blocks|key|27918|text|您对所有3个作用域的理解都是正确的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|27919|当组件无法共享时，将使用瞬态。非线程安全的数据库访问对象就是一个例子。|offset|length|style|BOLD|27920|作用域可用于实体框架数据库上下文。主要原因是，从数据库中获取的实体将附加到请求中的所有组件都能看到的相同上下文。当然，如果您计划使用它并行执行查询，则不能使用作用域。|27921|作用域对象的另一个例子是某种RequestContext类，它包含例如调用者的用户名。中间件/MVC过滤器可以请求它并填写信息，其他组件也可以请求它，它肯定会包含当前请求的信息。|CODE|27922|单例组件总是共享的，因此它们最适合不需要绑定到请求的线程安全组件。IOptions就是一个例子，它允许访问配置设置。在单个静态HttpClient实例上使用SendAsync的HttpClient包装类也是完全线程安全的，并且是一个很好的单例候选者。|27923|请注意，如果您有一个依赖于作用域组件的Singleton组件，那么它的依赖项将在它之前被释放。因此，一个组件不能依赖于另一个作用域小于其自身的组件。|27924|entityMap^0|0|C|2|0|0|3|0|E|E|0|0|2|X|8|1R|A|26|9|2G|A|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@$D|U|E|V|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|W|8|@$D|X|E|Y|F|G]]|9|@]|A|$]]|$1|J|3|K|5|6|7|Z|8|@$D|10|E|11|F|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|12|8|@$D|13|E|14|F|G]|$D|15|E|16|F|L]|$D|17|E|18|F|L]|$D|19|E|1A|F|L]|$D|1B|E|1C|F|L]]|9|@]|A|$]]|$1|O|3|P|5|6|7|1D|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|1E|8|@]|9|@]|A|$]]]|R|$]]

Your understanding of all 3 scopes is correct.

Transient would be used when the component cannot be shared. A non-thread-safe database access object would be one example.

Scoped can be used for Entity Framework database contexts. The main reason is that then entities gotten from the database will be attached to the same context that all components in the request see. Of course if you plan on doing queries with it in parallel, you can't use Scoped.

Another example of a Scoped object would be some kind of a <code>RequestContext</code> class, that contains e.g. the username of the caller. A middleware/MVC filter can request it and fill out the info, and other components down the line can also request for it, and it will surely contain the info for the current request.

Singleton components are shared always, so they are best for thread-safe components that do not need to be bound to a request. An example would be <code>IOptions</code>, which gives access to configuration settings. An <code>HttpClient</code> wrapper class that uses <code>SendAsync</code> on a single static <code>HttpClient</code> instance would also be completely thread-safe, and a good candidate for being a Singleton.

Note that if you have a Singleton component that depends on a Scoped component, its dependency would get disposed before it. Thus a component cannot depend on another component that has smaller scope than itself.

blocks|key|27939|text|我看过“只使用AddTransient<T>()”的观点，但我不同意。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|27940|考虑内存分配|27941|我讨厌在不必要的时候分配东西，所以如果我知道我正在创建线程安全的东西，或者我有明确的文档说明拥有一个单例实例是预期的用法，那么我就是在创建一个单例实例。|27942|AddSingleton()|27943|下面是作为单例的ApplicationInsights+TelemetryClient实例。他们的文档说这是可行的。|27944|telemetryClient+=+new+TelemetryClient(TelemetryConfiguration.Active);
services.AddSingleton<TelemetryClient>(telemetryClient);|code-block|syntax|javascript|27945|在这个项目中，我也使用了Azure+Table+Storage，我发现将CloudTableClient创建为单例效果很好。我不需要为每个请求都创建它的实例。|27946|CloudStorageAccount+storageAccount+=+CloudStorageAccount.Parse(Configuration.GetValue<string>("storageAccountConnectionString"));
CloudTableClient+someTableClient+=+storageAccount.CreateCloudTableClient();
services.AddSingleton<CloudTableClient>(someTableClient);|27947|在某种意义上，它等同于类的只读静态属性，例如：|27948|public+static+readonly+CloudTableClient+cloudTableClient+=+new+CloudTableClient(...);|27949|..。在整个应用程序中只有一个它的实例，但是通过使用services.AddSingleton<T>()，我们可以通过依赖注入直接访问它。|27950|AddScoped()|27951|对我来说，AddScoped<T>()的一个例子是，我想要将获取应用程序洞察所需的JavaScript嵌入到web页面中，但我使用的是Content-Security-Policy，因此我需要在页面上的任何JavaScript上放置一个现时值。我有一些代码可以帮助我做到这一点。|27952|services.AddScoped<ApplicationInsightsJsHelper>();|27953|AddTransient()|27954|我还没有发现需要使用AddTransient<T>()做任何事情。这可能是我不认为我必须创建的东西，每次我需要它们时，作为“服务”……它们只是我新添加的变量。从某种意义上说，AddTransient<T>()是工厂模式的一种隐藏用途……您可以(有效地)让依赖项注入为您做同样的事情，而不是调用静态MyServiceFactory.Create()函数。|27955|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src^0|7|H|0|0|0|0|0|0|0|0|0|0|Q|Q|0|0|5|E|1V|N|0|0|0|0|A|H|2F|H|44|P|0^^$0|@$1|2|3|4|5|6|7|1K|8|@$9|1L|A|1M|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1N|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|1O|8|@]|D|@]|E|$]]|$1|J|3|K|5|6|7|1P|8|@]|D|@]|E|$]]|$1|L|3|M|5|6|7|1Q|8|@]|D|@]|E|$]]|$1|N|3|O|5|P|7|1R|8|@]|D|@]|E|$Q|R]]|$1|S|3|T|5|6|7|1S|8|@]|D|@]|E|$]]|$1|U|3|V|5|P|7|1T|8|@]|D|@]|E|$Q|R]]|$1|W|3|X|5|6|7|1U|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|P|7|1V|8|@]|D|@]|E|$Q|R]]|$1|10|3|11|5|6|7|1W|8|@$9|1X|A|1Y|B|C]]|D|@]|E|$]]|$1|12|3|13|5|6|7|1Z|8|@]|D|@]|E|$]]|$1|14|3|15|5|6|7|20|8|@$9|21|A|22|B|C]]|D|@$9|23|A|24|1|25]]|E|$]]|$1|16|3|17|5|P|7|26|8|@]|D|@]|E|$Q|R]]|$1|18|3|19|5|6|7|27|8|@]|D|@]|E|$]]|$1|1A|3|1B|5|6|7|28|8|@$9|29|A|2A|B|C]|$9|2B|A|2C|B|C]|$9|2D|A|2E|B|C]]|D|@]|E|$]]|$1|1C|3|-4|5|6|7|2F|8|@]|D|@]|E|$]]]|1D|$1E|$5|1F|1G|1H|E|$1I|1J]]]]

I've seen the &quot;just use <code>AddTransient&lt;T&gt;()</code>&quot; view, but I don't agree.
<h2>Think about memory allocation</h2>
I hate allocating things when I don't have to, so if I know I'm creating something that's thread-safe, or that I have explicit documentation that having a singleton instance is the expected usage, then I'm creating a singleton.
<h2>AddSingleton()</h2>
Here's the ApplicationInsights TelemetryClient instance as a singleton. Their documentation says this works.
<pre><code>telemetryClient = new TelemetryClient(TelemetryConfiguration.Active);
services.AddSingleton&lt;TelemetryClient&gt;(telemetryClient);
</code></pre>
In this project I use Azure Table Storage as well, and I've found that creating a CloudTableClient as a singleton works just fine. I don't need to keep creating instances of it for every request.
<pre><code>CloudStorageAccount storageAccount = CloudStorageAccount.Parse(Configuration.GetValue&lt;string&gt;(&quot;storageAccountConnectionString&quot;));
CloudTableClient someTableClient = storageAccount.CreateCloudTableClient();
services.AddSingleton&lt;CloudTableClient&gt;(someTableClient);
</code></pre>
In some sense, it's the equivalent of a class's readonly static property, for instance:
<pre><code>public static readonly CloudTableClient cloudTableClient = new CloudTableClient(...);
</code></pre>
... there's only one instance of it in the whole application, but by using <code>services.AddSingleton&lt;T&gt;()</code> we get direct access to it using Dependency Injection.
<h2>AddScoped()</h2>
An example of <code>AddScoped&lt;T&gt;()</code> for me is that I want to embed the JavaScript that I need to get Application Insights into a web page, but I use <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src" rel="nofollow noreferrer">Content-Security-Policy</a>, so I need to put a nonce on any on-page JavaScript. I have a little bit of code that helps me do it.
<pre><code>services.AddScoped&lt;ApplicationInsightsJsHelper&gt;();
</code></pre>
<h2>AddTransient()</h2>
I haven't found a need to use <code>AddTransient&lt;T&gt;()</code> for anything yet. It could be that I don't think of things that I have to create, every time I need them, as &quot;services&quot;... they're just variables that I new up. In some sense, <code>AddTransient&lt;T&gt;()</code> is a hidden use of the Factory pattern... instead of calling a static <code>MyServiceFactory.Create()</code> function, you have Dependency Injection (effectively) do the same thing for you.

After reading <a href="https://stackoverflow.com/questions/38138100/what-is-the-difference-between-services-addtransient-service-addscope-and-servi">this</a> post I can understand the differences between <code>AddTransient</code>,<code>AddScoped</code> and <code>AddSingleton</code> however, I am unable to see the practical usage of each of them. 

My understanding is 

AddTransient

Creates a new instance every time when the client asks for it.

<pre><code>services.AddTransient&lt;IDataAccess, DataAccess&gt;();
</code></pre>

will return a new DataAccess object every time a client code asks for it. More likely a constructor.

Usage of AddTransient

In cases when we have to access a database to read and update it and destroy the access object (DataAccess) its best to use <code>AddTransient</code> - Not sure about the thread safty.

AddScoped

Creates a new instance for each http web request.

Usage of AddScoped

<pre><code> services.AddScoped&lt;ShoppingCart&gt;(serviceProvider =&gt; ShoppingCart.GetShoppingCart(serviceProvider));
</code></pre>

this mean each web request will be having its own shopping cart instance which intern means each user / client will be having its own shoping cart instance for that http web request.

AddSingleton

Create single instance for all the http web requests.

Usage of AddSingleton

Found this code in an sample application but I dont understand how it is being useful. 

<pre><code>services.AddSingleton&lt;IHttpContextAccessor, HttpContextAccessor&gt;(); 
</code></pre>

Can someone please give a decent practical example when to use AddSingleton and check if my understanding of AddTransient and AddScoped is correct?

What are the practical scenarios to use IServiceCollection.AddTransient, IServiceCollection.AddSingleton and IServiceCollectionAddScoped Methods?

中间件

数据库

在阅读了的帖子后，我可以理解AddTransient，AddScoped和AddSingleton之间的区别，但是我看不到它们各自的实际用法。我的理解是AddTransient每次客户端请求时，都会创建一个新实例。services.AddTransient<IDataAccess, DataAccess>();每次客户...

问使用IServiceCollection.AddTransient、IServiceCollection.AddSingleton和IServiceCollectionAddScoped方法的实际场景是什么？
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