.NET 網路下載三大核心類別：WebRequest、WebResponse、WebClient 實戰解析

.NET Framework 提供三層網路存取抽象：WebRequest/WebResponse（協議不可知的請求/回應基礎架構）與 WebClient（高階便利封裝）。理解其關係與適用場景，能寫出更健壯的下載/上傳程式。

類別關係速覽

核心方法詳解

1. WebRequest.Create(string uri) — 工廠方法

依 URI Scheme 自動回傳對應具體子類：https:// → HttpWebRequest，file:// → FileWebRequest。可透過 WebRequest.RegisterPrefix 擴充自訂協議。

2. WebRequest.GetResponse() — 同步發送請求

回傳 WebResponse，需強制轉型存取協議特定屬性（如 HttpWebResponse.StatusCode）。

3. WebClient.DownloadFile(string url, string path) — 一鍵下載

內部自動建立請求、開啟串流、寫入檔案，支援 DownloadFileAsync 非同步版本與 DownloadProgressChanged 進度事件。

實戰一：HTTP 網頁抓取並另存

using System;
using System.IO;
using System.Net;
using System.Text;
using System.Windows.Forms;

public class HttpDownloader
{
    public string FetchHtml(string url)
    {
        var request = (HttpWebRequest)WebRequest.Create(url);
        request.Credentials = CredentialCache.DefaultCredentials;
        request.UserAgent = "Mozilla/5.0 (compatible; .NET Downloader)";

        using (var response = (HttpWebResponse)request.GetResponse())
        using (var stream = response.GetResponseStream())
        using (var reader = new StreamReader(stream, Encoding.UTF8))
        {
            return reader.ReadToEnd();
        }
    }

    public void SaveHtml(string url, string filePath)
    {
        using (var client = new WebClient())
        {
            client.Encoding = Encoding.UTF8;
            client.DownloadFile(url, filePath); // 同步阻塞
            // 非同步版：client.DownloadFileAsync(new Uri(url), filePath);
        }
    }
}

// UI 使用範例
private void btnFetch_Click(object sender, EventArgs e)
{
    if (string.IsNullOrWhiteSpace(txtUrl.Text)) return;
    try { rtbHtml.Text = new HttpDownloader().FetchHtml(txtUrl.Text.Trim()); }
    catch (Exception ex) { MessageBox.Show("錯誤：" + ex.Message); }
}

private void btnSave_Click(object sender, EventArgs e)
{
    if (saveFileDialog.ShowDialog() == DialogResult.OK)
    {
        new HttpDownloader().SaveHtml(txtUrl.Text.Trim(), saveFileDialog.FileName);
        MessageBox.Show("儲存完成");
    }
}

實戰二：FILE 協議本機寫入（需目標機器權限）

public void WriteToFileShare(string fileUri, string content, int timeoutMs = 5000)
{
    var uri = new Uri(fileUri); // 格式：file://server/share/path.txt
    var request = (FileWebRequest)WebRequest.CreateDefault(uri);
    request.Method = "POST"; // 或 "PUT"
    request.Timeout = timeoutMs;

    using (var stream = request.GetRequestStream())
    using (var writer = new StreamWriter(stream, Encoding.UTF8))
    {
        writer.Write(content);
    }
    // 注意：FileWebRequest 不提供 GetResponse，寫入成功即無例外
}

進階議題與最佳實務

HttpWebRequest 進階控制

• 自動重導向：AllowAutoRedirect = true（預設）
• Cookie 容器：CookieContainer = new CookieContainer() 維持 Session
• 壓縮支援：AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate
• Keep-Alive：KeepAlive = true 重用連線
• 逾時設定：ReadWriteTimeout 與 Timeout 分開設定

WebClient 限制與替代

遷移建議：拥抱 HttpClient

.NET Core / .NET 5+ 專案**強烈建議直接使用 HttpClient**；唯有維護 .NET Framework 4.x 老專案且無法引入 System.Net.Http 時，才繼續使用 WebClient/HttpWebRequest。

// 現代標準寫法
private static readonly HttpClient http = new HttpClient();
public async Task DownloadStringAsync(string url) => await http.GetStringAsync(url);
public async Task DownloadFileAsync(string url, string path)
{
    using var stream = await http.GetStreamAsync(url);
    using var fs = File.Create(path);
    await stream.CopyToAsync(fs);
}