简介

HttpClient 是 .NET 中用于发送 HTTP 请求和接收 HTTP 响应的现代化 API，它取代了过时的 WebClient 和 HttpWebRequest 类。

HttpClient 是 .NET Framework 4.5 + 和 .NET Core/.NET 5+ 中提供的、基于消息处理管道（message handler pipeline）的现代 HTTP 客户端库。

相比早期的 HttpWebRequest，它更易用、支持异步、可扩展性强，并且在 .NET Core 中底层使用 SocketsHttpHandler，性能与可配置性都有显著提升。

底层架构

• HttpClient
• 封装请求创建、发送及响应处理逻辑，提供 GetAsync、PostAsync、SendAsync 等方法。
• HttpMessageHandler 管道
• 默认使用 SocketsHttpHandler（.NET Core/.NET 5+），.NET Framework 下为 HttpClientHandler。
• 可以通过继承 DelegatingHandler 串联多个自定义中间件（例如日志、重试、认证）。
• 连接池与长连接
• SocketsHttpHandler 内置连接池和 HTTP/2 支持，同一目标主机复用 TCP 连接，减少握手与重建成本。

HttpClient 核心特性

常用方法

使用单例

public static class HttpClientSingleton
{
    public static readonly HttpClient Instance = new HttpClient();
}

// 使用
await HttpClientSingleton.Instance.GetAsync(url);

• 全局唯一，避免重复创建 handler，复用连接池。
• 需注意：设置到期（Timeout）、默认头（DefaultRequestHeaders）等全局属性变更会影响所有调用。

使用HttpClientFactory工厂

services.AddHttpClient("GitHub", client =>
{
    client.BaseAddress = new Uri("https://api.github.com/");
    client.DefaultRequestHeaders.Add("User-Agent", "MyApp");
})
.AddTransientHttpErrorPolicy(policy =>
    policy.WaitAndRetryAsync(3, _ => TimeSpan.FromSeconds(2)));

// 使用
var client = _httpClientFactory.CreateClient("GitHub");

• 框架管理生命周期、稳定重用 SocketsHttpHandler。
• 支持命名客户端、类型化客户端、配置管道、添加 Polly 弹性策略。

类型化客户端

public class GitHubService {
    private readonly HttpClient _client;
    public GitHubService(HttpClient client) => _client = client;
    // 封装API方法
}
// 注册
services.AddHttpClient<GitHubService>(client => { /* 配置 */ });

GET请求

var client = HttpClientSingleton.Instance;
var response = await client.GetAsync("https://api.example.com/items/1");
response.EnsureSuccessStatusCode();
string json = await response.Content.ReadAsStringAsync();

POST请求

var content = new StringContent(
    JsonSerializer.Serialize(new { Name = "测试" }),
    Encoding.UTF8, "application/json");
var postResp = await client.PostAsync("https://api.example.com/items", content);
postResp.EnsureSuccessStatusCode();

SendAsync自定义请求

var request = new HttpRequestMessage(HttpMethod.Put, #34;items/{id}")
{
    Content = new StringContent(json, Encoding.UTF8, "application/json")
};
request.Headers.Add("X-Custom", "Value");

using var resp = await client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead);
resp.EnsureSuccessStatusCode();

流式上传/下载

// 下载
using var resp = await client.GetAsync(fileUrl, HttpCompletionOption.ResponseHeadersRead);
using var stream = await resp.Content.ReadAsStreamAsync();
// 将 stream 写入文件 ...

// 上传
using var fs = File.OpenRead(localFilePath);
var streamContent = new StreamContent(fs);
var uploadResp = await client.PostAsync(uploadUrl, streamContent);

DelegatingHandler 管道

public class LoggingHandler : DelegatingHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, CancellationToken ct)
    {
        Console.WriteLine(#34;Request: {request}");
        var resp = await base.SendAsync(request, ct);
        Console.WriteLine(#34;Response: {resp}");
        return resp;
    }
}

// 注册（HttpClientFactory）
services.AddHttpClient("WithLogging")
    .AddHttpMessageHandler<LoggingHandler>();

Polly 弹性策略

services.AddHttpClient("GitHub")
    .AddTransientHttpErrorPolicy(p =>
        p.WaitAndRetryAsync(3, retry => TimeSpan.FromSeconds(1)));

• AddTransientHttpErrorPolicy 捕获 5xx、408、HttpRequestException 等 transient 错误；
• 可组合断路器、超时、隔离策略。

认证与配置

• Bearer Token

client.DefaultRequestHeaders.Authorization =
    new AuthenticationHeaderValue("Bearer", accessToken);

• Client Certificates

var handler = new HttpClientHandler();
handler.ClientCertificates.Add(new X509Certificate2("cert.pfx", "pwd"));
var client = new HttpClient(handler);

• 代理

handler.Proxy = new WebProxy("http://127.0.0.1:8888");
handler.UseProxy = true;

设置超时和请求头

httpClient.Timeout = TimeSpan.FromSeconds(30); // 设置全局超时
httpClient.DefaultRequestHeaders.Add("User-Agent", "MyApp/1.0");
httpClient.DefaultRequestHeaders.Authorization = 
    new AuthenticationHeaderValue("Bearer", "your-token");

处理不同类型的响应

// 获取 JSON 响应
var jsonResponse = await httpClient.GetStringAsync("https://api.example.com/json");

// 获取二进制响应（如文件下载）
var bytes = await httpClient.GetByteArrayAsync("https://example.com/file.pdf");

// 获取流响应（大文件下载，避免内存溢出）
await using var stream = await httpClient.GetStreamAsync("https://example.com/largefile.zip");

await using var fileStream = File.Create("largefile.zip");
await stream.CopyToAsync(fileStream);

发送带参数的请求

// GET 请求带查询参数
var queryParams = new Dictionary<string, string>
{
    { "page", "1" },
    { "size", "20" }
};
var uri = new UriBuilder("https://api.example.com/data")
{
    Query = new FormUrlEncodedContent(queryParams).ReadAsStringAsync().Result
};
var response = await httpClient.GetAsync(uri.Uri);

// POST 请求带表单数据
var formContent = new FormUrlEncodedContent(new[]
{
    new KeyValuePair<string, string>("username", "test"),
    new KeyValuePair<string, string>("password", "pass")
});
await httpClient.PostAsync("https://example.com/login", formContent);

处理 HTTP 错误

try
{
    var response = await httpClient.GetAsync("https://api.example.com/resource");
    response.EnsureSuccessStatusCode(); // 非 200 状态码会抛出异常
}
catch (HttpRequestException ex)
{
    Console.WriteLine(#34;HTTP Error: {ex.StatusCode}");
    Console.WriteLine(#34;Message: {ex.Message}");
}

处理 gzip/deflate 压缩响应

var httpClientHandler = new HttpClientHandler
{
    AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate
};
var httpClient = new HttpClient(httpClientHandler);

取消操作

var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10)); // 10秒超时
try {
    var response = await client.GetAsync("https://slow-api.com", cts.Token);
} catch (TaskCanceledException) {
    // 处理超时
}