.NET Core中自定义HttpMessageHandler注入服务注册后Header不生效

问题场景 #

我编写了服务注册类InternalSymbolServiceRegister用于注册ISymbolService，尝试通过注入自定义HttpMessageHandler为请求添加自定义头，但调用API时请求头为空。

服务注册类代码 #

public static class InternalSymbolServiceRegister
{
    public const string DefaultConfigSectionName = "myService";

    public static IServiceCollection AddSymbols(
        this IServiceCollection services, 
        IConfiguration configuration,
        HttpMessageHandler customHttpMessageHandler = null,
        string configSectionName = DefaultConfigSectionName)
    {
        services.Configure<SymbolOptions>(configuration.GetSection(configSectionName));
        services.AddHttpClient<ISymbolService, InternalSymbolService>((sp, client) =>
        {
            var options = sp.GetRequiredService<IOptions<SymbolOptions>>().Value;
            client.BaseAddress = new Uri(options.ServiceUrl);
            client.Timeout = TimeSpan.FromMilliseconds(options.TimeoutMs);
          
        }).AddPolicyHandler(GetCircuitBreakerPolicy());
        if (customHttpMessageHandler != null)
        {
            services.Configure<HttpClientFactoryOptions>(nameof(ISymbolService), options =>
            {
                options.HttpMessageHandlerBuilderActions.Add(builder =>
                {
                    builder.PrimaryHandler = customHttpMessageHandler;
                });
            });
        }

        return services;
    }

    private static IAsyncPolicy<HttpResponseMessage> GetCircuitBreakerPolicy()
        => HttpPolicyExtensions
            .HandleTransientHttpError()
            .CircuitBreakerAsync(handledEventsAllowedBeforeBreaking: 5, durationOfBreak: TimeSpan.FromSeconds(7));

}

自定义Header处理器代码 #

public sealed class InternalSymbolHeaderHandler : DelegatingHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        request.Headers.Add("admin_header", "_11111");
        return await base.SendAsync(request, cancellationToken);
    }
}

Startup注册代码 #

services.AddSymbols(Configuration, customHttpMessageHandler: new InternalSymbolHeaderHandler());

问题原因 #

1. DelegatingHandler使用错误：DelegatingHandler是作为请求处理链中的一环设计的，必须通过InnerHandler指向链的下一个处理器。直接将其设为PrimaryHandler时，InnerHandler未初始化，调用base.SendAsync无法继续传递请求，导致请求头添加逻辑失效。
2. Handler配置逻辑缺陷：通过Configure<HttpClientFactoryOptions>替换PrimaryHandler会覆盖默认的HttpClientHandler，但由于DelegatingHandler缺少后续处理节点，整个请求流程被中断。此外，直接传入new InternalSymbolHeaderHandler()会造成实例单例复用，存在线程安全隐患。

解决方案 #

方案1：将DelegatingHandler作为附加处理器添加（推荐） #

保留默认PrimaryHandler，将自定义处理器加入请求链，由HttpClientFactory自动管理处理器间的关联：

修改AddSymbols方法的自定义Handler配置逻辑：

public static IServiceCollection AddSymbols(
    this IServiceCollection services, 
    IConfiguration configuration,
    Func<IServiceProvider, DelegatingHandler> customDelegatingHandlerFactory = null,
    string configSectionName = DefaultConfigSectionName)
{
    services.Configure<SymbolOptions>(configuration.GetSection(configSectionName));
    var httpClientBuilder = services.AddHttpClient<ISymbolService, InternalSymbolService>((sp, client) =>
    {
        var options = sp.GetRequiredService<IOptions<SymbolOptions>>().Value;
        client.BaseAddress = new Uri(options.ServiceUrl);
        client.Timeout = TimeSpan.FromMilliseconds(options.TimeoutMs);
    }).AddPolicyHandler(GetCircuitBreakerPolicy());

    // 将自定义DelegatingHandler添加到处理链
    if (customDelegatingHandlerFactory != null)
    {
        httpClientBuilder.AddHttpMessageHandler(customDelegatingHandlerFactory);
    }

    return services;
}

Startup注册时传入工厂委托，让容器管理Handler生命周期：

services.AddSymbols(Configuration, customDelegatingHandlerFactory: sp => new InternalSymbolHeaderHandler());

方案2：自定义PrimaryHandler，改用HttpClientHandler #

如果必须替换默认PrimaryHandler，则自定义Handler需继承HttpClientHandler而非DelegatingHandler：

修改自定义Handler：

public sealed class InternalSymbolHeaderHandler : HttpClientHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        request.Headers.Add("admin_header", "_11111");
        return await base.SendAsync(request, cancellationToken);
    }
}

同时优化AddSymbols的配置逻辑，推荐通过容器注入Handler：

// 先注册Handler到容器
services.AddSingleton<HttpMessageHandler, InternalSymbolHeaderHandler>();
// 再注册服务
services.AddSymbols(Configuration, customHttpMessageHandler: sp => sp.GetRequiredService<HttpMessageHandler>());

验证 #

修改后重新运行程序，调用API时即可看到自定义请求头admin_header被正确添加到请求中。

内容的提问来源于stack exchange，提问作者Ehsan Akbar