Question

尝试在StructureMap ala中定义HttpClient单例：

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

这会导致运行时出现以下错误（依赖项注入时）：

   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)

如果我们还像这样配置HttpMessageHandler：

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

然后问题解决了。问题是为什么？ HttpClient的默认构造函数负责其自己的依赖项注入：

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

我在这里想念东西吗？

Answer 1

摘自documentation上的structuremap文档

如果一个混凝土上有多个公共构造函数类，StructureMap的默认行为是选择“最贪婪” 构造函数，即参数最多的构造函数。

如果您查看HttpClient的可能构造函数，则应该为

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

Answer 2

扩展@Brad M的答案，对我有用的是.SelectConstructor(() => new HttpClient())。指定应显式使用哪个构造函数。

通过StructureMap定义“ HttpClient”单例会导致有关在运行时未配置“ HttpMessageHandler”的错误

2 个答案: