Difference between static class and singleton patt

Question

What real (i.e. practical) difference exists between a static class and a singleton pattern?

Both can be invoked without instantiation, both provide only one "Instance" and neither of them is thread-safe. Is there any other difference?

Answer 1

A static class is one that has only static methods, for which a better word would be "functions". The design style embodied in a static class is purely procedural.

Singleton, on the other hand, is a pattern specific to OO design. It is an instance of an object (with all the possibilities inherent in that, such as polymorphism), with a creation procedure that ensures that there is only ever one instance of that particular role over its entire lifetime.

Answer 2

I'm not a great OO theorist, but from what I know, I think the only OO feature that static classes lack compared to Singletons is polymorphism. But if you don't need it, with a static class you can of course have inheritance ( not sure about interface implementation ) and data and function encapsulation.

The comment of Morendil, "The design style embodied in a static class is purely procedural" I may be wrong, but I disagree. In static methods you can access static members, which would be exactly the same as singleton methods accessing their single instance members.

edit:
I'm actually thinking now that another difference is that a Static class is instantiated at program start* and lives throughout the whole life span of the program, while a singleton is explicitly instantiated at some point and can be destroyed also.

* or it may be instantiated at first use, depending on the language, I think.

Answer 3

Well a singleton is just a normal class that IS instantiated but just once and indirectly from the client code. Static class is not instantiated. As far as I know static methods (static class must have static methods) are faster than non-static.

Edit:
FxCop Performance rule description: "Methods which do not access instance data or call instance methods can be marked as static (Shared in VB). After doing so, the compiler will emit non-virtual call sites to these members which will prevent a check at runtime for each call that insures the current object pointer is non-null. This can result in a measurable performance gain for performance-sensitive code. In some cases, the failure to access the current object instance represents a correctness issue."
I don't actually know if this applies also to static methods in static classes.

Answer 4

static classes should not do anything need state, it is useful for putting bunch of functions together i.e Math (or Utils in projects). So the class name just give us a clue where we can find the functions and there's nothing more.

Singleton is my favorite pattern and use it to manage something at a single point. It's more flexible than static classes and can maintain state. It can implement interfaces, inherit from other classes and allow inheritance.

My rule for choosing between static and singleton:

If there are bunch of functions should be kept together, then static is the choice. Anything else which needs single access to some resources, could be implemented singleton.

Answer 5

Static Class:-

1. You cannot create the instance of static class.

2. Loaded automatically by the .NET Framework common language runtime (CLR) when the program or namespace containing the class is loaded.

3. Static Class cannot have constructor.

4. We cannot pass the static class to method.

5. We cannot inherit Static class to another Static class in C#.

6. A class having all static methods.

7. Better performance (static methods are bonded on compile time)

Singleton:-

1. You can create one instance of the object and reuse it.

2. Singleton instance is created for the first time when the user requested.

3. Singleton class can have constructor.

4. You can create the object of singleton class and pass it to method.

5. Singleton class does not say any restriction of Inheritance.

6. We can dispose the objects of a singleton class but not of static class.

7. Methods can be overridden.

8. Can be lazy loaded when need (static classes are always loaded).

9. We can implement interface(static class can not implement interface).

Answer 6

We have our DB framework that makes connections to Back end.To Avoid Dirty reads across Multiple users we have used singleton pattern to ensure we have single instance available at any point of time.

In c# a static class cannot implement an interface. When a single instance class needs to implement an interface for a business contracts or IoC purposes, this is where I use the Singleton pattern without a static class

Singleton provides a way to maintain state in stateless scenarios

Hope that helps you..