编辑“︁C Sharp HTTP 客户端”︁（章节）

= C# HTTP客户端 =

HTTP客户端是C#网络编程中用于发送HTTP请求和接收HTTP响应的核心组件。.NET框架提供了多种方式来实现HTTP通信，包括传统的'''HttpWebRequest'''、'''WebClient'''，以及现代推荐的'''HttpClient'''类。本章将重点介绍这些技术的使用方法、最佳实践和实际应用场景。

== 概述 ==
HTTP客户端允许应用程序与Web服务器进行通信，发送请求（如GET、POST等）并处理响应。在C#中，主要使用以下类：

* '''HttpWebRequest'''：早期的底层HTTP客户端实现
* '''WebClient'''：更高级别的封装，简化常见操作
* '''HttpClient'''：.NET 4.5+引入的现代HTTP客户端，支持异步操作

随着.NET的发展，'''HttpClient'''已成为推荐选择，因为它：
* 支持异步编程模型
* 可重用连接（减少资源消耗）
* 更灵活的请求/响应处理
* 更好的性能

== HttpClient 基础 ==
HttpClient位于'''System.Net.Http'''命名空间，是处理HTTP请求的主要类。

=== 基本用法 ===
<syntaxhighlight lang="csharp">
using System;
using System.Net.Http;
using System.Threading.Tasks;

class Program
{
    static async Task Main(string[] args)
    {
        // 创建HttpClient实例
        using HttpClient client = new HttpClient();
        
        // 发送GET请求
        HttpResponseMessage response = await client.GetAsync("https://api.example.com/data");
        
        // 确保请求成功
        response.EnsureSuccessStatusCode();
        
        // 读取响应内容
        string responseBody = await response.Content.ReadAsStringAsync();
        
        Console.WriteLine(responseBody);
    }
}
</syntaxhighlight>

'''输出示例'''：
<pre>
{
    "userId": 1,
    "id": 1,
    "title": "示例数据",
    "completed": false
}
</pre>

=== 关键组件 ===
* '''HttpClient'''：发送请求和接收响应的主类
* '''HttpRequestMessage'''：表示HTTP请求消息
* '''HttpResponseMessage'''：表示HTTP响应消息
* '''HttpContent'''：表示HTTP内容（请求体或响应体）

== 高级特性 ==

=== 自定义请求头 ===
<syntaxhighlight lang="csharp">
using (HttpClient client = new HttpClient())
{
    // 添加自定义请求头
    client.DefaultRequestHeaders.Add("User-Agent", "MyApp/1.0");
    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
    
    // 添加授权头
    client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", "your_token_here");
    
    var response = await client.GetAsync("https://api.example.com/protected");
    // 处理响应...
}
</syntaxhighlight>

=== POST请求与JSON内容 ===
<syntaxhighlight lang="csharp">
var user = new { Name = "John", Age = 30 };
string json = JsonSerializer.Serialize(user);

using (var content = new StringContent(json, Encoding.UTF8, "application/json"))
using (HttpClient client = new HttpClient())
{
    var response = await client.PostAsync("https://api.example.com/users", content);
    
    if (response.IsSuccessStatusCode)
    {
        var responseJson = await response.Content.ReadAsStringAsync();
        Console.WriteLine($"服务器响应: {responseJson}");
    }
}
</syntaxhighlight>

=== 处理不同响应类型 ===
<syntaxhighlight lang="csharp">
// 处理JSON响应
var response = await client.GetAsync("https://api.example.com/data");
if (response.IsSuccessStatusCode)
{
    var data = await response.Content.ReadFromJsonAsync<MyDataClass>();
    Console.WriteLine($"Received: {data}");
}

// 处理二进制数据
var imageResponse = await client.GetAsync("https://example.com/image.png");
if (imageResponse.IsSuccessStatusCode)
{
    byte[] imageData = await imageResponse.Content.ReadAsByteArrayAsync();
    File.WriteAllBytes("downloaded.png", imageData);
}
</syntaxhighlight>

== 最佳实践 ==

=== HttpClient生命周期管理 ===
HttpClient设计为可重用，不应为每个请求创建新实例。最佳实践包括：

* 在应用程序生命周期内重用单个HttpClient实例
* 或使用'''IHttpClientFactory'''（在ASP.NET Core中）

=== 超时设置 ===
<syntaxhighlight lang="csharp">
var client = new HttpClient
{
    Timeout = TimeSpan.FromSeconds(30) // 设置30秒超时
};
</syntaxhighlight>

=== 取消支持 ===
<syntaxhighlight lang="csharp">
var cts = new CancellationTokenSource();
cts.CancelAfter(TimeSpan.FromSeconds(10)); // 10秒后取消

try
{
    var response = await client.GetAsync("https://slow-api.example.com", cts.Token);
    // 处理响应...
}
catch (TaskCanceledException)
{
    Console.WriteLine("请求已取消");
}
</syntaxhighlight>

== 实际应用案例 ==

=== 天气API客户端 ===
<syntaxhighlight lang="csharp">
public class WeatherService
{
    private readonly HttpClient _httpClient;
    
    public WeatherService(HttpClient httpClient)
    {
        _httpClient = httpClient;
        _httpClient.BaseAddress = new Uri("https://api.weatherapi.com/v1/");
    }
    
    public async Task<WeatherData> GetCurrentWeatherAsync(string location)
    {
        var response = await _httpClient.GetAsync($"current.json?key=YOUR_API_KEY&q={location}");
        response.EnsureSuccessStatusCode();
        
        return await response.Content.ReadFromJsonAsync<WeatherData>();
    }
}

// 使用示例
var weatherService = new WeatherService(new HttpClient());
var weather = await weatherService.GetCurrentWeatherAsync("London");
Console.WriteLine($"当前温度: {weather.Current.TempC}°C");
</syntaxhighlight>

=== 文件下载进度报告 ===
<syntaxhighlight lang="csharp">
public async Task DownloadFileWithProgressAsync(string fileUrl, string localPath, IProgress<double> progress)
{
    using (var client = new HttpClient())
    {
        // 只获取头部信息来确定内容长度
        using (var response = await client.GetAsync(fileUrl, HttpCompletionOption.ResponseHeadersRead))
        {
            response.EnsureSuccessStatusCode();
            
            var totalBytes = response.Content.Headers.ContentLength ?? -1L;
            var receivedBytes = 0L;
            
            using (var contentStream = await response.Content.ReadAsStreamAsync())
            using (var fileStream = new FileStream(localPath, FileMode.Create))
            {
                var buffer = new byte[8192];
                var isMoreToRead = true;
                
                do
                {
                    var read = await contentStream.ReadAsync(buffer, 0, buffer.Length);
                    if (read == 0)
                    {
                        isMoreToRead = false;
                    }
                    else
                    {
                        await fileStream.WriteAsync(buffer, 0, read);
                        
                        receivedBytes += read;
                        if (totalBytes > 0)
                        {
                            progress.Report((double)receivedBytes / totalBytes);
                        }
                    }
                }
                while (isMoreToRead);
            }
        }
    }
}

// 使用示例
var progress = new Progress<double>(p => Console.WriteLine($"下载进度: {p:P}"));
await DownloadFileWithProgressAsync("https://example.com/largefile.zip", "largefile.zip", progress);
</syntaxhighlight>

== 性能考虑 ==

=== 连接池 ===
HttpClient内部使用连接池来重用HTTP连接。可以通过'''ServicePointManager'''进行配置：

<syntaxhighlight lang="csharp">
ServicePointManager.DefaultConnectionLimit = 20; // 增加默认连接限制
</syntaxhighlight>

=== HTTP/2 支持 ===
从.NET Core 3.0开始，HttpClient支持HTTP/2协议：

<syntaxhighlight lang="csharp">
var client = new HttpClient()
{
    DefaultRequestVersion = HttpVersion.Version20
};
</syntaxhighlight>

== 常见问题与解决方案 ==

{| class="wikitable"
|-
! 问题 !! 解决方案
|-
| '''Socket耗尽''' || 重用HttpClient实例或使用IHttpClientFactory
|-
| '''DNS变更不生效''' || 设置HttpClient的BaseAddress或使用HttpClientHandler的PooledConnectionLifetime
|-
| '''大文件下载内存问题''' || 使用流式处理(ReadAsStreamAsync)而非缓冲(ReadAsByteArrayAsync)
|-
| '''SSL/TLS错误''' || 配置正确的SecurityProtocol或处理证书验证
|}

== 总结 ==
C#中的HTTP客户端功能强大且灵活，从简单的GET请求到复杂的API交互都能胜任。现代应用程序应优先使用HttpClient类，遵循最佳实践来确保性能、可靠性和安全性。通过合理配置和错误处理，可以构建健壮的HTTP通信组件，满足各种网络编程需求。

[[Category:编程语言]]
[[Category:C Sharp]]
[[Category:C Sharp 网络编程]]