实体框架核心服务默认生命周期

是的，DbContext 的默认生命周期是有范围的.这是这样的.

Yes, the default life time for DbContext is scoped. This is intended this way.

实例化 DbContext 非常便宜，它确保您不使用许多资源.如果您有一个具有单例生命周期的 DbContext，那么您读取一次的所有记录都将被 DbContext 跟踪，除非您明确禁用跟踪.这将需要更多的内存使用量，并且会继续增长.

Instantiating DbContext is pretty cheap and it makes sure that the your do not use to many resources. If you'd have a DbContext with a singleton lifetime, then all records that you read once will be tracked by the DbContext, unless you specifically disable tracking. This will require much more memory usage and it will keep growing.

DbContext 跟踪的越多，性能就越低.这就是为什么您经常看到 DbContext 仅在 using(var context = new AppDbContext()) 块中使用的原因.

And the more the DbContext tracks, the lower the performance will be. That's why you often see DbContext only used within a using(var context = new AppDbContext()) block.

然而，在 web 应用程序中，使用 using 块是不好的，因为生命周期由 framework 并且如果您提前处置它，之后的调用将失败并出现异常.

In web applications however, using the using block is bad, because the lifetime is managed by the framework and if you dispose it to early the calls after that will fail with an exception.

如果你在另一边使用瞬态生命周期，你将失去交易"功能.有了作用域，DbContext 有一个与请求一样长的事务作用域.

If you use transient lifetime on the other side, you will lose the "transaction" functionality. With scoped, the DbContext has a transaction scope that's as long as the request.

如果您需要更细粒度的控制，则必须使用工作单元模式(DbContext 已经在使用).

If you need more fine-grained control, you have to use the Unit of Work pattern (which DbContext already kind of utilize).

对于你的第二个问题:

如果您创建一个服务，它的生命周期必须等于范围之一或更短(阅读:范围或瞬态).

If you create a service, it must have a lifetime that's equal to the one of the scope or shorter (read: Scoped or transient).

如果您明确需要更长的服务生命周期，您应该将 DbContext 工厂服务或工厂方法注入您的服务.

If you explicitly need a longer life-time for a service, you should inject a DbContext factory service or factory method into your service.

你可以用像

services.AddTransient<Func<AppDbContext>>( (provider) => new Func<MyDbContext>( () => new AppDbContext()));
services.AddSingleton<IMySingletonService, MySingletonService>();

您的服务可能如下所示:

And your service may look like this:

public class MySingletonService : IMySingletonService, IDisposable
{
    private readonly AppDbContext context;

    public MySingletonService(Func<AppDbContext> contextFactory)
    {
        if(contextFactory == null)
            throw new ArgumentNullException(nameof(contextFactory));

        // it creates an transient factory, make sure to dispose it in `Dispose()` method.
        // Since it's member of the MySingletonService, it's lifetime
        // is effectively bound to it. 
        context = contextFactory();
    }
}