Singleton: Ammendment 2

I've found an intersting question regarding the Singleton pattern that might be worth examining: "What if the class that is supposed to become a Singleton doesn't have parameter less constructor?"
(If you need to read explanations on the pattern read here and here)
Hmmm! No why would you have a singleton where its constructor accepts parameters? It kind of sounds useless since no one is supposed to create instances of the singleton class.
But just for the sake of argument lets assume we have a singleton class with such properties: for example we've decided to create a log helper as a singleton. It's a class that's going to be used all over our program and anyone who needs to log something will send it to this class. The log helper would take care of writing the entry to all the different log output devices. Again for argument's sake let's assume that we have created the constructor of this class so that it accepts some settings in regards to how it's supposed to behave (don't ask why!):

class LogHelper
{
  /* Logging method implementations */ 

  private LogHelper(some-configuration-value)
  {
    //use the some-configuration-value or store it somewhere
  }
  
  public static LogHelper Instance
  {
    get
    {
      return _instance;
    }
  }
  private static LogHelper _instance = new LogHelper(/*how to provide the param?*/);
}

As you can see the big question is how do we provide the parameters to the constructor. Well the issue here is a little bit of a bad design: WHY would you want to pass something to the constructor of the class your designing when no one can actually call the constructor but yourself? Weren't you better off just extracting the parameters or config values or what ever it was supposed to be passed to the constructor in the constructor itself? That would be a better implementation.

Now the above example is very simple (and stupid) but there are situations where a constructor with parameters could help a Singleton design! Surprised? Well here is a situation:
Suppose we were building a class called PrintManager, this class is going to manage the queue of documents that are waiting to be printed to a specific printer and it is also going to handle the printing operations (don't really care about that part). The only thing important about this design is that the PrintManager is going to be a multi-instance singleton meaning that multiple instances will be available, each instance managing a specific printer. Now for simplicities sake we are going to assume that all the printers are the same (one class can handle them all) but each printer is on a different port so when we create the printer object we need to tell it (via the constructor) on what port to print to.
Even in this case the code responsible for creating the PrintManager object is still part of that class so if the PrintManager object has no parameters in the constructor or has 10 parameters it doesn't really affect anyone and it's an internal thing. See the code below:

class PrintManager
{
  private int Port;
  /* printing & queue management code */

  private PrintManager(int p)
  {
    Port = p;
  }

  public static PrintManager GetInstance(int forPort)
  {
    lock(Instances)
    {
      if (Instances.ContainsKey(forPort) == false)
        Instances.Add(forPort, new PrintManager(forPort));
    }

    return Instances[forPort];
  }
  private Dictionary Instances = new Dictionary();
}

As can be seen in the above code the internal code of the PrintManager class has to deal with the constructor so again having parameters or not having them doesn't really matter.

The much debated Singleton: Part II

As I mentioned in my previous post regarding the singleton pattern I've seen a lot of talk on different blogs/forums regarding the evils of the singleton pattern. I've tried to cover the general concepts of where this pattern should be used and where it should be avoided in the previous post I made regarding this issue (read here) and in this post I'm going to quote a couple of the stuff that I've found and post my reply to them in here. So here goes: My responses in bold.

The items that I found on http://c2.com/cgi/wiki?SingletonsAreEvil

• Someone had mentioned that when you use the singleton pattern you suddenly have two types of objects:
  

  "those that can be instantiated in a standard fashion and those that cannot be created at all. I would personally rather use a container which governs the number of a given object that can exist in a system and acquire the objects from the container"
  

  We already have a lot of different types of objects that can be instantiated (or not instantiated) in different ways this isn't a particular problem with the singleton pattern but quite the opposite, the singleton pattern is using one of these different types of language mechanisms to obtain what its aiming for (the fact that no one should be able to instantiate the object unless they go through the single point of access). As an example we have interfaces, abstract classes, sealed classes, protected & private constructors, static classes and a whole lot of different types of classes that some you can instantiate in the regular way (using new) and some you can't (and some you are not allowed to instantiate period; e.g. the abstract class). Anyway this isn't a really good argument against the use of the singleton pattern.
  

• Someone else had mentioned this:
  

  "Singletons usually are used to provide a single point of access to a global service. I always make the singleton separate from the class itself so the class can be used any way you want. The singleton can then use the class. The singleton also doesn't have to instantiate the object"
  

  One of the points of the singleton pattern is to prevent accidental or purposeful creation of the singleton object, that's why it has a private constructor so one can create it except for the singleton class itself which knows when and where to create it. In regards to the second part I have seen C++ code that uses the "friend" keyword to have a separate singleton factory than the singleton object itself but I really can't see any use for it. You need a single point of access you also need a class that can be instantiated in a controlled manner why are you making your design complex and the programmer using your design confused by making him deal with (and understand) two classes (albeit one being very small and simple). I guess you might come up with very rare cases where this would be useful but in general I think sticking with a singleton class that also provides the single point of access is the best way to go.
  

• A couple of people where mentioning coupling issues with singleton class names and/or polymorphism issues with the singleton:
  

  "Subsystem's coupling to Singleton's Class Name" or "Singletons aren't polymorphic"
  

  Let's make something very clear singletons are polymorphic (please read the section regarding polymorphic singletons in this post) and let's also focus on something else the whole reason we are using a singleton as opposed to a global variable (or static member) is because it allows us to deal with the single point of access issue through an object oriented fashion. It allows us to have all the OO design techniques, design patterns etc. at our disposal when we are dealing with a singleton just as we would when dealing with other classes & objects. So all general OO design techniques and features apply to a singleton.
  

  Regarding the first argument: your different subsystem's are going to be dependent on some single point of access (because you've made that design decision) now it's a lot better that you implement it using the polymorphic singleton pattern just to make sure that they are only dependant on an abstract class (the singleton being that abstract class) not on the actual implementation.
  

• An amusing one:
  

  "Doesn't a C# static class, with its static constructor, provide all the functionality you would ever need from a singleton?"
  

  OK I think this guy has missed the whole point of the singleton. If you use a static class that means all member variables/methods are static and then you might use the static constructor to initialize some of these member variables, BUT this isn't object oriented programming this is a feature simulating the procedural way of design & implementation. Why have the designers of the C# language decided to include it there? Well one reason being the fact that you will need utility classes (methods) that are pure static classes and you don't need a real object to respond to the calls (another design decision) but a utility (or pure static class) is just a bunch of methods but a singleton is a full blown object instantiated from a class the only difference being that only one instance (or a limited number of instances) exist of it.
  

  If you think this argument is correct you should try to first understand the singleton pattern and then come back and re-read what the argument is saying!
  

These items I've found on http://www-128.ibm.com/developerworks/webservices/library/co-single.html

• "There is one implementation anti-pattern that flourishes in an application with too many singletons: I know where you live anti-pattern. This occurs when, among collaborating classes, one class knows where to get instances of the other."
  

  This will be a problem when you overuse the singleton pattern, but correct usage of the pattern especially in places where there is a strong domain reason or a legitimate technical requirement backing its existence but otherwise over-use of the pattern will cause the above mentioned problem.
  

• "Where's the harm? Coupling among classes is vastly increased when classes know where to get instances of their collaborators. First, any change in how the supplier class is instantiated ripples into the client class. This violates the Liskov Substitution Principle, which states that you should allow any application the freedom to tell the client class to collaborate with any subclass of the supplier. This violation is felt by unit tests, but more importantly, it makes it difficult to enhance the supplier in a backward-compatible way. First, the unit tests cannot pass the client class a mock supplier instance for the purposes of simulating the supplier's behavior, as described above. Next, no one can enhance the supplier without changing the client code, which requires access to the supplier's source. Even when you have access to the supplier's source, do you really want to change all 178 clients of the supplier? Weren't you planning on having a nice, relaxing weekend?
  

  Collaborating classes should be built to allow the application to decide how to wire them together. This increases the flexibility of the application and makes unit testing simpler, faster, and generally more effective. Remember that the easier it is to test a class, the more likely a developer will test it."
  

  Again I think this a case of misuse of the pattern. Let's straighten one thing out the fact that a singleton is a class that by default you're not suppose to instantiate doesn't mean that it can't have sub-classes and doesn't mean that replacing it with "stub" classes for the purpose of testing would be impossible. I feel I'm repeating myself too much but a polymorphic singleton especially when it's combined with other patterns such as bridge or strategy it can give you all the flexibility in the world and doesn't affect testing or any other type of pluggable behavior. Completely loosely coupled!
  

• "To decide whether a class is truly a singleton, you must ask yourself some questions: (1) Will every application use this class exactly the same way? (exactly is the key word) (2) Will every application ever need only one instance of this class? (ever and one are the key words) (3) Should the clients of this class be unaware of the application they are part of?"
  

  I don't agree with the rules set above although they might be very helpful in situations where there is going to be only one instance of the singleton but as Gamma mentions in his book a singleton might be polymorphic (therefore question 1 is not really the case if you need a polymorphic singleton), a singleton might be designed because we need a controlled number of objects (therefore question 2 is not true) and I don't understand question 3! J
  

This item is from http://www.sitepoint.com/forums/showthread.php?t=386447

• "It's hard coupling. Singletons are essentially static functions - the difference is syntactical sugar. Whenever you reference a static symbol in your code, you make it more dependent on the context. It's really the same as with a global variable - A global variable is bad because it has a global scope. A static function is also global in scope, so it has the same problems associated. The difference between the two, which accounts for the assumption that singletons are slightly better than global variables, is that functions are more abstract than variables, and this gives you some flexibility. This flexibility can then be used to excersise some damage control. Still it can't beat narrowing the scope, which is what objects provides."
  

  Hard coupling!! First time I heard that (I think he meant tight coupling). Anyhow this description is very good, and especially relevant to misuse of the pattern. If you use a singleton as an excuse to be lazy and not think about your design, if you use a singleton as an excuse to make everything global then yes there is no difference between a singleton and static members BUT if you use the singleton where it's supposed to be used it will be a whole lot more than "syntactical sugar." Plus we are forgetting that part of using a pattern isn't just because it's implemented with for example a static member or it isn't, but it's the whole knowledge that comes packed up with a pattern and is transferred to the next person (read this post). The above comparison is like saying using functions & procedures is "syntactical sugar" to using a goto statement because they are eventually a jump to a piece of code and a jump back! But that's not the case, functions and procedures provide a higher level of abstraction, so do classes, interfaces etc. which provide an even higher level of abstraction. Patterns are pretty much the same thing they too provide a higher level of abstraction albeit being implemented using mundane "static" members or similar constructs of a lower level of abstraction.
  

Items from http://home.earthlink.net/~huston2/dp/singleton.html

• "It's relatively easy to protect Singleton against multiple instantiations because there is good language support in this area. The most complicated problem is managing a Singleton's lifetime, especially its destruction ... There are serious threading issues surrounding the pattern."
  

  Who says we don't want multiple instantiation of a singleton? There are cases that we need multiple instantiation (but controlled number of instances). I totally agree that in C++ and other languages that don't have garbage collection managing the lifetime of a Singleton or a lot of other patterns could be hard but not impossible. Yes threading could also be a serious problem to consider, but with singletons there are usually two scenarios that occur: you either have a singleton instance per thread then you have a multi-instanced singleton which returns an instance based on the calling active thread or you have on instance of the singleton replying to all threads which means the class must be thread-aware.
  

• "The Singleton design pattern is one of the most inappropriately used patterns. Singletons are intended to be used when a class must have exactly one instance, no more, no less ... frequently use Singletons in a misguided attempt to replace global variables ... A Singleton is, for intents and purposes, a global variable. The Singleton does not do away with the global; it merely renames it."
  

  One instance and no more? I don't agree and if you read Gamma's book you'll see that it's not what he intended when he created the pattern. I agree that singleton is usually misused and it is not supposed to fix global variable problems (i.e. common coupling) but the advantages it provides as opposed to using a global variable are tremendous. Read my post on this issue here & here.
  

  "When is Singleton unnecessary? Short answer: most of the time. Long answer: when it's simpler to pass an object resource as a reference to the objects that need it, rather than letting objects access the resource globally ... The real problem with Singletons is that they give you such a good excuse not to think carefully about the appropriate visibility of an object. Finding the right balance of exposure and protection for an object is critical for maintaining flexibility."
  

  I agree completely! Very well said.
  

• "Our group had a bad habit of using global data, so I did a study group on Singleton. The next thing I know Singletons appeared everywhere and none of the problems related to global data went away. The answer to the global data question is not, "Make it a Singleton." The answer is, "Why in the hell are you using global data?" Changing the name doesn't change the problem. In fact, it may make it worse because it gives you the opportunity to say, "Well I'm not doing that, I'm doing this" – even though this and that are the same thing."
  

  Very well said.

The much debated Singleton

After creating my post about the Singleton pattern I noticed a lot of hype around this pattern and its use (or misuse) over the internet. I also found some very interesting and sometimes amusing posts/comments on the matter (which I've included at the end of this post). Interestingly there was a lot of negative content about this pattern and the side effects that it has on your design. Intrigued by all of this information I decided to write my version of why & when you should use the Singleton pattern and also try to answer a lot of misconceptions that I read about during the past two weeks. So here goes:

The original thought behind the conception of the singleton pattern is to provide an object oriented approach to alleviate the pain of common coupling (global data) and provide a different strategy when dealing with global data. Also the pattern was intended as a solution when you absolutely needed to limit the number of objects of a certain type (class). Note that the singleton pattern will not totally remove common coupling from your design and to solve the problem completely, in most cases, requires the designer to completely re-think their design. Let's understand common coupling and it's evils before we delve any deeper:

Common coupling is a situation where function/component/class/module A & B are dependent on one another (coupled together) through the existence of some sort of external (and shared) data area between them in which one can change the content and the other will read from it (or both will read and change). Now the two coupled entities might be calling each other or might not be using one another directly at all. It doesn't matter; they are coupled to each other due to their shared use of the common data area. The perfect example being global data shared between multiple functions/components/classes/modules etc. (just for the sake of brevity I'm going to refer to all these different sections of code that coupling might relate to as components from now on).

Why is common coupling or global data bad? Well let's start by answering this question: why do we want loose coupling between our components (as opposed to tight coupling)?

• When multiple components are tightly coupled we lose the chance for component reuse: If we want to reuse component A we must also take component B for the ride, because component B changes the global data and if the global data doesn't contain a certain state we will not be able to get the functionality we need from component A.
  
• Due to tightly coupled components we increase our cost of maintenance and tests. When something changes instead of only needing a replacement of component A's implementation (which relates to the global area), since other components are tightly coupled to it we need to change them as well. Once we change component A and test it we must also test the other components tightly coupled with it.
  
• When there is too much coupling we reduce the probability of independent design, coding & testing therefore creating a tougher environment as far as technical & project management is concerned.
  
• Too many tightly coupled components mean a bigger thing to understand: more complexity.
  

As we can see common coupling (which is a level of tight coupling) can become a major reuse hindering factor, especially in cases where future change and evolution of the system is expected. How many times have we all designed something using global data only to find out in later versions that the data that we assumed (wrongly assumed!) to only have one running instance in the system needed to actually have multiple running instances. For example suppose we were to design a word processor similar to WordPad that ships with Windows. One of the immediate design decisions that come to mind when designing this word processor is to keep the current document information in a global variable where all the different routines of our program can access it and so we don't have to pass it around (common coupling). Making this design decision will cause many problems down the road but let's just examine two:

• Components that we design (i.e. the spell checker) all depend on the existence of a globally available document to do their work and their reuse is hindered by this fact when we need to make use of them in a different project. As an example suppose we need a spell checker to check product information before it was saved to the database. If I had had the foresight to design my spell checker as an independent component that accepts what it's supposed to check as input (a rather abstract form of input but never the less an input not a global data area) and performed the checks on the input (as opposed to a global data area), I wouldn't have had this problem when the time came for its reuse.
  
• If in the future the stakeholders decide that we are going to change our SDI (single document interface) word processor into a fully fledged MDI (multiple document interface) application where the user can open and work on multiple documents simultaneously we are going to be in trouble. Again we would need to rethink our single point of access because now we have multiple documents being loaded and it no longer makes sense to have one single document in memory where everyone will access and use.
  

Having said all this, one can't deny the fact that using global data in some cases is absolutely essential. But the scenarios where use of global data as a design decision is correct and invaluable are rare and will only occur once or twice in a moderately sized program. So the big problem is detecting when you are to allow a global shared piece of data and when not to. Chances are that in most cases you shouldn't allow it and we'll try to figure out when to do this and when not to, but before we get into that let's get back to the singleton pattern.

Based on the discussion so far the singleton pattern is useful when their exists enough reasons to share something as global data in your program, at that point instead of sharing it as global data (or a static member variable of a class) you should expose it as a singleton. This way you have more of a control over how that piece of global data will be accessed and used and in the future you can maintain it a lot easier (since singletons are regular objects instantiated from classes) by using OO techniques and other design patterns. You can also make sure that only one instance of that global object exists and no one (on purpose or through a mistake) can create a second instance which could have dire consequences (depending on your object's role in the system).

OK now we can go back to the definition of the singleton pattern and figure out when to use it:

The singleton design pattern is used in places where you want to ensure that only a single (or a controlled number) of instances of a specific class exist. The client code using the object shouldn't be allowed to create new instances (since it doesn't make sense to create new instances) and all of the different areas of client codes should access that single object through a well defined access point.

As can be seen nowhere in the definition of the singleton pattern (in any documentation about the pattern) is there a reference to use a singleton whenever you feel like using global data! Or use a singleton whenever you're lazy and don't feel like thinking about your design so you just expose some object as a global object and let everyone in all the different layers of your software access it through the single point of access. Or any other similar excuse for singleton's (mis)use. In order to use a Singleton correctly one cannot break other design rules (such as Common Coupling) a singleton does not justify having common coupling in your design!

So where can we use a singleton? Well there are situations where hardware or some other fact from the domain or a non-functional (technical) requirement dictates the use of the singleton pattern. A perfect example is the polymorphic singleton that I've used in the Abstract Factory example so that we have one single point of access to our abstract factory and that single point is configured with one of its subclasses. The technical requirements of that design imply that throughout the execution of the software the singleton will only contain one object that never changes. Other such examples would be when you are managing one piece of hardware (i.e. a modem, printer, etc.) and even in this case it is always wise to think about the consequences of your design decisions if we end up supporting more than one of those hardware pieces (i.e. two modems at the same time). But even for places where we have a controlled number of hardware (multiple modems) we can use the multi-instance singleton (please read my post on the singleton pattern) to fix this problem.

Another perfect use of the singleton pattern is to make sure that from a specific class we only get one object per thread. An example is the Connection Manager (which in some cases we might need more than one per thread but for the time being we'll assume one per thread will do) in this case the singleton pattern is there to make sure that nobody creates an extra copy of the ConnectionManager and cause problems in the way we communicate with the DB but the singleton itself might have multiple instances (actually one for every active thread that is running) so code in each thread will get a different ConnectionManager object hence making sure that one thread wouldn't use another thread's database connection.

There are a lot of perfectly legitimate reasons to use a singleton and I've only touched on a couple of them here but to round things up I believe the following list will help make an informed decision. As with any engineering design decision you as the designer have to consider the tradeoffs and you will ultimately decide if the singleton will help you out in a specific scenario but you should also take care not to misuse it since its negative effects on your design (especially form a reuse point of view) can be disastrous:

1. Is the candidate object a truly single object from a problem domain point of view? Will it always be that way (considering future growth of the software)? If yes by all means implement it as a singleton.
   
2. Will the candidate object becoming a singleton cause the common coupling problem (as described above)? If so be very careful you might be better off not implementing this as a singleton. If it doesn't create a common coupling problem (as is the case with many technical requirements which point us to a singleton and won't create a common coupling problem) then a singleton would be a lot better than a global variable (or a static member variable).
   
3. Is the data stored in the singleton object being used from multiple locations in your software and a change from one location will be picked up in another location? If this is the case go back and think really seriously about item #2 in this list, since it looks like your causing a common coupling problem. If it's not a common coupling problem but the question still holds true then ask yourself are the multiple locations that are going to be accessing this singleton located in different layers of the software possible deployed in different processes or across machine boundaries? Will they ever be deployed like that? A nice example of this is current user information. Suppose we are building a multi-layered distributed windows application. The UI layer authenticates the user and puts the current user credentials in a singleton object hoping that one of the back end layers (for example the DAL) will be able to pick it up and use it. In this case your code will work when you're testing it and everything is running in-process but will immediately break once you deploy it to the live environment where the UI resides on client machines and the DAL resides on a server. Why? Because singletons don't automatically get marshaled across process/machine boundaries. Actually that's the case for any global (static) variable and you would have to come up with some solution to get them across. In this kind of a scenario a singleton might work but you need extra effort and code to make sure the data it's holding gets marshaled across to the other process/machine.
   
4. Does the problem domain consider a class to only have one instance, for simplicities sake or is that an immutable fact in the domain? For example suppose we're developing a financial accounting system and in this system we decide to create a singleton called OwnerCorporation: The OwnerCorporation is considered a singleton since the financial accounting system will be sold to one corporation and they will use it to keep a record of their financial records. But this decision (having only one OwnerCorporation object) has been made for simplicities sake it's really not true in the real world and the moment this app has to support other different scenarios in the real world this design decision will cause it to break. For example suppose down the line we decide to sell the app to an accounting firm where it will be used to track the financial accounts of multiple corporations hence needing multiple instances of the OwnerCorporation class. Or in the future we decide to sell our financial accounting services online and the same back-end routines need to be used to support multiple owner corporations each accessing the site to manipulate their specific information, again implementing the OwnerCorporation as a singleton will cause headaches in this scenario.
   

In general you should pay attention to the fact that some object might seem like a singleton from a certain view point but from a different viewpoint it's not a singleton at all (as in item #4 above). This is the case with many other design decisions that we have to make and the cost/time needed to design a perfect scenario that will fit all possible viewpoints is usually so prohibiting that most designers don't really bother. The same case is true for the singleton pattern BUT the issue that should be stressed time and again is that the singleton is not a valid replacement for good (or decent) design or we shouldn't use the singleton as an excuse to create global data shared by everyone and therefore creating common coupling in our system.

I know I promised to review some posts & comments that I found on the internet here but this post is very long as it is so I'll work on it in the next post.

Singleton: Ammendment 1

An excellent issue was raised by Farzad in his comment to my previous post (Singleton) regarding the fact that when you have a multi-instanced singleton and the singleton is going to decide based on the current thread what object to return it's better to store it in the thread using SetData & GetData.
I absolutely agree with Farzad since this will save us from the hassel of clean up issues. Consider the example of the multi-instanced connection manager:

public class ConnectionManager
{
 /* ConnectionManager implementation */

 //Singleton logic:
 private ConnectionManager() { }

 private static Dictionary _instances = new Dictionary();

 public static ConnectionManager Instance
 {
  get
  {
   lock (_instances)
   {
    if (_instances.ContainsKey(System.Threading.Thread.CurrentThread) == false)
     _instances.Add(System.Threading.Thread.CurrentThread,new ConnectionManager());
   }
   return _instances[System.Threading.Thread.CurrentThread];
  }
 }
}

One problem with the above code that you would have to resolve is what happens when a thread is terminated? The singleton will still contain a reference to the ConnectionManager associated with the thread and over time this could cause a memory leak. Now in my previous post this was only supposed to be an example, but in real life you should take this stuff into account.
There are multiple ways to solve this. In some systems you wouldn't have a problem because a very limited number of threads will be created and maintained throughout the lifetime of the system. You could also use weak references that will automatically get cleaned up when the garbage collector is doing a round of clean up.
But a very elegant solution would be using the Thread.SetData & GetData methods. So the result would look like this:

public class ConnectionManager
{
 /* ConnectionManager implementation */

 //Singleton logic:
 private ConnectionManager() { }

 public static ConnectionManager Instance
 {
  get
  {
   if (Thread.GetData(Thread.GetNamedDataSlot("ConnectionManager")) == null)
    Thread.SetData(Thread.GetNamedDataSlot("ConnectionManager"), new ConnectionManager());

   return (ConnectionManager)System.Threading.Thread.GetData(Thread.GetNamedDataSlot("ConnectionManager"));
  }
 }
}

As can be seen in the above code we aren't using an explicit data structure to store our ConnectionManager objects but we are using a place that the thread will provide for storing data specific to this thread. The other interesting bit about the above code is there is no need for locking due to the fact that the SetData & GetData function are operating on the current thread's data and there is no way two threads can access one thread's data at the same time :)

Singleton

As I promised the first pattern that I'm going to write about is Singleton. Now I know that this might be the simplest and the most well known pattern ever existed but I think there are a lot of points that haven't been collected in one location so I'm going to focus on that stuff. Let me re-iterate that I'm not going to get into in depth discussion of what is Singleton but to more advanced topics and discussions, but to start off we need an introduction so here goes:

In software design there are a lot of situations where we need only one instance of an object to exist or a limited control number of objects to exist (of a specific class). This might be due to resource issues or just the simple fact that it doesn't make sense to have more than one object of a specific class. Many consider the singleton pattern an excellent solution to common coupling (global data). This pattern provides a very easy solution to solve the problems of common coupling and at the same time have the ease of use of global data.

OK so let's take a look at a very simple singleton. Let's assume that we have a PrintManager class that we can call its Print method and pass a document to it for printing. Now this class should queue all the documents that it receives and print them in order. For the sake of simplicity let's assume we have only one printer to print to and therefore only one queue. The singleton pattern says that if you want to make a class a singleton write your logic as if you were going to have multiple instances of it (as a regular class) and then perform the following steps on it:

1. Make the constructor private so no one can create an instance.
   
2. Create a private class variable (static) of your class.
   
3. Create a public class method (static) to access the private variable and instantiate it on demand.
   

For example:

public class PrintManager
{
 //The following code is regular implementation not part of
 //the singleton pattern
 private Queue PrintQue; //Sample member variable
 
 public void Print(Document doc)
 {
  //code that would take care of queing
  //and other boring stuff we don't care about
 }
 
 
 //The singleton pattern implementation:
 private PrintManager()
 {
 }
 
 private static PrintManager _instance = null;
 
 public static PrintManager getInstance()
 {
  if (_instance == null)
   _instance = new PrintManager();
  
  return _instance;
 }
}

Obviously the client code needing to use the single instance of the above class will access it through code similar to this:

PrintManager.getInstance().Print(myDoc);

This way no one can create a new instance or remove the one instance that exists, but it's also available for everyone's use. It is also created on first use of the PrintManager.

Items to discuss:

1) Almost all implementations of the singleton pattern should safe guard against a race condition in multi-threaded environments. So the correct implementation of the above code would be:

public class PrintManager
{
 /* Ommited for simplicity */
 
 //The singleton pattern implementation:
 private PrintManager()
 {
 }
 
 private static PrintManager _instance = null;
 private static object LockObject = typeof(PrintManager);
 
 public static PrintManager getInstance()
 {
  lock(LockObject)
  {
   if (_instance == null)
    _instance = new PrintManager();
  }
  
  return _instance;
 }
}

The above code will make sure that no two threads can accidentally enter the getInstance method at the same time and create two instances of the PrintManager object, one overwriting the other.

2) Using C# syntax and relying on a couple of .net CLR features we can rewrite the above code like this:

public class PrintManager
{
 /* Ommited for simplicity */
 
 //The singleton pattern implementation:
 private PrintManager()
 {
 }
 
 private static PrintManager _instance = new PrintManager();
 
 public static PrintManager Instance
 {
  get
  {
   return _instance;
  }
 }
}

And the client code would look like this:

PrintManager.Instance.Print(myDoc);

This is a lot nicer and more readable than the previous code. Also notice that since we are relying on the CLR's static member initialization routines we don't need to worry about a race condition or similar multi threading issues.

3) Another situation that exists is a singleton that might have multiple controlled objects (instead of only one object). To keep with the above example we might want to implement our PrintManager so that it can manage multiple printers. Each printer would have a name and would work completely separately from other printers (it will have its own separate queue etc.). One solution that might immediately pop into mind is changing the Print method to accept two parameters a printer name and a document. But this means changing the logic of our code, a logic that might be working fine and we only needed to extend it. Obviously a bad solution and if you really want to know why go look up functional cohesion. Yes if we mix the code to separate different printers & queues with the printing logic we have basically downgraded our design from functional cohesion level to a lower less cohesive level (in this design it looks like a downgrade to logical cohesion; very badJ).
OK to solve this we need a multi-instance singleton

The Multi-Instanced Singleton:

This variation of the singleton pattern which can be implemented in many different ways is basically a singleton that has more than one instance of its class, but each instance is created and controlled by the singleton. For our current example assuming that each printer would be differentiated using a string name our multi-instance singleton would look like this:

public class PrintManager
{
 /*Same logic as before (no changes needed) */
 
 //The singleton pattern implementation:
 private PrintManager()
 {
 }
 
 //We need a method to keep multiple instances of the PrintManager class:
 private static Dictionary _instances = new Dictionary();
 
 public static PrintManager GetInstance(string printerName)
 {
  lock (_instances)
  {
   if (_instances.ContainsKey(printerName) == false)
    _instances.Add(printerName, new PrintManager());
  }
  
  return _instances[printerName];
 }
}

In all variations of the multi-instanced singleton we always face two design decisions:

• How am I going to store the multiple objects (the data structure needed)?
  
• How am I going to differentiate between the different instances?
  

The first question is usually easy to answer we might need a list or a dictionary or similar data structures. In rare cases we might not even need to store the different instances directly and they would get stored in some other sort of runtime available structure (discussed later).

The second question is a lot more interesting. The above example is a perfect example where the client code will decide which instance it needs to access and the singleton will check if it has that instance, if it does it's returned otherwise it's created (or loaded) and then returned.

Another widely used variety of the pattern is when the singleton class itself can decide which instance to return (the client code just uses the Instance property oblivious to which instance is actually returned. In these cases there has got to be some external way of figuring out which instance to return. As an example suppose we are writing an MDI document processing application. When the user clicks on the Tools menu and selects "Spell Check" we want to invoke the spell check routine passing it the current active document. Now suppose we have implemented Document class as a singleton. When accessing this Document singleton the client code doesn't need to tell it which document all it needs to do is say Document.Instance (whatever object is returned will be the active document).

Another perfectly useful example of this second variation is when you are developing objects that should only exist one per thread. The calling client doesn't need to tell the singleton which object it requires, all that is needed is to ask for the Instance and the active instance will be decided based on the calling thread. See below:

public class ConnectionManager
{
 /* ConnectionManager implementation */
 
 //Singleton logic:
 private ConnectionManager()
 {
 }
 
 private static Dictionary _instances = new Dictionary();
 
 public static ConnectionManager Instance
 {
  get
  {
   lock (_instances)
   {
    if (_instances.ContainsKey(System.Threading.Thread.CurrentThread) == false)
     _instances.Add(System.Threading.Thread.CurrentThread, new ConnectionManager());
   }
   
   return _instances[System.Threading.Thread.CurrentThread];
  }
 }
}

4) Other situations exist that we need a singleton object but this singleton object might be implemented in different ways or need to use mechanisms such as polymorphism to allow different implementation of the singleton. As an example suppose that we were implementing the above ConnectionManager in a single instance singleton but we needed to provide multiple implementations of it. For example an implementation to work with a SQL Server DB and another to work with an Oracle DB. These implementations would differ a lot in the actual logic of the class but would require the same singleton logic and the same access point for clients. In other words we want our clients to be able to say:

ConnectionManager.Instance.OpenConnection();

regardless of whether the SQL Server DB is configured for use or the Oracle version is going to be used. To achieve this we would need a mechanism to decide which version of our ConnectionManager is going to be used at instantiation but that is irrelevant in this example and for the sake of simplicity I'm going to assume that we will use reflection to create the currently configured version of our ConnectionManager and start using it as the only instance available. Please see the following piece of code:

The Polymorphic Singleton:

public abstract class ConnectionManager
{
 //Abstract methods that need to be implemented by concrete ConnectionManagers:
 public abstract void OpenConnection();
 public abstract void CloseConnection();
 // .
 // .
 // .
 
 //Singleton logic:
 
 //Note that in this version the constructor should be protected
 protected ConnectionManager()
 {
 }
 
 private static ConnectionManager _instance = CreateInstance();
 
 public static ConnectionManager Instance
 {
  get
  {
   return _instance;
  }
 }
 
 private static ConnectionManager CreateInstance()
 {
  //let's assume the assembly name and class name of the currently
  //active ConnectionManager is stored in the .config file
  Assembly asm = Assembly.Load(ConfigurationManager.AppSettings["Assembly"]);
  return (ConnectionManager)asm.CreateInstance(ConfigurationManager.AppSettings["CMFullName"]);
 }
}

public class SQLConnectionManager : ConnectionManager
{
 public SQLConnectionManager() : base()
 {
 }
 
 public override void OpenConnection()
 {
  //SQL implementation
 }
 
 public override void CloseConnection()
 {
  //SQL implementation
 }
}

public class OracleConnectionManager : ConnectionManager
{
 public OracleConnectionManager() : base()
 {
 }
  
 public override void OpenConnection()
 {
  //Oracle implementation
 }
 
 public override void CloseConnection()
 {
  //Oracle implementation
 }
}

As is obvious in the above implementation the constructor of our singleton class must be protected (otherwise no one can inherit from it) and the sub-classes need to have a public constructor so that the singleton CreateInstance method can create them on demand.

5) Finally you might even think of a situation where we need a multi-instanced polymorphic singleton as the final type of singleton.

Singletons in Web Apps

A question that I have been asked time and again is what happens in web based applications. Do we still need the singleton pattern there? Can't we just use the Application/Session objects?

YES you can and many people do and feel that it's a lot easier dealing with Application/Session than with a singleton, but I'd like to make these couple of points regarding web based scenarios:

1. In many cases you might be developing a library or a reusable piece of code that will be run in multiple environments. You might need to use it in a web based scenario and a windows based scenario and other similar situations. For these kinds of reusable parts using the Application/Session will tightly couple them with the web-based environment and will prevent their reuse in non-web based environments. To get over this we can implement our singleton using the methods discussed above or implement a polymorphic singleton that depending on the environment that it's in will choose a web based or a windows based implementation.
   
2. Another big question in web based scenarios is what are you using the singleton for? Are you using it to store user specific information, in other words a multi-instanced singleton that has an instance per user storing that users' information or are you designing a singleton (or multi-instanced one) that has no relation to the user. For the former you have to use the Session object otherwise in server farm scenarios you would have to implement a lot of code to replicate your singleton's data across all servers. But for the latter situation a regular singleton should suffice.
   
3. Speed & performance might be another reason you would pick a regular singleton over the Session/Application data. Session data (especially in a server farm environment) could be really slow (object serialization, transfer to state server, storage there and the reverse process). Again if you don't need to replicate that data across all servers in the farm why use a Session object.
   
4. As a general best practice it's better to wrap all your Session/Application needs in a class similar to a singleton implementation so you don't have to deal with strings in accessing Session/Application data and you have strongly typed variables (preventing many runtime time errors). The wrapper you would create around the session/application object can be a singleton in itself.
   

Other patterns:

Many patterns can be combined with the singleton object to solve more complex problems and I will try to touch on them in the next posts, but maybe one of the most famous ones is the Abstract Factory pattern. This pattern in most cases gets combined with a polymorphic singleton and we'll get into this on the next design pattern post I'm going to get into.