Asp.Net Core中的HttpContext.Features与HttpContext.Items

问题描述

这两个属性之间有什么区别?

我可以使用HttpContext.Items代替HttpContext.Features在middlewares之间共享数据.我看到的唯一区别是，我告诉Items一个密钥，它给了我对象，因此我必须强制转换它.可以自动在Features中完成投射.

I can use HttpContext.Items instead of HttpContext.Features to share data between middlewares. The only difference i see is that i tell Items for a key and it gives me object and i have to cast it. This casting can be done in Features automatically.

他们身后还有别的东西吗?

Is there something else behind them?

推荐答案

最大的区别是HttpContext.Items设计为存储Key-Value-Pair，而HttpContext.Features设计为存储Type-Instance-Pair.

The biggest difference is that the HttpContext.Items is designed to store Key-Value-Pair, while the HttpContext.Features is designed to store Type-Instance-Pair.

更清楚地说，HttpContext.Items旨在共享当前请求范围内的项目，而作为IFeatureCollection实例的HttpContext.Features绝不能那样使用.

To be more clear, HttpContext.Items is designed to share items within the scope of current request, while the HttpContext.Features, which is an instance of IFeatureCollection, is by no means to be used like that .

IFeatureCollection接口表示HTTP功能的集合，例如:

The IFeatureCollection interface represents a collection of HTTP features, such as:

1. IAuthenticationFeature，用于存储原始PathBase和原始Path.
2. ISessionFeature存储当前会话.
3. IHttpConnectionFeature存储基础连接.
4. 以此类推.

1. IAuthenticationFeature which stores original PathBase and original Path.
2. ISessionFeature which stores current Session.
3. IHttpConnectionFeature which stores the underlying connection.
4. and so on.

为帮助存储和检索Type-Instance-Pair，该界面具有三种重要方法:

To help store and retrieve a Type-Instance-Pair, the interface has three important methods:

public interface IFeatureCollection : IEnumerable<KeyValuePair<Type, object>>{
    // ...
    object this[Type key] { get; set; }
    TFeature Get<TFeature>();
    void Set<TFeature>(TFeature instance);
}

和实现(FeatureCollection)会将值简单地转换为所需的类型:

and the implementation (FeatureCollection) will simply cast the value into required type:

public class FeatureCollection : IFeatureCollection
{
    // ... get the required type of feature
    public TFeature Get<TFeature>()
    {
        return (TFeature)this[typeof(TFeature)];    // note: cast here!
    }

    public void Set<TFeature>(TFeature instance)
    {
        this[typeof(TFeature)] = instance;          // note!
    }
}

这是设计使然.因为不需要存储两个IHttpConnectionFeature实例或两个ISession实例.

This is by design. Because there's no need to store two IHttpConnectionFeature instances or two ISession instances.

虽然可以用FeatureCollection存储一些Type-Value对，但最好不要.如您所见，如果集合中已经存在某种类型的Set<TFeature>(TFeature instance)，它将简单地替换旧的.如果您有两个相同的类型，这也意味着会出现错误.

While you can store some Type-Value pairs with FeatureCollection, you'd better not . As you see, the Set<TFeature>(TFeature instance) will simply replace the old one if the some type already exists in the collection; it also means there will be a bug if you have two of the same type.

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