Question

我研究了如何在.NET Core中创建HttpContext。然后，我发现有一个名为HttpContextFactory的类，它创建HttpContext对象并将其分配到HttpContext类的HttpContextAccessor属性中。为了在代码中使用HttpContext对象，我们将IHttpContextAccessor注入到需要该对象的类的构造函数中。

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，然后在HttpContextAccessor上将其注册为 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，以这种方式代替使用 Scope 服务有什么好处？在我看来，两者都将使对象在请求范围内可用。

如果它是一个范围服务，我认为HttpContextAccessor会看起来像这样

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点和缺点，以便为项目创建库时了解何时将Singleton与AsyncLocal或Scope一起使用。

Answer 1

只要是单例，已解析的IHttpContextAccessor实例可以由单例服务永久保存并正常工作，而如果单例服务解析范围为IHttpContextAccessor的实例可能会引起问题。 / p>

Answer 2

我猜想原因之一可能是Asp.Net Core IServiceProvider不允许将作用域依赖项注入到Singleton类中。那可能是背后的重大决定。如果事情已经按照您的建议进行了范围划分，那么使用它的所有类都必须进行范围划分。但是有趣的是，一旦请求得到处理，HTTPContext就会为空。

具有AsyncLocal与范围服务的Singleton

2 个答案: