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Simple Injector auto-wiring with an Initialize()-Method

I'm using Simple injector successfully with constructor injection and I love the auto-wiring feature for the ctr-Arguments. Is there a similar auto-wire way for a initialize method?
So like instead of providing arguments to the constructor where doing async stuff is often impossible, things are provided to an Initialize() method with auto-wiring
public class A
{
// only std-ctor ()
[Init] // maybe mark needed?
void /*async*/ Initialize(IC c, IB b)
{
// get auto called by container and params auto-wired
// maybe do async stuff with b
}
}
// in Composition root (for simplicity):
container.RegisterSingleton<A>();
So the async possibility would be a plus feature.
[Update]
So async is reasonably not the feature you want in object graph creation. So let's not put the focus on that. But sometimes it may still be desireable to rather use init (i.e. if a ()-constructor is required). I realize that auto calling Initialize() by simple injector leads to nonsense. I just see the need sometimes to split object creation and initialization and auto-wiring is just way too cool. How would one then activate or implement that? So that simple Injector does auto-wire it for you? I hope I do not have to write an "initializer-class" where I have to 'pipe through' all dependencies. So that is the actual direction of the question (get ahold of Magic.AutoWire()).
public class SomeClassWithA
{
A _a;
public SomeClassWithA(A a)
{
_a=a;
}
void CalledLater()
{
Magic.AutoWire(_a);
// Initialize with auto-wire parameters was called
}
}

Although it is possible to extend Simple Injector to call an initialize method during object graph initialization, it is impossible to make this truly asynchronous, simply because Simple Injector lacks an asynchronous API. Without a "Task<T> GetInstanceAsync<T>()" method, the application would still have wait for I/O by blocking a thread, instead of using an I/O completion port, which would completely waste the benefits of using async.
But Simple Injector lacks such async feature and this is deliberate (and AFAIK most DI libraries lack such feature for the exact same reason). The construction of object graphs should be fast and should not consist of any I/O. Constructors (or initializers with dependencies) should do nothing more than store incoming dependencies, as Mark Seemann clearly stated here.
This basically means that you should delay any I/O till after object graph construction until the moment that an object is used for the first time. This not only keeps object construction simple, fast and reliable, it also makes it possible to verify the construction of your onject graph in isolation (without the existence of any I/O components such as a database or the file system).

Testable code: Attach event handler in constructor

as for my understanding, part of writing (unit-)testable code, a constructor should not do real work in constructor and only assigning fields. This worked pretty well so far. But I came across with a problem and I'm not sure what is the best way to solve it. See the code sample below.
class SomeClass
{
private IClassWithEvent classWithEvent;
public SomeClass(IClassWithEvent classWithEvent)
{
this.classWithEvent = classWithEvent;
// (1) attach event handler in ctor.
this.classWithEvent.Event += OnEvent;
}
public void ActivateEventHandling()
{
// (2) attach event handler in method
this.classWithEvent.Event += OnEvent;
}
private void OnEvent(object sender, EventArgs args)
{
}
}
For me option (1) sounds fine, but it the constructor should only assign fields. Option (2) feels a bit "too much".
Any help is appreciated.

A unit test would test SomeClass at most. Therefore you would typically mock classWithEvent. Using some kind of injection for classWithEvent in ctor is fine.
Just as Thomas Weller said wiring is field assignment.
Option 2 is actually bad IMHO. As if you omit a call to ActivateEventHandling you end up with a improperly initialized class and need to transport knowledge of the requirement to call ActivateEventHandling in comments or somehow else, which make the class harder to use and probably results in a class-usage that was not even tested by you, as you have called ActivateEventHandling and tested it but an uninformed user omitting the activation didn't, and you have certainly not tested your class when ActivateEventHandling was not called, right? :)
Edit: There may be alternative approaches here which are worth mentioning it
Depending on the paradigm it may be wise to avoid wiring events in the class at all. I need to relativize my comment on Stephen Byrne's answer.
Wiring can be regarded as context knowledge. The single responsibility principle says a class should do only one task. Furthermore a class can be used more versatile if it does not have a dependency to something else. A very loosely coupled system would provide many classes witch have events and handlers and do not know other classes.
The environment is then responsible for wiring all the classes together to connect events properly with handlers.
The environment would create the context in which the classes interact with each-other in a meaningful way.
A class in this case does therefore not know to whom it will be bound and it actually does not care. If it requires a value, it asks for it, whom it asks should be unknown to it. In that case there wouldn't even be an interface injected into the ctor to avoid a dependency. This concept is similar to neurons in a brain as they also emit messages to the environment and expect answer not knowing neighbouring neurons.
However I regard a dependency to an interface, if it is injected by some means of a dependency injection container just another paradigm and not less wrong.
The non trivial task of the environment to wire up all classes on start may lead to runtime errors (which are mitigated by a very good test coverage of functional and integration tests, which may be a hard task for large projects) and it gets very annoying if you need to wire dozens of classes and probably hundreds of events on startup manually.
While I agree that wiring in an environment and not in the class itself can be nice, it is not practical for large scale code.
Ralf Westphal (one of the founders of the clean code developer initiative (sorry german only)) has written a software that performs the wiring automatically in a concept called "event based components" (not necessarily coined by himself). It uses naming conventions and signature matching with reflection to bind events and handlers together.

Wiring events is field assignment (because delegates are nothing but simple reference variables that point to methods).
So option(1) is fine.

The point of constructor is not to "assign fields". It is to establish invariants of your object, i. e. something that never changes during its lifetime.
So if in other methods of class you depend on being always subscribed to some object, you'd better do it in the constructor.
On the other hand, if subscriptions come and go (probably not the case here), you can move this code to another method.

The single responsibility principle dictates that that wiring should be avoided. Your class should not care how, or where from it receives data. It would make sense to rename OnEvent method to something more meaningful, and make it public.
Then some other class (bootstrapper, configurator, whatever) should be responsible for the wiring. Your class should only be responsible for what happens when a new data come's in.
Pseudo code:
public interface IEventProvider //your IClassWithEvent
{
Event MyEvent...
}
public class EventResponder : IEventResponder
{
public void OnEvent(object sender, EventArgs args){...}
}
public class Boostrapper
{
public void WireEvent(IEventProvider eventProvider, IEventResponder eventResponder)
{
eventProvider>event += eventResponder.OnEvent;
}
}
Note, the above is pseudo code, and it's only for the purpose to describe the idea.
How your bootstrapper actually is implemented depends on many things. It can be your "main" method, or your global.asax, or whatever you have in place to actually configure and prepare your application.
The idea is, that whatever is responsible to prepare the application to run, should compose it, not the classes themselves, as they should be as single purpose as possible, and should not care too much about how and where they are used.

Good or bad practice? Initializing objects in getter

I have a strange habit it seems... according to my co-worker at least. We've been working on a small project together. The way I wrote the classes is (simplified example):
[Serializable()]
public class Foo
{
public Foo()
{ }
private Bar _bar;
public Bar Bar
{
get
{
if (_bar == null)
_bar = new Bar();
return _bar;
}
set { _bar = value; }
}
}
So, basically, I only initialize any field when a getter is called and the field is still null. I figured this would reduce overload by not initializing any properties that aren't used anywhere.
ETA: The reason I did this is that my class has several properties that return an instance of another class, which in turn also have properties with yet more classes, and so on. Calling the constructor for the top class would subsequently call all constructors for all these classes, when they are not always all needed.
Are there any objections against this practice, other than personal preference?
UPDATE: I have considered the many differing opinions in regards to this question and I will stand by my accepted answer. However, I have now come to a much better understanding of the concept and I'm able to decide when to use it and when not.
Cons:
Thread safety issues
Not obeying a "setter" request when the value passed is null
Micro-optimizations
Exception handling should take place in a constructor
Need to check for null in class' code
Pros:
Micro-optimizations
Properties never return null
Delay or avoid loading "heavy" objects
Most of the cons are not applicable to my current library, however I would have to test to see if the "micro-optimizations" are actually optimizing anything at all.
LAST UPDATE:
Okay, I changed my answer. My original question was whether or not this is a good habit. And I'm now convinced that it's not. Maybe I will still use it in some parts of my current code, but not unconditionally and definitely not all the time. So I'm going to lose my habit and think about it before using it. Thanks everyone!

What you have here is a - naive - implementation of "lazy initialization".
Short answer:
Using lazy initialization unconditionally is not a good idea. It has its places but one has to take into consideration the impacts this solution has.
Background and explanation:
Concrete implementation:
Let's first look at your concrete sample and why I consider its implementation naive:
It violates the Principle of Least Surprise (POLS). When a value is assigned to a property, it is expected that this value is returned. In your implementation this is not the case for null:
foo.Bar = null;
Assert.Null(foo.Bar); // This will fail
It introduces quite some threading issues: Two callers of foo.Bar on different threads can potentially get two different instances of Bar and one of them will be without a connection to the Foo instance. Any changes made to that Bar instance are silently lost.
This is another case of a violation of POLS. When only the stored value of a property is accessed it is expected to be thread-safe. While you could argue that the class simply isn't thread-safe - including the getter of your property - you would have to document this properly as that's not the normal case. Furthermore the introduction of this issue is unnecessary as we will see shortly.
In general:
It's now time to look at lazy initialization in general:
Lazy initialization is usually used to delay the construction of objects that take a long time to be constructed or that take a lot of memory once fully constructed.
That is a very valid reason for using lazy initialization.
However, such properties normally don't have setters, which gets rid of the first issue pointed out above.
Furthermore, a thread-safe implementation would be used - like Lazy<T> - to avoid the second issue.
Even when considering these two points in the implementation of a lazy property, the following points are general problems of this pattern:
Construction of the object could be unsuccessful, resulting in an exception from a property getter. This is yet another violation of POLS and therefore should be avoided. Even the section on properties in the "Design Guidelines for Developing Class Libraries" explicitly states that property getters shouldn't throw exceptions:
Avoid throwing exceptions from property getters.
Property getters should be simple operations without any preconditions. If a getter might throw an exception, consider redesigning the property to be a method.
Automatic optimizations by the compiler are hurt, namely inlining and branch prediction. Please see Bill K's answer for a detailed explanation.
The conclusion of these points is the following:
For each single property that is implemented lazily, you should have considered these points.
That means, that it is a per-case decision and can't be taken as a general best practice.
This pattern has its place, but it is not a general best practice when implementing classes. It should not be used unconditionally, because of the reasons stated above.
In this section I want to discuss some of the points others have brought forward as arguments for using lazy initialization unconditionally:
Serialization:
EricJ states in one comment:
An object that may be serialized will not have it's contructor invoked when it is deserialized (depends on the serializer, but many common ones behave like this). Putting initialization code in the constructor means that you have to provide additional support for deserialization. This pattern avoids that special coding.
There are several problems with this argument:
Most objects never will be serialized. Adding some sort of support for it when it is not needed violates YAGNI.
When a class needs to support serialization there exist ways to enable it without a workaround that doesn't have anything to do with serialization at first glance.
Micro-optimization:
Your main argument is that you want to construct the objects only when someone actually accesses them. So you are actually talking about optimizing the memory usage.
I don't agree with this argument for the following reasons:
In most cases, a few more objects in memory have no impact whatsoever on anything. Modern computers have way enough memory. Without a case of actual problems confirmed by a profiler, this is pre-mature optimization and there are good reasons against it.
I acknowledge the fact that sometimes this kind of optimization is justified. But even in these cases lazy initialization doesn't seem to be the correct solution. There are two reasons speaking against it:
Lazy initialization potentially hurts performance. Maybe only marginally, but as Bill's answer showed, the impact is greater than one might think at first glance. So this approach basically trades performance versus memory.
If you have a design where it is a common use case to use only parts of the class, this hints at a problem with the design itself: The class in question most likely has more than one responsibility. The solution would be to split the class into several more focused classes.

It is a good design choice. Strongly recommended for library code or core classes.
It is called by some "lazy initialization" or "delayed initialization" and it is generally considered by all to be a good design choice.
First, if you initialize in the declaration of class level variables or constructor, then when your object is constructed, you have the overhead of creating a resource that may never be used.
Second, the resource only gets created if needed.
Third, you avoid garbage collecting an object that was not used.
Lastly, it is easier to handle initialization exceptions that may occur in the property then exceptions that occur during initialization of class level variables or the constructor.
There are exceptions to this rule.
Regarding the performance argument of the additional check for initialization in the "get" property, it is insignificant. Initializing and disposing an object is a more significant performance hit than a simple null pointer check with a jump.
Design Guidelines for Developing Class Libraries at http://msdn.microsoft.com/en-US/library/vstudio/ms229042.aspx
Regarding Lazy<T>
The generic Lazy<T> class was created exactly for what the poster wants, see Lazy Initialization at http://msdn.microsoft.com/en-us/library/dd997286(v=vs.100).aspx. If you have older versions of .NET, you have to use the code pattern illustrated in the question. This code pattern has become so common that Microsoft saw fit to include a class in the latest .NET libraries to make it easier to implement the pattern. In addition, if your implementation needs thread safety, then you have to add it.
Primitive Data Types and Simple Classes
Obvioulsy, you are not going to use lazy-initialization for primitive data type or simple class use like List<string>.
Before Commenting about Lazy
Lazy<T> was introduced in .NET 4.0, so please don't add yet another comment regarding this class.
Before Commenting about Micro-Optimizations
When you are building libraries, you must consider all optimizations. For instance, in the .NET classes you will see bit arrays used for Boolean class variables throughout the code to reduce memory consumption and memory fragmentation, just to name two "micro-optimizations".
Regarding User-Interfaces
You are not going to use lazy initialization for classes that are directly used by the user-interface. Last week I spent the better part of a day removing lazy loading of eight collections used in a view-model for combo-boxes. I have a LookupManager that handles lazy loading and caching of collections needed by any user-interface element.
"Setters"
I have never used a set-property ("setters") for any lazy loaded property. Therefore, you would never allow foo.Bar = null;. If you need to set Bar then I would create a method called SetBar(Bar value) and not use lazy-initialization
Collections
Class collection properties are always initialized when declared because they should never be null.
Complex Classes
Let me repeat this differently, you use lazy-initialization for complex classes. Which are usually, poorly designed classes.
Lastly
I never said to do this for all classes or in all cases. It is a bad habit.

Do you consider implementing such pattern using Lazy<T>?
In addition to easy creation of lazy-loaded objects, you get thread safety while the object is initialized:
http://msdn.microsoft.com/en-us/library/dd642331.aspx
As others said, you lazily-load objects if they're really resource-heavy or it takes some time to load them during object construction-time.

I think it depends on what you are initialising. I probably wouldn't do it for a list as the construction cost is quite small, so it can go in the constructor. But if it was a pre-populated list then I probably wouldn't until it was needed for the first time.
Basically, if the cost of construction outweighs the cost of doing an conditional check on each access then lazy create it. If not, do it in the constructor.

Lazy instantiation/initialization is a perfectly viable pattern. Keep in mind, though, that as a general rule consumers of your API do not expect getters and setters to take discernable time from the end user POV (or to fail).

The downside that I can see is that if you want to ask if Bars is null, it would never be, and you would be creating the list there.

I was just going to put a comment on Daniel's answer but I honestly don't think it goes far enough.
Although this is a very good pattern to use in certain situations (for instance, when the object is initialized from the database), it's a HORRIBLE habit to get into.
One of the best things about an object is that it offeres a secure, trusted environment. The very best case is if you make as many fields as possible "Final", filling them all in with the constructor. This makes your class quite bulletproof. Allowing fields to be changed through setters is a little less so, but not terrible. For instance:
class SafeClass
{
String name="";
Integer age=0;
public void setName(String newName)
{
assert(newName != null)
name=newName;
}// follow this pattern for age
...
public String toString() {
String s="Safe Class has name:"+name+" and age:"+age
}
}
With your pattern, the toString method would look like this:
if(name == null)
throw new IllegalStateException("SafeClass got into an illegal state! name is null")
if(age == null)
throw new IllegalStateException("SafeClass got into an illegal state! age is null")
public String toString() {
String s="Safe Class has name:"+name+" and age:"+age
}
Not only this, but you need null checks everywhere you might possibly use that object in your class (Outside your class is safe because of the null check in the getter, but you should be mostly using your classes members inside the class)
Also your class is perpetually in an uncertain state--for instance if you decided to make that class a hibernate class by adding a few annotations, how would you do it?
If you make any decision based on some micro-optomization without requirements and testing, it's almost certainly the wrong decision. In fact, there is a really really good chance that your pattern is actually slowing down the system even under the most ideal of circumstances because the if statement can cause a branch prediction failure on the CPU which will slow things down many many many more times than just assigning a value in the constructor unless the object you are creating is fairly complex or coming from a remote data source.
For an example of the brance prediction problem (which you are incurring repeatedly, nost just once), see the first answer to this awesome question: Why is it faster to process a sorted array than an unsorted array?

Let me just add one more point to many good points made by others...
The debugger will (by default) evaluate the properties when stepping through the code, which could potentially instantiate the Bar sooner than would normally happen by just executing the code. In other words, the mere act of debugging is changing the execution of the program.
This may or may not be a problem (depending on side-effects), but is something to be aware of.

Are you sure Foo should be instantiating anything at all?
To me it seems smelly (though not necessarily wrong) to let Foo instantiate anything at all. Unless it is Foo's express purpose to be a factory, it should not instantiate it's own collaborators, but instead get them injected in its constructor.
If however Foo's purpose of being is to create instances of type Bar, then I don't see anything wrong with doing it lazily.

Using Funcs instead of instances in frameworks

When looking at the source code of a couple of projects I found a pattern I can not quite understand.
For instance in FubuMVC and Common Service Locator a Func is used when a static provider is changed.
Can anyone explain what the benefit is of using:
private static Func<IServiceLocator> currentProvider;
public static IServiceLocator Current
{
get { return currentProvider(); }
}
public static void SetLocatorProvider(Func<IServiceLocator> newProvider)
{
currentProvider = newProvider;
}
instead of:
private static IServiceLocator current;
public static IServiceLocator Current
{
get { return current; }
}
public static void SetLocator(IServiceLocator newInstance)
{
current = newInstance;
}

The major advantage of the first model over the second is what's called "lazy initialization". In the second example, as soon as SetLocator is called, you must have an IServiceLocator instance loaded in memory and ready to go. If such instances are expensive to create, and/or created along with a bunch of other objects at once (like on app startup), it's a good idea to try to delay actual creation of the object to reduce noticeable delays to the user. Also, if the dependency may not be used by the dependent class (say it's only needed for certain operations, and the class can do other things that don't require the dependency), it would be a waste to instantiate one.
The solution is to provide a "factory method" instead of an actual instance. When the instance is actually needed, the factory method is called, and the instance is created at the last possible moment before it's used. This reduces front-end loading times and avoids creating unneeded dependencies.

Good answer by #KeithS. Another thing to note here is what happens under the covers of the initialization of certain instances. Keeping a reference to intentionally volatile objects can be tricky.
FubuMVC, for instance, spins up a nested StructureMap container per HTTP request which scopes all service location to that specific request. If you have classes running within that pipeline that have been built up, you'll want to use the contextual injection provided to you via THAT instance of IServiceLocator.

Theres a lot more flexibility to the implementer of newProvider. They can lazy load, async load (and and then if it's not loaded by the time the func is called it can have code to wait), they can allow it change based on runtime parameters etc.

A func allows several things
The locator creation can be delayed until it is needed. It is therefore lazy.
The provider object does not contain any state. It is not its responsiblity to shut down the locator does anything with it except to return the current locator when needed.
When the locator is reconfigured at run time or it decides that a different instance is needed it can control the lifetime of the locator as long as the calling code does not store a reference to locator.
Since the locator is returned by a method it has more flexibility e.g. to create a thread local locator so it can create many objects in each thread without the need to coordinate object creation in one global object which could become a bottleneck when many threads are involved.
I am sure the designers did could give you more points than I did why it can be a good idea to abstract away "simple" things like to return an instance of a service locator.
Yours,
Alois Kraus

Is it a code smell for one method to depend on another?

I am refactoring a class so that the code is testable (using NUnit and RhinoMocks as testing and isolations frameworks) and have found that I have found myself with a method is dependent on another (i.e. it depends on something which is created by that other method). Something like the following:
public class Impersonator
{
private ImpersonationContext _context;
public void Impersonate()
{
...
_context = GetContext();
...
}
public void UndoImpersonation()
{
if (_context != null)
_someDepend.Undo();
}
}
Which means that to test UndoImpersonation, I need to set it up by calling Impersonate (Impersonate already has several unit tests to verify its behaviour). This smells bad to me but in some sense it makes sense from the point of view of the code that calls into this class:
public void ExerciseClassToTest(Impersonator c)
{
try
{
if (NeedImpersonation())
{
c.Impersonate();
}
...
}
finally
{
c.UndoImpersonation();
}
}
I wouldn't have worked this out if I didn't try to write a unit test for UndoImpersonation and found myself having to set up the test by calling the other public method. So, is this a bad smell and if so how can I work around it?

Code smell has got to be one of the most vague terms I have ever encountered in the programming world. For a group of people that pride themselves on engineering principles, it ranks right up there in terms of unmeasurable rubbish, and about as useless a measure, as LOCs per day for programmer efficiency.
Anyway, that's my rant, thanks for listening :-)
To answer your specific question, I don't believe this is a problem. If you test something that has pre-conditions, you need to ensure the pre-conditions have been set up first for the given test case.
One of the tests should be what happens when you call it without first setting up the pre-conditions - it should either fail gracefully or set up it's own pre-condition if the caller hasn't bothered to do so.

Well, there is a bit too little context to tell, it looks like _someDepend should be initalized in the constructor.
Initializing fields in an instance method is a big NO for me. A class should be fully usable (i.e. all methods work) as soon as it is constructed; so the constructor(s) should initialize all instance variables. See e.g. the page on single step construction in Ward Cunningham's wiki.
The reason initializing fields in an instance method is bad is mainly that it imposes an implicit ordering on how you can call methods. In your case, TheMethodIWantToTest will do different things depending on whether DoStuff was called first. This is generally not something a user of your class would expect, so it's bad :-(.
That said, sometimes this kind of coupling may be unavoidable (e.g. if one method acquires a resource such as a file handle, and another method is needed to release it). But even that should be handled within one method if possible.
What applies to your case is hard to tell without more context.

Provided you don't consider mutable objects a code smell by themselves, having to put an object into the state needed for a test is simply part of the set-up for that test.

This is often unavoidable, for instance when working with remote connections - you have to call Open() before you can call Close(), and you don't want Open() to automatically happen in the constructor.
However you want to be very careful when doing this that the pattern is something readily understood - for instance I think most users accept this kind of behaviour for anything transactional, but might be surprised when they encounter DoStuff() and TheMethodIWantToTest() (whatever they're really called).
It's normally best practice to have a property that represents the current state - again look at remote or DB connections for an example of a consistently understood design.
The big no-no is for this to ever happen for properties. Properties should never care what order they are called in. If you have a simple value that does depend on the order of methods then it should be a parameterless method instead of a property-get.

Yes, I think there is a code smell in this case. Not because of dependencies between methods, but because of the vague identity of the object. Rather than having an Impersonator which can be in different persona states, why not have an immutable Persona?
If you need a different Persona, just create a new one rather than changing the state of an existing object. If you need to do some cleanup afterwards, make Persona disposable. You can keep the Impersonator class as a factory:
using (var persona = impersonator.createPersona(...))
{
// do something with the persona
}

To answer the title: having methods call each other (chaining) is unavoidable in object oriented programming, so in my view there is nothing wrong with testing a method that calls another. A unit test can be a class after all, it's a "unit" you're testing.
The level of chaining depends on the design of your object - you can either fork or cascade.
Forking:
classToTest1.SomeDependency.DoSomething()
Cascading:
classToTest1.DoSomething() (which internally would call SomeDependency.DoSomething)
But as others have mentioned, definitely keep your state initialisation in the constructor which from what I can tell, will probably solve your issue.