Question

我们有一个dotnet-core 2.1 web-api项目设置，如下所示。我们正在尝试将MediatR与HttpClient一起使用，并在HttpMagicService中发送事件以响应不同的响应。在调用HttpClient.SendAsync()之后尝试使用注入的服务时，问题再次出现。

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)
    {
        // ...
    }       
}

对于我们的第一个呼叫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。

从this.mediator看，我们发现有针对该服务的解决方案。

当我们运行var response = await this.httpClient.SendAsync(request).ConfigureAwait(false);时，httpClient似乎决定为所有人分配所有服务。 ?

在此行之后查看this.mediator时，我们发现一切都已清除。

由于没有服务，因此呼叫await this.mediator.Publish(new MagicAddIntegrationEvent(data));将无法运行，并且我们会收到异常消息。

这是this.httpClient.SendAsync的预期行为吗？我们应该用另一种方式吗？

一种可行的解决方案是忽略设置services.AddHttpClient<IMagicService, HttpMagicService>();，而为HttpClient设置单例服务，这似乎可行，但是我不确定这是否是正确的方法

解决方案

使用了Panagiotis Kanavos的建议并使用了范围限定的httpClient，所以我的解决方案改为了此。

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>();
}

如果在不使用var mediator= scope.ServiceProvider.GetRequiredService<IMediator>();的情况下尝试在范围内使用IMediator，则会出现与以前相同的问题。因此，我错过了DI容器存在的范围问题。

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作用域服务时，经常会发生此类异常。 This question about MediatR and scopes可能是相关的。无论如何，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，它将在某个时候被处理掉。

无论哪种情况，您都可以通过将IServiceProvider注入HttpMagicService并显式创建作用域来使用HttpClient。选中consuming a scoped service in a background task。

您可以注入IServiceProvider而不是HttpClient，并在AddAsync方法内创建范围，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>();
         ...
        }

HttpClient.SendAsync（）之后无法访问已处置的对象

1 个答案: