1. HttpClient的使用

public static async void UseHttp()
{
    HttpClient httpClient = new HttpClient(); //静态实例,主要是为了复用Tcp连接,用在项目中要进行封装
    httpClient.BaseAddress = new Uri("http://localhost:5000"); //要访问的基地址,ip加端口

    string result = await httpClient.GetStringAsync("/Home/GetHtmlData");
    Console.WriteLine(result);
}

2.在Asp.Net Core使用HttpClientFactory

//在容器中注册HttpClient中间件
services.AddHttpClient("baidu", client =>
{
    client.BaseAddress = new Uri("http://localhost:5000");  
    //还可以请求定制化
});

/// <summary>
/// 
/// </summary>
/// <param name="logger">日志</param>
/// <param name="peopleService">注入IPeopleService的实现</param>
/// <param name="service">注入IService的实现</param>
/// <param name="cache">缓存</param>
/// <param name="httpClientFactory">容器在实例化HomeController的时候,会先实现IHttpClientFactory的实现DefaultHttpClientFactory(AddHttpClient在容器中注册)</param>
public HomeController(ILogger<HomeController> logger,
                        IPeopleService peopleService,
                        AOP.IService service,
                        IMemoryCache cache,
                        IHttpClientFactory httpClientFactory)
{
    Console.WriteLine("有参数构造 HomeController");
    this._logger = logger;
    this._peopleService = peopleService;
    this._service = service;
    this._cache = cache;
    this._httpClientFactory = httpClientFactory;
}

/// <summary>
/// 使用HttpClient请求
/// </summary>
/// <returns></returns>
public async Task<string> GetHtmlData()
{
    //通过DefaultHttpClientFactory的CreateClient获取HttpClient实例
    var client = _httpClientFactory.CreateClient("baidu");
    return await client.GetStringAsync("/");
}

在Asp.Net Core的HttpClient中间件源码

/// <summary>
/// Adds the <see cref="IHttpClientFactory"/> and related services to the <see cref="IServiceCollection"/>.
/// </summary>
/// <param name="services">The <see cref="IServiceCollection"/>.</param>
/// <returns>The <see cref="IServiceCollection"/>.</returns>
public static IServiceCollection AddHttpClient(this IServiceCollection services)
{
    if (services == null)
    {
        throw new ArgumentNullException(nameof(services));
    }

    services.AddLogging();
    services.AddOptions();

    //
    // Core abstractions
    //
    services.TryAddTransient<HttpMessageHandlerBuilder, DefaultHttpMessageHandlerBuilder>();
    services.TryAddSingleton<DefaultHttpClientFactory>();
    services.TryAddSingleton<IHttpClientFactory>(serviceProvider => serviceProvider.GetRequiredService<DefaultHttpClientFactory>());
    services.TryAddSingleton<IHttpMessageHandlerFactory>(serviceProvider => serviceProvider.GetRequiredService<DefaultHttpClientFactory>());

    //
    // Typed Clients
    //
    services.TryAdd(ServiceDescriptor.Transient(typeof(ITypedHttpClientFactory<>), typeof(DefaultTypedHttpClientFactory<>)));
    services.TryAdd(ServiceDescriptor.Singleton(typeof(DefaultTypedHttpClientFactory<>.Cache), typeof(DefaultTypedHttpClientFactory<>.Cache)));

    //
    // Misc infrastructure
    //
    services.TryAddEnumerable(ServiceDescriptor.Singleton<IHttpMessageHandlerBuilderFilter, LoggingHttpMessageHandlerBuilderFilter>());

    // This is used to track state and report errors **DURING** service registration. This has to be an instance
    // because we access it by reaching into the service collection.
    services.TryAddSingleton(new HttpClientMappingRegistry());

    // Register default client as HttpClient
    services.TryAddTransient(s =>
    {
        return s.GetRequiredService<IHttpClientFactory>().CreateClient(string.Empty);
    });

    return services;
}

/// <summary>
/// Adds the <see cref="IHttpClientFactory"/> and related services to the <see cref="IServiceCollection"/> and configures
/// a named <see cref="HttpClient"/>.
/// </summary>
/// <param name="services">The <see cref="IServiceCollection"/>.</param>
/// <param name="name">The logical name of the <see cref="HttpClient"/> to configure.</param>
/// <returns>An <see cref="IHttpClientBuilder"/> that can be used to configure the client.</returns>
/// <remarks>
/// <para>
/// <see cref="HttpClient"/> instances that apply the provided configuration can be retrieved using 
/// <see cref="IHttpClientFactory.CreateClient(string)"/> and providing the matching name.
/// </para>
/// <para>
/// Use <see cref="Options.Options.DefaultName"/> as the name to configure the default client.
/// </para>
/// </remarks>
public static IHttpClientBuilder AddHttpClient(this IServiceCollection services, string name)
{
    if (services == null)
    {
        throw new ArgumentNullException(nameof(services));
    }

    if (name == null)
    {
        throw new ArgumentNullException(nameof(name));
    }

    AddHttpClient(services);

    return new DefaultHttpClientBuilder(services, name);
}

/// <summary>
/// Adds the <see cref="IHttpClientFactory"/> and related services to the <see cref="IServiceCollection"/> and configures
/// a named <see cref="HttpClient"/>.
/// </summary>
/// <param name="services">The <see cref="IServiceCollection"/>.</param>
/// <param name="name">The logical name of the <see cref="HttpClient"/> to configure.</param>
/// <param name="configureClient">A delegate that is used to configure an <see cref="HttpClient"/>.</param>
/// <returns>An <see cref="IHttpClientBuilder"/> that can be used to configure the client.</returns>
/// <remarks>
/// <para>
/// <see cref="HttpClient"/> instances that apply the provided configuration can be retrieved using 
/// <see cref="IHttpClientFactory.CreateClient(string)"/> and providing the matching name.
/// </para>
/// <para>
/// Use <see cref="Options.Options.DefaultName"/> as the name to configure the default client.
/// </para>
/// </remarks>
public static IHttpClientBuilder AddHttpClient(this IServiceCollection services, string name, Action<HttpClient> configureClient)
{
    if (services == null)
    {
        throw new ArgumentNullException(nameof(services));
    }

    if (name == null)
    {
        throw new ArgumentNullException(nameof(name));
    }

    if (configureClient == null)
    {
        throw new ArgumentNullException(nameof(configureClient));
    }

    AddHttpClient(services);

    var builder = new DefaultHttpClientBuilder(services, name);
    builder.ConfigureHttpClient(configureClient);
    return builder;
}

在.Net 6加入Minimal API

var builder = WebApplication.CreateBuilder(args);


builder.Services.AddRazorPages();
//在.Net 6中,使用Minimal api
builder.Services.AddHttpClient("test", client =>
{
    client.BaseAddress = new Uri("http://localhost:5000");
});


var app = builder.Build();

// Configure the HTTP request pipeline.
if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler("/Error");
}
app.UseStaticFiles();

app.UseRouting();

app.UseAuthorization();

app.MapRazorPages();

//在这里注入IHttpClientFactory
app.MapGet("/", async (IHttpClientFactory httpClientFactory) =>
{
    var client = httpClientFactory.CreateClient("test");
    return await client.GetStringAsync("/");
});

app.Run();