在 HttpClient.SendAsync() 之后无法访问已释放的对象

Question

我们有一个 dotnet-core 2.1 web-api 项目设置如下。 我们正在尝试将 MediatR 与 HttpClient 一起使用，并在 HttpMagicService 中为不同的响应发送事件。在调用 HttpClient.SendAsync() 后尝试使用注入服务时出现问题。

IMagicService 和 MediatR 的初始设置。

public void ConfigureServices(IServiceCollection services)
{

    // other services configured above
    services.AddTransient(typeof(IPipelineBehavior<,>), typeof(RequestLoggingBehavior<,>));
    services.AddTransient(typeof(IPipelineBehavior<,>), typeof(RequestPerformanceBehaviour<,>));
    services.AddTransient(typeof(IPipelineBehavior<,>), typeof(RequestValidationBehavior<,>));
    services.AddMediatR(typeof(ApplicaitonCommand).GetTypeInfo().Assembly); 
    services.AddHttpClient<IMagicService, HttpMagicService>();
}

IMagicService 的实现。

public class HttpMagicService : IMagicService
{
    private readonly IMediator mediator;
    private readonly HttpClient httpClient;

    public HttpMagicService(IMediator mediator, HttpClient httpClient)
    {
        this.mediator = mediator ?? throw new ArgumentNullException(nameof(mediator));
        this.httpClient = httpClient ?? throw new ArgumentNullException(nameof(httpClient));
    }

    public async Task AddAsync(Magic data, Metadata metadata)
    {
        // This one works
        await this.mediator.Publish(new MagicAddInitIntegrationEvent(data));

        var url = metadata.RemoteUri + "/command/AddMagic";
        using (var request = new HttpRequestMessage(HttpMethod.Post, url))
        {
            try
            {
                var httpContent = CreateHttpContent(data);
                request.Content = httpContent;
                request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", metadata.AuthKey);

                // httpClient clears the ResolvedServices?!
                var response = await this.httpClient.SendAsync(request).ConfigureAwait(false);
                if (response.StatusCode == System.Net.HttpStatusCode.OK)
                {
                    // This one fails
                    await this.mediator.Publish(new MagicAddIntegrationEvent(data));
                }
                else
                {
                    await this.mediator.Publish(new MagicAddFailedIntegrationEvent(data));
                }
            }
            catch (Exception ex)
            {
                await this.mediator.Publish(new MagicAddFailedIntegrationEvent(meeting, ex));
            }
        }
    }

    private static HttpContent CreateHttpContent(object content)
    {
        // ...
    }       
}

对于我们的第一次调用await this.mediator.Publish(new MagicAddInitIntegrationEvent(data));，事件已发布并且处理程序可以处理该事件。

查看this.mediator，我们看到有一个已解决的服务。

当我们运行var response = await this.httpClient.SendAsync(request).ConfigureAwait(false); 时，似乎httpClient 只是决定为每个人处理所有服务。 ????

在这行之后查看this.mediator 时，我们看到所有内容都已清理完毕。

由于没有服务，调用 await this.mediator.Publish(new MagicAddIntegrationEvent(data)); 将无法运行，我们得到一个异常。

这是this.httpClient.SendAsync 的预期行为吗？我们应该以另一种方式来做吗？

似乎可行的一种解决方案是忽略设置services.AddHttpClient<IMagicService, HttpMagicService>();，而是为HttpClient 设置单例服务，这似乎可行，但我不确定这是否是正确的方法。

---

解决方案

使用了 Panagiotis Kanavos 的建议并做了一个作用域 httpClient，所以我的解决方案变成了这个。

public void ConfigureServices(IServiceCollection services)
{

    // other services configured above
    services.AddTransient(typeof(IPipelineBehavior<,>), typeof(RequestLoggingBehavior<,>));
    services.AddTransient(typeof(IPipelineBehavior<,>), typeof(RequestPerformanceBehaviour<,>));
    services.AddTransient(typeof(IPipelineBehavior<,>), typeof(RequestValidationBehavior<,>));
    services.AddMediatR(typeof(ApplicaitonCommand).GetTypeInfo().Assembly); 
    services.AddScoped<HttpClient>();
}

如果尝试在范围内使用 IMediator 而不使用 var mediator= scope.ServiceProvider.GetRequiredService<IMediator>();，则会出现与之前相同的问题。所以我错过了 DI 容器的一些范围问题。

public class HttpMagicService : IMagicService
{
    private readonly IMediator mediator;
    private readonly IServiceProvider services;

    public HttpMagicService(IServiceProvider services)
    {
        this.services = services?? throw new ArgumentNullException(nameof(services));
    }

    public async Task AddAsync(Magic data, Metadata metadata)
    {
        using (var scope = Services.CreateScope())
        {
            var httpClient = scope.ServiceProvider.GetRequiredService<HttpClient>();
            var mediator= scope.ServiceProvider.GetRequiredService<IMediator>();
         ...
        }

Answer 1

当单例服务尝试使用范围服务时，经常会发生此类异常。 This question about MediatR and scopes 可能是相关的。在任何情况下，HttpClientFactory 都会定期回收 HttpClient 实例（特别是 handler 实例），这意味着如果HttpMagicService 缓存 HttpClient 实例足够长的时间，它将在某个时候被释放。

在任何一种情况下，您都可以通过将 IServiceProvider 注入 HttpMagicService 并显式创建范围来使用 HttpClient。检查consuming a scoped service in a background task。

您可以注入 IServiceProvider 而不是 HttpClient，并在 AddAsync 方法内创建一个作用域，例如：

public class HttpMagicService : IMagicService
{
    private readonly IMediator mediator;
    private readonly IServiceProvider services;

    public HttpMagicService(IMediator mediator, IServiceProvider services)
    {
        this.mediator = mediator ?? throw new ArgumentNullException(nameof(mediator));
        this.services = services?? throw new ArgumentNullException(nameof(services));
    }

    public async Task AddAsync(Magic data, Metadata metadata)
    {
        using (var scope = Services.CreateScope())
        {
            var httpClient = scope.ServiceProvider.GetRequiredService<HttpClient>();
         ...
        }