在 await 块中调用 Request.CreateResponse() 时 WebApi 会产生什么风险？

Question

我有一个异步 WebApi 控制器，在 await 块内调用 Request.CreateResponse(HttStatusCode.Created, result) 时会引发 StructureMapException，但仅在第一个请求期间。 StructureMap 正确解析了所有构造函数依赖项，但表现得好像在请求结束后调用了 Request.CreateResponse。在 await 块中使用 Request.CreateResponse 只是一个坏主意吗？这种策略可能会带来哪些其他风险？

我正在使用以下 NuGet 包：

• 结构图 3.1.6.186
• StructureMap.Web 3.1.0.133
• StructureMap.WebApi2 3.0.4.125
• WebActivator 2.0.5

代码如下：

public class EmailController : ApiController
{
    private readonly ISaveEmailNotification _saveEmailNotification;
    private readonly ISaveFile _saveFile;

    public EmailController(ISaveEmailNotification saveEmailNotification, ISaveFile saveFile)
    {
        _saveEmailNotification = saveEmailNotification;
        _saveFile = saveFile;
    }

    [HttpPost]
    public async Task<HttpResponseMessage> SaveEmail(NotificationRequest model)
    {
        return await Task.Run(() =>
        {
            var emailResult = _saveEmailNotification.Execute(model);
            var fileResult = _saveFile.Execute(model, emailResult);
            return Request.CreateResponse(HttpStatusCode.Created, fileResult);
        }
    }
}

这是错误：

StructureMap.StructureMapException was unhandled by user code
HResult=-2146233088
Message=You cannot use the HttpContextLifecycle outside of a web request. Try the HybridLifecycle instead.
1.) Container.GetInstance(System.Web.HttpContextBase)
2.) Container.TryGetInstance(System.Web.HttpContextBase)

Source=StructureMap.Web
Title=You cannot use the HttpContextLifecycle outside of a web request. Try the HybridLifecycle instead.
StackTrace:
   at StructureMap.Web.Pipeline.HttpContextLifecycle.findHttpDictionary() in c:\BuildAgent\work\996e173a8ceccdca\src\StructureMap.Web\Pipeline\HttpContextLifecycle.cs:line 57
   at StructureMap.Web.Pipeline.HttpContextLifecycle.FindCache(ILifecycleContext context) in c:\BuildAgent\work\996e173a8ceccdca\src\StructureMap.Web\Pipeline\HttpContextLifecycle.cs:line 20
   at StructureMap.BuildSession.ResolveFromLifecycle(Type pluginType, Instance instance) in c:\BuildAgent\work\996e173a8ceccdca\src\StructureMap\BuildSession.cs:line 102
   at StructureMap.SessionCache.GetObject(Type pluginType, Instance instance, ILifecycle lifecycle) in c:\BuildAgent\work\996e173a8ceccdca\src\StructureMap\SessionCache.cs:line 88
   at StructureMap.SessionCache.GetDefault(Type pluginType, IPipelineGraph pipelineGraph) in c:\BuildAgent\work\996e173a8ceccdca\src\StructureMap\SessionCache.cs:line 66
   at StructureMap.Container.GetInstance(Type pluginType) in c:\BuildAgent\work\996e173a8ceccdca\src\StructureMap\Container.cs:line 335
   at StructureMap.Container.TryGetInstance(Type pluginType) in c:\BuildAgent\work\996e173a8ceccdca\src\StructureMap\Container.cs:line 278
   at StructureMap.Container.TryGetInstance[T]() in c:\BuildAgent\work\996e173a8ceccdca\src\StructureMap\Container.cs:line 289
   at NotificationHubWeb.DependencyResolution.StructureMapDependencyScope.get_HttpContext() in C:\Users\Dave\Source\Workspaces\NotificationHub\Main\NotificationHubWeb\DependencyResolution\StructureMapDependencyScope.cs:line 68
   at NotificationHubWeb.DependencyResolution.StructureMapDependencyScope.get_CurrentNestedContainer() in C:\Users\Dave\Source\Workspaces\NotificationHub\Main\NotificationHubWeb\DependencyResolution\StructureMapDependencyScope.cs:line 55
   at NotificationHubWeb.DependencyResolution.StructureMapDependencyScope.DoGetInstance(Type serviceType, String key) in C:\Users\Dave\Source\Workspaces\NotificationHub\Main\NotificationHubWeb\DependencyResolution\StructureMapDependencyScope.cs:line 109
   at Microsoft.Practices.ServiceLocation.ServiceLocatorImplBase.GetInstance(Type serviceType, String key) in c:\Projects\CommonServiceLocator\main\Microsoft.Practices.ServiceLocation\ServiceLocatorImplBase.cs:line 49

Answer 1

当我这样重写代码时，看起来在 await 块之外使用 Request.CreateResponse 确实会在首次使用时避开 StructureMapException：

[HttpPost]
public async Task<HttpResponseMessage> SaveEmail(NotificationRequest model)
{
    var taskResult await Task.Run(() =>
    {
        var emailResult = _saveEmailNotification.Execute(model);
        return _saveFile.Execute(model, emailResult);
    }

    return Request.CreateResponse(HttpStatusCode.Created, taskResult);
}

现在，这看起来像是赢家，但如果你能解释为什么将 Request.CreateReponse 移到 await 块之外可以让代码无错误地运行，我会投票支持或宣布你为回答者。