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

The true answer is by Jon Skeet, on another forum here.

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.

Answer 2

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 3

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 4

In singleton pattern you can create the singleton as an instance of a derived type, you can't do that with a static class.

Quick Example:

if( useD3D )
    IRenderer::instance = new D3DRenderer
else
    IRenderer::instance = new OpenGLRenderer

Answer 5

To illustrate Jon's point 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 6

a. Serialization - Static members belong to the class and hence can't be serialized.

b. Though we have made the constructor private, static member variables still will be carried to subclass.

c. We can't do lazy initialization as everything will be loaded upon class loading only.