如何使用C#(.NET)中的POST变量登录HTML表单？

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

public async Task<string> LoginHtmlFormAsync(string url, string username, string password)
{
    using (var httpClient = new HttpClient())
    {
        var formContent = new FormUrlEncodedContent(new[]
        {
            new KeyValuePair<string, string>("username", username),
            new KeyValuePair<string, string>("password", password)
        });

        httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/x-www-form-urlencoded"));

        var response = await httpClient.PostAsync(url, formContent);

        if (response.IsSuccessStatusCode)
        {
            return await response.Content.ReadAsStringAsync();
        }
        else
        {
            throw new Exception($"Error {response.StatusCode}: {await response.Content.ReadAsStringAsync()}");
        }
    }
}

public async Task Main()
{
    try
    {
        string url = "https://example.com/login";
        string username = "your_username";
        string password = "your_password";

        string response = await LoginHtmlFormAsync(url, username, password);

        Console.WriteLine("Response: " + response);
    }
    catch (Exception ex)
    {
        Console.WriteLine("Error: " + ex.Message);
    }
}