具有 AsyncLocal 与范围服务的单例

Question

我查看了如何在 .NET Core 中创建 HttpContext。然后我发现有一个名为 HttpContextFactory 的 class 创建并分配 HttpContext 对象到 HttpContext 属性 of HttpContextAccessor class.为了在我们的代码中使用 HttpContext 对象，我们将 IHttpContextAccessor 注入到需要该对象的 class 的构造函数中。

当我查看 HttpContextAccessor 的实现时，显然它的 HttpContext 属性从私有 AsyncLocal 变量获取 HttpContext 对象值，后来 HttpContextAccessor 被注册为 Singleton.

https://github.com/aspnet/AspNetCore/blob/master/src/Http/Http/src/HttpContextAccessor.cs

// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System.Threading;

namespace Microsoft.AspNetCore.Http
{
    public class HttpContextAccessor : IHttpContextAccessor
    {
        private static AsyncLocal<HttpContextHolder> _httpContextCurrent = new AsyncLocal<HttpContextHolder>();

        public HttpContext HttpContext
        {
            get
            {
                return  _httpContextCurrent.Value?.Context;
            }
            set
            {
                var holder = _httpContextCurrent.Value;
                if (holder != null)
                {
                    // Clear current HttpContext trapped in the AsyncLocals, as its done.
                    holder.Context = null;
                }

                if (value != null)
                {
                    // Use an object indirection to hold the HttpContext in the AsyncLocal,
                    // so it can be cleared in all ExecutionContexts when its cleared.
                    _httpContextCurrent.Value = new HttpContextHolder { Context = value };
                }
            }
        }

        private class HttpContextHolder
        {
            public HttpContext Context;
        }
    }
}

我很好奇，这样做而不是使用 Scope 服务有什么好处？在我看来，两者都会使对象在请求范围内可用。

如果它是范围服务，我认为 HttpContextAccessor 看起来像这样

using System.Threading;

namespace Microsoft.AspNetCore.Http
{
    public class HttpContextAccessor : IHttpContextAccessor
    {
        private HttpContextHolder _httpContextCurrent;

        public HttpContext HttpContext
        {
            get
            {
                return  _httpContextCurrent?.Context;
            }
            set
            {
                if (value != null)
                {
                    _httpContextCurrent = new HttpContextHolder { Context = value };
                }
            }
        }

        private class HttpContextHolder
        {
            public HttpContext Context;
        }
    }
}

然后将其用作范围服务

services.TryAddScope<IHttpContextAccessor, HttpContextAccessor>();

我想知道每种方法的优缺点，以便在为我的项目创建库时了解何时将 Singleton 与 AsyncLocal 或 Scope 结合使用。

Answer 1

只要是单例，已解析的 IHttpContextAccessor 实例就可以由单例服务永久持有并正常工作，而如果单例服务解析范围 IHttpContextAccessor 则可能会导致问题.

Answer 2

我想原因之一可能是 Asp.Net 核心 IServiceProvider 不允许将作用域依赖项注入单例中 class。这可能是其背后的一个重大决定。如果按照您的建议确定了范围，那么所有使用它的 classes 可能都必须确定范围。但有趣的是，一旦请求被送达，HTTPContext 就变为 null。

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