blocks|key|138368|text|与大容量网站相关，但不直接与HttpClient相关。我们在所有的服务中都有下面的代码片段。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|138369|++++++++//+number+of+milliseconds+after+which+an+active+System.Net.ServicePoint+connection+is+closed.
++++++++const+int+DefaultConnectionLeaseTimeout+=+60000;

++++++++ServicePoint+sp+=
++++++++++++++++ServicePointManager.FindServicePoint(new+Uri("http://<yourServiceUrlHere>"));
++++++++sp.ConnectionLeaseTimeout+=+DefaultConnectionLeaseTimeout;|code-block|syntax|javascript|138370|来自https://msdn.microsoft.com/query/dev14.query?appId=Dev14IDEF1&l=EN-US&k=k(System.Net.ServicePoint.ConnectionLeaseTimeout);k(TargetFrameworkMoniker-.NETFramework,Version%253Dv4.5.2);k(DevLang-csharp)&rd=true|offset|length|138371|“您可以使用这个属性来确保ServicePoint对象的活动连接不会无限期地保持打开状态。这个属性适用于应该定期删除和重新建立连接的情况，例如负载平衡情况。|138372|默认情况下，当请求的KeepAlive为true时，MaxIdleTime属性将设置由于不活动而关闭ServicePoint连接的超时时间。如果ServicePoint具有活动连接，则MaxIdleTime无效，并且连接将无限期地保持打开状态。|138373|当ConnectionLeaseTimeout属性设置为非-1的值时，在经过指定的时间后，通过将请求中的KeepAlive设置为false，活动的ServicePoint连接将在为该请求提供服务后关闭。设置此值会影响ServicePoint对象管理的所有连接。“|138374|当您有要故障转移的CDN或其他端点后的服务时，此设置可帮助呼叫者跟随您转到新目标。在此示例中，在故障转移后60秒，所有调用方都应重新连接到新端点。它确实要求您知道您的依赖服务(您调用的那些服务)及其端点。|138375|entityMap|0|LINK|mutability|MUTABLE|url|https://msdn.microsoft.com/query/dev14.query?appId=Dev14IDEF1&l=EN-US&k=k(System.Net.ServicePoint.ConnectionLeaseTimeout);k(TargetFrameworkMoniker-.NETFramework,Version=v4.5.2);k(DevLang-csharp)&rd=true^0|0|0|2|5O|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|11|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|12|8|@]|9|@$I|13|J|14|1|15]]|A|$]]|$1|K|3|L|5|6|7|16|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|17|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|18|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|19|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]]|T|$U|$5|V|W|X|A|$Y|Z]]]]

Related to high-volume web sites but not directly to HttpClient. We have the snippet of code below in all of our services.

<pre><code> // number of milliseconds after which an active System.Net.ServicePoint connection is closed.
 const int DefaultConnectionLeaseTimeout = 60000;

 ServicePoint sp =
 ServicePointManager.FindServicePoint(new Uri("http://&lt;yourServiceUrlHere&gt;"));
 sp.ConnectionLeaseTimeout = DefaultConnectionLeaseTimeout;
</code></pre>

From <a href="https://msdn.microsoft.com/query/dev14.query?appId=Dev14IDEF1&amp;l=EN-US&amp;k=k(System.Net.ServicePoint.ConnectionLeaseTimeout);k(TargetFrameworkMoniker-.NETFramework,Version%3Dv4.5.2);k(DevLang-csharp)&amp;rd=true" rel="nofollow noreferrer">https://msdn.microsoft.com/query/dev14.query?appId=Dev14IDEF1&amp;l=EN-US&amp;k=k(System.Net.ServicePoint.ConnectionLeaseTimeout);k(TargetFrameworkMoniker-.NETFramework,Version%3Dv4.5.2);k(DevLang-csharp)&amp;rd=true</a>

"You can use this property to ensure that a ServicePoint object's active connections do not remain open indefinitely. This property is intended for scenarios where connections should be dropped and reestablished periodically, such as load balancing scenarios.

By default, when KeepAlive is true for a request, the MaxIdleTime property sets the time-out for closing ServicePoint connections due to inactivity. If the ServicePoint has active connections, MaxIdleTime has no effect and the connections remain open indefinitely.

When the ConnectionLeaseTimeout property is set to a value other than -1, and after the specified time elapses, an active ServicePoint connection is closed after servicing a request by setting KeepAlive to false in that request.
Setting this value affects all connections managed by the ServicePoint object."

When you have services behind a CDN or other endpoint that you want to failover then this setting helps callers follow you to your new destination. In this example 60 seconds after a failover all callers should re-connect to the new endpoint. It does require that you know your dependent services (those services that YOU call) and their endpoints.

blocks|key|4645024|text|正如其他答案所述，HttpClient是用于重用的。但是，在多线程应用程序中重用单个HttpClient实例意味着您不能更改其有状态属性的值，如BaseAddress和DefaultRequestHeaders+(因此，只有当它们在应用程序中保持不变时才能使用它们)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4645025|绕过这一限制的一种方法是用一个类包装HttpClient，该类复制您需要的所有HttpClient方法(GetAsync、PostAsync等)，并将它们委托给一个单独的HttpClient。然而，这是相当繁琐的(您将需要包装extension+methods，幸运的是there+is+another+way+-不断创建新的HttpClient实例，但重用底层HttpClientHandler。只需确保您不会丢弃处理程序：|4645026|HttpClientHandler+_sharedHandler+=+new+HttpClientHandler();+//never+dispose+this
HttpClient+GetClient(string+token)
{
++++//client+code+can+dispose+these+HttpClient+instances
++++return+new+HttpClient(_sharedHandler,+disposeHandler:+false)+++++++++
++++{
+++++++DefaultRequestHeaders+=+
+++++++{
++++++++++++Authorization+=+new+AuthenticationHeaderValue("Bearer",+token)+
+++++++}+
++++};
}|code-block|syntax|javascript|4645027|entityMap|0|LINK|mutability|MUTABLE|url|https://msdn.microsoft.com/en-us/library/system.net.http.httpclientextensions(v=vs.118%2529.aspx)%2520too|1|https://github.com/dotnet/corefx/issues/5811^0|9|A|16|A|20|B|2C|L|0|I|A|13|A|1G|8|1P|9|2D|A|4J|A|51|H|35|H|0|3R|K|1|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|15|8|@$9|16|A|17|B|C]|$9|18|A|19|B|C]|$9|1A|A|1B|B|C]|$9|1C|A|1D|B|C]|$9|1E|A|1F|B|C]|$9|1G|A|1H|B|C]|$9|1I|A|1J|B|C]]|D|@$9|1K|A|1L|1|1M]|$9|1N|A|1O|1|1P]]|E|$]]|$1|H|3|I|5|J|7|1Q|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|1R|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]|U|$5|P|Q|R|E|$S|V]]]]

As the other answers state, <code>HttpClient</code> is meant for reuse. However, reusing a single <code>HttpClient</code> instance across a multi-threaded application means you can't change the values of its stateful properties, like <code>BaseAddress</code> and <code>DefaultRequestHeaders</code> (so you can only use them if they are constant across your application).

One approach for getting around this limitation is wrapping <code>HttpClient</code> with a class that duplicates all the <code>HttpClient</code> methods you need (<code>GetAsync</code>, <code>PostAsync</code> etc) and delegates them to a singleton <code>HttpClient</code>. However that's pretty tedious (you will need to wrap the <a href="https://msdn.microsoft.com/en-us/library/system.net.http.httpclientextensions(v=vs.118).aspx" rel="noreferrer">extension methods</a> too), and fortunately <a href="https://github.com/dotnet/corefx/issues/5811" rel="noreferrer">there is another way</a> - keep creating new <code>HttpClient</code> instances, but reuse the underlying <code>HttpClientHandler</code>. Just make sure you don't dispose the handler:

<pre><code>HttpClientHandler _sharedHandler = new HttpClientHandler(); //never dispose this
HttpClient GetClient(string token)
{
 //client code can dispose these HttpClient instances
 return new HttpClient(_sharedHandler, disposeHandler: false) 
 {
 DefaultRequestHeaders = 
 {
 Authorization = new AuthenticationHeaderValue("Bearer", token) 
 } 
 };
}
</code></pre>

blocks|key|4645063|text|需要指出的一件事是，“不要使用”博客注意事项中没有一条是您需要考虑的不仅仅是BaseAddress和DefaultHeader。一旦您将HttpClient设置为静态，就会有内部状态在请求之间传递。举个例子:你使用HttpClient向第三方认证以获得一个FedAuth令牌(忽略为什么不使用OAuth/OWIN/等)，该响应消息有一个用于FedAuth的Set-Cookie头，这将被添加到你的HttpClient状态。下一个登录到您的应用编程接口的用户将发送最后一个人的FedAuth+cookie，除非您在每次请求时管理这些cookie。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4645064|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

One thing to point out, that none of the "don't use using" blogs note is that it is not just the BaseAddress and DefaultHeader that you need to consider. Once you make HttpClient static, there are internal states that will be carried across requests. An example: You are authenticating to a 3rd party with HttpClient to get a FedAuth token (ignore why not using OAuth/OWIN/etc), that Response message has a Set-Cookie header for FedAuth, this is added to your HttpClient state. The next user to login to your API will be sending the last person's FedAuth cookie unless you are managing these cookies on each request.

What should be the <code>HttpClient</code> lifetime of a WebAPI client? 
 Is it better to have one instance of the <code>HttpClient</code> for multiple calls?

What's the overhead of creating and disposing a <code>HttpClient</code> per request, like in example below (taken from <a href="http://www.asp.net/web-api/overview/web-api-clients/calling-a-web-api-from-a-net-client" rel="noreferrer">http://www.asp.net/web-api/overview/web-api-clients/calling-a-web-api-from-a-net-client</a>): 

<pre><code>using (var client = new HttpClient())
{
 client.BaseAddress = new Uri("http://localhost:9000/");
 client.DefaultRequestHeaders.Accept.Clear();
 client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

 // New code:
 HttpResponseMessage response = await client.GetAsync("api/products/1");
 if (response.IsSuccessStatusCode)
 {
 Product product = await response.Content.ReadAsAsync&lt;Product&gt;();
 Console.WriteLine("{0}\t${1}\t{2}", product.Name, product.Price, product.Category);
 }
}
</code></pre>

What is the overhead of creating a new HttpClient per call in a WebAPI client?

OAuth

WebAPI客户端的HttpClient生存期应该是多久？让一个HttpClient实例用于多个调用是不是更好？创建和处理每个请求的HttpClient的开销是多少，如下面的示例(取自)：using (var client = new HttpClient()){    client.BaseAddress = new...

问在WebAPI客户端中每次调用创建一个新的HttpClient的开销是多少？
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问在WebAPI客户端中每次调用创建一个新的HttpClient的开销是多少？
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