClient实例，并将Cookie添加到请求头中
                using HttpClient loggedInClient = new HttpClient();
                loggedInClient.DefaultRequestHeaders.Add("Cookie", cookie);

                // 发送请求以获取登录后的网页内容
                HttpResponseMessage contentResponse = await loggedInClient.GetAsync("https://example.com/secure_page");

                // 确保内容请求成功
                if (contentResponse.IsSuccessStatusCode)
                {
                    // 读取响应内容并输出到控制台
                    string content = await contentResponse.Content.ReadAsStringAsync();
                    Console.WriteLine(content);
                }
                else
                {
                    Console.WriteLine("Error: Unable to retrieve secure page content.");
                }
            }
            else
            {
                Console.WriteLine("Error: Unable to login.");
            }
        }
    }
}