WCF Custom ServiceBehavior/InstanceProvider with parameterless constructor

We're trying to use Dependency Injection for a WCF Service. The Service has a dependency on a Unity Container. The container is used to find the appropriate class that implements an IJob Interface (based on a JobKey parameter in the method call) and calls a method on it.

The Service is being hosted in MVC2. I've omitted as much irrelevant stuff as possible from the snippets below. Full code available if required...

What I've done so far:

Based on this MSDN Article, I've created a custom InstanceProvider which should instantiate my service and pass it a container.

I then created a very noddy ServiceBehavior to use the InstanceProvider and finally a BehaviorExtension which just returns the ServiceBehavior.

Public Class WCFDIInstanceProvider
    Implements IInstanceProvider

    Private ServiceType As Type

    Private Property _Container As IUnityContainer
    Private ReadOnly Property Container As IUnityContainer
        Get
            If _Container Is Nothing Then
                _Container = InitialiseContainer()
            End If
            Return _Container
        End Get
    End Property

    Public Sub New(ByVal ServiceType As Type)
        Me.ServiceType = ServiceType
    End Sub

    Private Function InitialiseContainer() As IUnityContainer
            'Code which scans assemblies and populates the container as appropriate
            'I'm confident this code works as I've tested it elsewhere
        Return Container
    End Function

    Public Function GetInstance(ByVal instanceContext As System.ServiceModel.InstanceContext) As Object Implements System.ServiceModel.Dispatcher.IInstanceProvider.GetInstance
        Return GetInstance(instanceContext, Nothing)
    End Function

    Public Function GetInstance(ByVal instanceContext As System.ServiceModel.InstanceContext, ByVal message As System.ServiceModel.Channels.Message) As Object Implements System.ServiceModel.Dispatcher.IInstanceProvider.GetInstance
        Return Container.Resolve(Me.ServiceType)
    End Function

End Class

And the ServiceBehavior:

Public Class WCFDIServiceBehavior
Implements IServiceBehavior

    Public Sub ApplyDispatchBehavior(ByVal serviceDescription As System.ServiceModel.Description.ServiceDescription, ByVal serviceHostBase As System.ServiceModel.ServiceHostBase) Implements System.ServiceModel.Description.IServiceBehavior.ApplyDispatchBehavior
        For Each ChannelDispatcherBase As ChannelDispatcherBase In serviceHostBase.ChannelDispatchers
            Dim ChannelDispatcher As ChannelDispatcher = TryCast(ChannelDispatcherBase, ChannelDispatcher)
            If ChannelDispatcher IsNot Nothing Then
                For Each Dispatcher As EndpointDispatcher In ChannelDispatcher.Endpoints
                    Dispatcher.DispatchRuntime.InstanceProvider = New WCFDIInstanceProvider(serviceDescription.ServiceType)
                Next
            End If
        Next
    End Sub

And finally, the really noddy BehaviorExtension:

Public Class WCFDIBehaviorExtension
    Inherits BehaviorExtensionElement

    Public Overrides ReadOnly Property BehaviorType As System.Type
        Get
            Return GetType(WCFDIServiceBehavior)
        End Get
    End Property

    Protected Overrides Function CreateBehavior() As Object
        Return New WCFDIServiceBehavior
    End Function
End Class

The WCF Config tool seems to like all of the above and has generated the following Config XML:

  <serviceHostingEnvironment multipleSiteBindingsEnabled="true"
                             aspNetCompatibilityEnabled="true">
      <serviceActivations>
          <add relativeAddress="WebJob.svc"
               service="MyApplication.WebJobService"
               factory="System.ServiceModel.Activation.ServiceHostFactory" />
      </serviceActivations>
  </serviceHostingEnvironment>
<standardEndpoints>
  <mexEndpoint>
    <standardEndpoint name="WebJobServiceMex" />
  </mexEndpoint>
</standardEndpoints>
<behaviors>
  <serviceBehaviors>
    <behav开发者_StackOverflow社区ior name="WCFDIServiceBehavior">
      <serviceMetadata httpGetEnabled="true" />
      <serviceDebug includeExceptionDetailInFaults="true" />
      <WCFDIBehavior />
    </behavior>
  </serviceBehaviors>
</behaviors>
<services>
  <service name="WebJobService">
    <endpoint address="" binding="basicHttpBinding" bindingConfiguration="httpBinding"
      name="HTTPEndpoint" contract="MyApplication.JobService.Common.IWebJobService" />
    <endpoint binding="mexTcpBinding" bindingConfiguration="" name="mexEndpoint" />
  </service>
</services>

The exception I get is:

System.ServiceModel.ServiceActivationException: The service '/MyAppDir/WebJob.svc' cannot be activated due to an exception during compilation. The exception message is: The service type provided could not be loaded as a service because it does not have a default (parameter-less) constructor. To fix the problem, add a default constructor to the type, or pass an instance of the type to the host.

System.InvalidOperationException: The service type provided could not be loaded as a service because it does not have a default (parameter-less) constructor. To fix the problem, add a default constructor to the type, or pass an instance of the type to the host.

WebHost failed to process a request.
 Sender Information: System.ServiceModel.ServiceHostingEnvironment+HostingManager/13982700
 Exception: System.ServiceModel.ServiceActivationException: The service '/MyAppDir/WebJob.svc' cannot be activated due to an exception during compilation.  The exception message is: The service type provided could not be loaded as a service because it does not have a default (parameter-less) constructor. To fix the problem, add a default constructor to the type, or pass an instance of the type to the host.. ---> System.InvalidOperationException: The service type provided could not be loaded as a service because it does not have a default (parameter-less) constructor. To fix the problem, add a default constructor to the type, or pass an instance of the type to the host.
   at System.ServiceModel.Dispatcher.InstanceBehavior..ctor(DispatchRuntime dispatch, ImmutableDispatchRuntime immutableRuntime)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime..ctor(DispatchRuntime dispatch)
   at System.ServiceModel.Dispatcher.DispatchRuntime.GetRuntimeCore()
   at System.ServiceModel.Dispatcher.ChannelDispatcher.OnOpened()
   at System.ServiceModel.Channels.CommunicationObject.Open(TimeSpan timeout)
   at System.ServiceModel.ServiceHostBase.OnOpen(TimeSpan timeout)
   [Blah]
 Process Name: w3wp
 Process ID: 2108

Which makes sense assuming it's not applying my custom ServiceBehavior - The default ServiceBehavior's InstaceProvider can't instantiate the service.

Some things to note: If I add a parameterless constructor to my service, I don't get an exception (but of course, I don't get passed a container either) so I'm fairly confident I've found the source of the problem

Can someone please point out what I'm doing wrong?

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I guess that problem is in <service name="WebJobService">. Service element does not contain behaviorConfiguration. Also name attribute must normally contain type name with namespaces. So in your case it should be MyApplication.WebJobService.

You can also check these articles for alternative implementations 1, 2.

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I'd simplify the problem. It's not about Unity, it's about instantiating your service and passing a parameter to the constructor. If that's an issue, consider using property injection.

I'd normally have the service instantiate the Unity container, which avoids the issue entirely.

"Code which scans assemblies and populates the container" makes it sound like MEF might be a better fit for you than Unity.

EDIT: I think I'm wrong to recommend property injection. You don't want Unity to create the instance of the service at all. Since UnityContainer.Resolve is thread safe (although configuration requires locking), you could have the service create and own a static instance of the container. The container instance will resolve other dependencies.