c# - 通过 StructureMap 定义 'HttpClient' 单例导致运行时未配置 'HttpMessageHandler' 的错误

Question

尝试在 StructureMap ala 中定义一个 HttpClient 单例:

For<HttpClient>().Singleton().UseIfNone<HttpClient>();

这会导致运行时出现以下错误(依赖注入(inject)时):

   StructureMap.StructureMapConfigurationException: No default Instance is registered and cannot be automatically determined for type 'System.Net.Http.HttpMessageHandler'

   There is no configuration specified for System.Net.Http.HttpMessageHandler

   1.) new HttpClient(*Default of HttpMessageHandler*)
   2.) System.Net.Http.HttpClient
   3.) Instance of System.Net.Http.HttpClient
   4.) new AdmanAdapter(*Default of HttpClient*)
   5.) Organotiki.vNext.PostEval.Data.Adapters.ADMAN.AdmanAdapter
   6.) Instance of [....]

      at lambda_method(Closure , IBuildSession , IContext )
      at StructureMap.Building.BuildPlan.Build(IBuildSession session, IContext context)
      at StructureMap.BuildSession.BuildNewInSession(Type pluginType, Instance instance)
      at StructureMap.Pipeline.NulloTransientCache.Get(Type pluginType, Instance instance, IBuildSession session)
      at StructureMap.BuildSession.ResolveFromLifecycle(Type pluginType, Instance instance)
      at StructureMap.SessionCache.GetObject(Type pluginType, Instance instance, ILifecycle lifecycle)

如果我们也像这样配置 HttpMessageHandler:

For<HttpClient>().Singleton().UseIfNone<HttpClient>();
For<HttpMessageHandler>().UseIfNone(x => new HttpClientHandler());

然后问题就解决了。问题是为什么？ HttpClient 的默认构造函数负责其自身的依赖注入(inject):

/// <summary>Initializes a new instance of the <see cref="T:System.Net.Http.HttpClient" /> class.</summary>
[__DynamicallyInvokable]
public HttpClient()
  : this((HttpMessageHandler) new HttpClientHandler())
{
}

我是不是漏掉了什么？

Answer 1

来自 http://structuremap.github.io/registration/constructor-selection 的结构图文档

If there are multiple public constructor functions on a concrete class, StructureMap's default behavior is to select the "greediest" constructor, i.e., the constructor function with the most parameters.

如果您查看HttpClient 的可能构造函数，它应该是

public HttpClient();
public HttpClient(HttpMessageHandler handler);
public HttpClient(HttpMessageHandler handler, bool disposeHandler);