C# 網站可用性檢測：HttpWebRequest/HttpClient 完整實作與最佳實務

現代首選：HttpClient (單例/池化)

private static readonly HttpClient _http = new HttpClient
{
    Timeout = TimeSpan.FromSeconds(10)
};

public static async Task IsSiteAliveAsync(string url)
{
    try
    {
        using var resp = await _http.GetAsync(url, HttpCompletionOption.ResponseHeadersRead);
        return resp.IsSuccessStatusCode; // 2xx
    }
    catch (HttpRequestException)
    {
        return false; // DNS失敗、連線拒絕、逾時、SSL錯誤等
    }
    catch (TaskCanceledException) when (!Task.CurrentId.HasValue) // 僅捕獲逾時
    {
        return false;
    }
}

// 使用
bool ok = await IsSiteAliveAsync("https://example.com");

舊版：HttpWebRequest (相容 .NET Framework)

public static bool CheckSite(string url)
{
    try
    {
        var req = (HttpWebRequest)WebRequest.Create(url);
        req.Method = "HEAD"; // 僅取標頭，不下載內文
        req.Timeout = 10000;
        req.AllowAutoRedirect = true;

        using (var resp = (HttpWebResponse)req.GetResponse())
        {
            return resp.StatusCode == HttpStatusCode.OK; // 200
        }
    }
    catch (WebException)
    {
        return false;
    }
}

關鍵差異與選擇

進階健康檢查策略

public class HealthCheckResult
{
    public bool IsHealthy { get; set; }
    public int StatusCode { get; set; }
    public TimeSpan Latency { get; set; }
    public string Error { get; set; }
}

public static async Task CheckDetailedAsync(string url)
{
    var sw = Stopwatch.StartNew();
    try
    {
        using var req = new HttpRequestMessage(HttpMethod.Head, url);
        using var resp = await _http.SendAsync(req, HttpCompletionOption.ResponseHeadersRead);
        sw.Stop();
        return new HealthCheckResult
        {
            IsHealthy = resp.IsSuccessStatusCode,
            StatusCode = (int)resp.StatusCode,
            Latency = sw.Elapsed
        };
    }
    catch (Exception ex)
    {
        sw.Stop();
        return new HealthCheckResult { IsHealthy = false, Latency = sw.Elapsed, Error = ex.Message };
    }
}

生產級建議

• 單例 HttpClient：避免 `SocketExhaustion`，DI 註冊 `AddHttpClient`。
• HEAD 請求：僅驗證可用性時用 `HEAD`，節省頻寬；需驗證內容完整性用 `GET`。
• 重試策略：Polly `WaitAndRetryAsync` (指數退避 + 抖動)。
• 熔斷器：Polly `CircuitBreakerAsync` 避免雪崩效應。
• 多節點檢測：從不同地域/ISP 發起檢測，排除單點誤判。
• 憑證驗證：生產環境**勿**關閉 `ServerCertificateCustomValidation`，測試環境可用 `DangerousAcceptAnyServerCertificateValidator`。

總結

網站可用性檢測核心：**`HttpClient` 單例 + `HEAD` 請求 + `IsSuccessStatusCode` 判斷 + 例外捕獲 = 布林結果**。進階加入延遲測量、重試、熔斷、多點檢測，構建生產級健康檢查系統。舊專案維護用 `HttpWebRequest`，新專案**全面採用 `HttpClient`**。