Is the DI pattern limiting wrt expensive object cr

问题:

I'm having a hard time getting my head around what seems like an obvious pattern problem/limitation when it comes to typical constructor dependency injection. For example purposes, lets say I have an ASP.NET MVC3 controller that looks like:

Public Class MyController
    Inherits Controller

    Private ReadOnly mServiceA As IServiceA
    Private ReadOnly mServiceB As IServiceB
    Private ReadOnly mServiceC As IServiceC

    Public Sub New(serviceA As IServiceA, serviceB As IServiceB, serviceC As IServiceC)
        Me.mServiceA = serviceA
        Me.mServiceB = serviceB
        Me.mServiceC = serviceC
    End Sub

    Public Function ActionA() As ActionResult
        ' Do something with Me.mServiceA and Me.mServiceB
    End Function

    Public Function ActionB() As ActionResult
        ' Do something with Me.mServiceB and Me.mServiceC
    End Function
End Class

The thing I'm having a hard time getting over is the fact that the DI container was asked to instantiate all three dependencies when at any given time only a subset of the dependencies may be required by the action methods on this controller.

It's seems assumed that object construction is dirt-cheep and there are no side effects from object construction OR all dependencies are consistently utilized. What if object construction wasn't cheep or there were side effects? For example, if constructing IServiceA involved opening a connection or allocating other significant resources, then that would be completely wasted time/resources when ActionB is called.

If these action methods used a service location pattern (or other similar pattern), then there would never be the chance to unnecessarily construct an object instance that will go unused, of course using this pattern has other issues attached making it unattractive.

Does using the canonical constructor injection + interfaces pattern of DI basically lock the developer into a "limitation" of sorts that implementations of the dependency must be cheep to instantiate or the instance must be significantly utilized? I know all patterns have their pros and cons, is this just one of DI's cons? I've never seen it mentioned before, which I find curious.

回答1:

If you have a lot of fields that aren't being used by every member this means that the class' cohesion is low. This is a general programming concept - Constructor Injection just makes it more visible. It's usually a pretty good indicator that the Single Responsibility Principle is being violated.

If that's the case then refactor (e.g. to Facade Services).

You don't have to worry about performance when creating object graphs.

When it comes to side effects, (DI) constructors should be simple and not have side effects.

回答2:

Generally speaking, there should be no major costs or side effects of object construction. This is a general statement that I believe applies to most (not all) objects, but is especially true for services that you would inject via DI. In other words, constructing a service class automatically makes a database/service call, or changes the state of your system in a way that would have side effects is (at least) a code smell.

Regarding instances that go unused: it's hard to create a system that has perfect utilization of instances within dependent classes, regardless of whether you use DI or not. I'm not sure achieving this is very important, as long as you are adhering to the Single Responsibility Principle. If you find that your class has too many services injected, or that utilization is really uneven, it might be a sign that your class is doing too much and needs to be split into two or more smaller classes with more focused responsibilities.

回答3:

No you are not tied to the limitations you have listed. As of .net 4 you do have Lazy(Of T) at your disposal, which will allow you to defer instantiation of your dependencies until required.

It is not assumed that object construction is dirt-cheap and consequently some DI containers support Lazy(Of T) out of the box. Whilst Unity 2.0 supports lazy initialization out of the box through automatic factories, there is a good article here on an extension supporting Lazy(Of T) the author has on MSDN.

回答4:

Isn't your controller a singleton though? That is the normal way to do it in Java. There is only one instance created. Also you could split the controller into multiple controllers if the roles of the actions is so distinct.