asp.net mvc 4 IDependencyResolver与Unity

Question

我是MVC 4世界的新手（昨天刚刚开始）而且我是 看着DI团结一致。

我正在使用IDependencyResolver来获取/解决依赖关系。

如果我使用此实现，一切正常

Public Class UnityDependencyResolver : Implements IDependencyResolver

    Private _unity As IUnityContainer

    Public Sub New(ByVal icontainer As IUnityContainer)
        _unity = icontainer
    End Sub

    Public Function GetService(serviceType As Type) As Object Implements IDependencyResolver.GetService
        Dim _obj As Object = Nothing

        Try
            _obj = _unity.Resolve(serviceType)
        Catch ex As Exception
        End Try

        Return _obj
    End Function

    Public Function GetServices(serviceType As Type) As IEnumerable(Of Object) Implements IDependencyResolver.GetServices
        Dim _list As List(Of Object) = New List(Of Object)

        If _unity.IsRegistered(serviceType) Then
            _list = _unity.ResolveAll(serviceType)
        End If

        Return _list
    End Function
End Class

但如果我使用它，我会收到异常

Public Class UnityDependencyResolver : Implements IDependencyResolver

    Private _unity As IUnityContainer

    Public Sub New(ByVal icontainer As IUnityContainer)
        _unity = icontainer
    End Sub

    Public Function GetService(serviceType As Type) As Object Implements IDependencyResolver.GetService
        Dim _obj As Object = Nothing

        If _unity.IsRegistered(serviceType) Then
            _obj = _unity.Resolve(serviceType)
        End If

        Return _obj
    End Function

    Public Function GetServices(serviceType As Type) As IEnumerable(Of Object) Implements IDependencyResolver.GetServices
        Dim _list As List(Of Object) = New List(Of Object)

        If _unity.IsRegistered(serviceType) Then
            _list = _unity.ResolveAll(serviceType)
        End If

        Return _list
    End Function
End Class

在Global.asax文件中，我设置了

...
DependencyResolver.SetResolver(New UnityDependencyResolver(unity))

如果我不把_unity.Resolve放进去，为什么我会得到例外 尝试抓住？