What is the difference between static class and singleton pattern?

The true difference between static class and singleton pattern is the following:

A singleton allows access to a single created instance – that instance (or rather, a reference to that instance) can be passed as a parameter to other methods, and treated as a normal object.

A static class allows only static methods.

The big difference between a singleton and a bunch of static methods is that singletons can implement interfaces (or derive from useful base classes, although that’s less common IME), so you can pass around the singleton as if it were “just another” implementation.

Singletons are easier to work with when unit testing a class. Wherever you pass singletons as a parameter (constructors, setters or methods) you can instead substitute a mocked or stubbed version of the singleton.

Difference between static class and singleton pattern:

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. We cannot pass the static class to method.
4. We cannot inherit Static class to another Static class in C#.
5. A class having all static methods.
6. 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. You can create the object of singleton class and pass it to method.
4. Singleton class does not say any restriction of Inheritance.
5. We can dispose the objects of a singleton class but not of static class.
6. Methods can be overridden.
7. Can be lazy loaded when need (static classes are always loaded).
8. We can implement interface(static class can not implement interface).

Answer #2:

static classes are not for anything that needs state. It is useful for putting a bunch of functions together i.e Math (or Utils in projects). So the class name just gives us a clue where we can find the functions and nothing more.

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

My rule for choosing between static and singleton:

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

Answer #3:

What makes you say that either a singleton or a static method isn’t thread-safe? Usually both should be implemented to be thread-safe.

The big difference between a singleton and a bunch of static methods is that singletons can implement interfaces (or derive from useful base classes, although that’s less common, in my experience), so you can pass around the singleton as if it were “just another” implementation.

Static class vs singleton pattern- Answer #4:

1. Singleton objects are stored in Heap, but static objects are stored in stack.
2. We can clone (if the designer did not disallow it) the singleton object, but we can not clone the static class object .
3. Singleton classes follow the OOP (object oriented principles), static classes do not.
4. We can implement an interface with a Singleton class, but a class’s static methods (or e.g. a C# static class) cannot.

Answer #5:

The Singleton pattern has several advantages over static classes. First, a singleton can extend classes and implement interfaces, while a static class cannot (it can extend classes, but it does not inherit their instance members). A singleton can be initialized lazily or asynchronously while a static class is generally initialized when it is first loaded, leading to potential class loader issues. However the most important advantage, though, is that singletons can be handled polymorphically without forcing their users to assume that there is only one instance.

Answer #6:

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 #7:

To illustrate the first answer, what’s shown below cannot be done if Logger was a static class.The class SomeClass expects an instance of ILogger implementation to be passed into its constructor.

Singleton class is important for dependency injection to be possible.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {

            var someClass = new SomeClass(Logger.GetLogger());
        }


    }

    public class SomeClass 
    {
        public SomeClass(ILogger MyLogger)
        {

        }
    }

    public class Logger : ILogger
    {
        private static Logger _logger;
        private Logger() { }

        public static Logger GetLogger()
        {
            if (_logger==null)
            {
                _logger = new Logger();
            }

            return _logger;
        }

        public void Log()
        {

        }

    }


    public interface ILogger
    {
         void Log();
    }
}

Answer #8:

The main differences are:

• Singleton has an instance/object while static class is a bunch of static methods
• Singleton can be extended e.g. through an interface while static class can’t be.
• Singleton can be inherited which supports open/close principles in SOLID principles on the other hand static class can’t be inherited and we need to make changes in itself.
• Singleton object can be passed to methods while static class as it does not have instance can’t be passed as parameters

Answer #9:

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.

Hope you learned something from this post.

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