Static Classes vs Singleton Pattern

2017-02-10

Classes and objects

In object-oriented programming, classes are templates containing any combination of methods and variables. A class definition determines the components of the instances of that class, which are called objects. Objects exhibit behaviours defined by its class.

To illustrate, the snippet below defines a class, instantiates an object, and shows how an object behaves according to its class definition:

// Class definition
public class President()
{
    // Instance constructor
    public President(string lastName, int yearElected)
    {
        LastName = lastName;
        YearElected = yearElected;
    }

    // Class Fields
    private string LastName;
    private int YearElected;

    // Class Methods
    public string GetLastName()
    {
        return LastName;
    }

    public string PrintInfo()
    {
        return $"President {LastName} was elected in {YearElected}.";
    }
}

// Object instantiation
President currentPresident = new President("Trump", 2016);

// Prints "Trump"
Console.WriteLine(currentPresident.GetLastName());

// Prints "President Trump was elected in 2016."
Console.WriteLine(currentPresident.PrintInfo());

Static classes (in C#)

While classes are usually written to be instantiated into objects, static classes cannot be instantiated. As such, they can have neither instance constructors nor non-static members. One way to think about static classes is that they are functionally the same as any regular class with only static members and a private constructor.

Here is a good example of a static class definition from MSDN. I also added a couple lines of code using the static methods. Note the static keyword in the class and member definitions:

// Source: MSDN (https://msdn.microsoft.com/en-CA/library/79b3xss3.aspx)
public static class TemperatureConverter
{
    public static double CelsiusToFahrenheit(string temperatureCelsius)
    {
        // Convert argument to double for calculations.
        double celsius = Double.Parse(temperatureCelsius);

        // Convert Celsius to Fahrenheit.
        double fahrenheit = (celsius * 9 / 5) + 32;

        return fahrenheit;
    }

    public static double FahrenheitToCelsius(string temperatureFahrenheit)
    {
        // Convert argument to double for calculations.
        double fahrenheit = Double.Parse(temperatureFahrenheit);

        // Convert Fahrenheit to Celsius.
        double celsius = (fahrenheit - 32) * 5 / 9;

        return celsius;
    }
}

// Using the methods inside the static class
// Since we don't instantiate static classes, methods are instead called on the class itself
double tempInF = TemperatureConverter.CelsiusToFahrenheit(25);
double tempInC = TemperatureConverter.FahrenheitToCelsius(70);
Console.WriteLine($"25c in F is {tempInF}.");
Console.WriteLine($"25F in c is {tempInC}.");

Singleton pattern

Whereas the static keyword is a feature of the C# (and Java) language, the Singleton is an architectural design pattern. Like its name suggests, a Singleton is a class that has a maximum of one instance. Through lazy initialization, a singleton class can be implemented such that it is created during runtime precisely the first time it is needed. This makes it inherently more lightweight than a static class, which is always created at compile time.

This is the simplest possible implementation of a Singleton from MSDN:

// Source: MSDN (https://msdn.microsoft.com/en-us/library/ms998558.aspx)
public class Singleton
{
    private static Singleton instance;

    private Singleton() {}

    public static Singleton Instance
    {
        get
        {
            if (instance == null)
            {
                instance = new Singleton();
            }
            return instance;
        }
    }
}

We can see from the code that the constructor is marked private, which allows only methods in the same class to create a new instance of the class. The Instance method is the the only public method that calls the constructor, and it is designed to only call that constructor if an instance of the class does not already exist.

Static classes vs Singleton pattern

Developers don’t necessarily need to choose between static components and a Singleton design pattern. In fact in C#, a good implementation of Singleton usually incorporates some static members. In general though, static classes are easier to write and use correctly. Personally, I like to write static classes for ancillary components in my projects, such as tools and utilities.

Singletons have the potential to offer better performance and extensibility — which often makes it a good choice over the long run. However, it is more difficult to write correctly, especially within applications that implement multithreading.