c# - IServiceCollection.AddHostedService<>();没有解决

Question

我在全新的 .net core 2 网络应用程序中发生了一件奇怪的事情。这是来自 Visual Studio 内置模板的标准 Web API。 VS 2017，所有的爵士乐。这是整个 startup.cs:

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Routing;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using VerySimpleAPI.Helpers;

namespace VerySimpleAPI
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddMvc();
            services.AddHostedService<MainLoop>();
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseMvc(routes => {
                routes.MapRoute(
                    name: "Default",
                    template: "{controller}/{action}/{id?}",
                    defaults: new { controller = "Home", action = "Index"}
                    );
            });
        }
    }
}

MainLoop工具 IHostedService和 IDisposable .

services.AddHostedService<MainLoop>();没有解决，产生错误“‘IServiceCollection’不包含‘AddHostedService’的定义，也没有扩展方法……”所有这些爵士乐。我检查了 Microsoft.Extensions.DependencyInjection来源，我清楚地看到 public static IServiceCollection AddHostedService<THostedService>(this IServiceCollection services) 的定义

没有托管服务引用，项目编译得很好。有什么我想念的吗？

Answer 1

AddHostedService 是 Microsoft.Extensions.Hosting.Abstractions 的一部分。

虽然它是在 Microsoft.Extensions.DependencyInjection 命名空间中定义的，但它属于 ServiceCollectionHostedServiceExtensions 中的 Microsoft.Extensions.Hosting.Abstractions类

using Microsoft.Extensions.DependencyInjection.Extensions;
using Microsoft.Extensions.Hosting;

namespace Microsoft.Extensions.DependencyInjection
{
    public static class ServiceCollectionHostedServiceExtensions
    {
        /// <summary>
        /// Add an <see cref="IHostedService"/> registration for the given type.
        /// </summary>
        /// <typeparam name="THostedService">An <see cref="IHostedService"/> to register.</typeparam>
        /// <param name="services">The <see cref="IServiceCollection"/> to register with.</param>
        /// <returns>The original <see cref="IServiceCollection"/>.</returns>
        public static IServiceCollection AddHostedService<THostedService>(this IServiceCollection services)
            where THostedService : class, IHostedService
        {
            services.TryAddEnumerable(ServiceDescriptor.Singleton<IHostedService, THostedService>());

            return services;
        }
    }
}

并确保安装和引用了相关包以访问扩展方法