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Asked: 2026-05-24T11:46:01+00:00

I’m having a hard time getting my head around what seems like an obvious

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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.

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1 Answer

  1. Editorial Team

    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.

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