Constructor

We use constructors in C# to create classes. Constructors help keep programs simple by making "initialize" methods standard.

In the C# language, a constructor carries the same name as its class. We invoke a constructor with "new." A constructor can have multiple (overloaded) versions.

First example

The Widget class here has a public constructor with an int parameter. To create a Widget, we must call this constructor with an int argument.

using System;

class Widget
{
    int _size;
    public Widget(int size)
    {
        // Part 2: execute constructor logic.
        this._size = size;
    }
}

class Program
{
    static void Main()
    {
        // Part 1: invoke constructor.
        Widget widget1 = new Widget(10);
        Widget widget2 = new Widget(20);
        Console.WriteLine("DONE");
    }
}
DONE

Base constructor

If we have to add a constructor to a derived class, the base keyword is often useful. This will have the derived constructor invoke the base class constructor.

using System;

public class Bird
{
    public Bird(int value)
    {
        Console.WriteLine($"Bird() called with {value}");
    }
}

public class Parrot : Bird
{
    public Parrot(int value) : base(value)
    {
        Console.WriteLine($"Parrot called with {value}");
    }
}

class Program
{
    static void Main()
    {
        Parrot parrot = new Parrot(450);
        Console.WriteLine(":::DONE:::");
    }
}
Bird() called with 450
Parrot called with 450
:::DONE:::

This constructor

Sometimes in a class we have many constructors. We can use "this" to have one constructor invocation call another constructor method. This reduces code bloat.

using System;

class Mouse
{
    public Mouse() : this(-1, "")
    {
        // Uses constructor initializer.
    }

    public Mouse(int weight, string name)
    {
        // Constructor implementation.
        Console.WriteLine("Constructor weight = {0}, name = {1}",
            weight,
            name);
    }
}

class Program
{
    static void Main()
    {
        // Test the 2 constructors for Mouse type.
        Mouse mouse1 = new Mouse();
        Mouse mouse2 = new Mouse(10, "Sam");
    }
}
Constructor weight = -1, name =
Constructor weight = 10, name = Sam

Default constructor

Classes have default constructors. These constructors are injected into all class declarations that do not introduce other constructors. This simplifies syntax.

using System;

class Test // Has default parameterless constructor
{
    public int Value { get; set; }
}

class Program
{
    static void Main()
    {
        // Call the default constructor.
        Test test = new Test();
        test.Value = 1;
        Console.WriteLine(test != null);
    }
}
True

Notes, default constructor

In .NET, code is compiled into an intermediate representation. The default constructors in this program are added there.

.method public hidebysig specialname rtspecialname
    instance void .ctor() cil managed
{
    .maxstack 8
    L_0000: ldarg.0
    L_0001: call instance void [mscorlib]System.Object::.ctor()
    L_0006: ret
}

Public constructor

Public constructors allow a class to be instantiated from an external location in a program. Most constructors will be in the public accessibility domain.

using System;

class Test
{
    public Test(int a)
    {
        if (a == 0)
        {
            throw new ArgumentException("Error", "a");
        }
    }
}

class Program
{
    static void Main()
    {
        Test test = new Test(5);
        Console.WriteLine("DONE");
    }
}
DONE

Private constructor

A private constructor cannot be externally called. It can ensure higher-quality code. It forces the class to provide a controlled and unified access pattern.

using System;

public sealed class Test
{
    public static readonly Test Instance = new Test(); // Singleton pattern.
    public int A; // Instance field.
    private Test() // This is the private constructor.
    {
        this.A = 5;
    }
}

class Program
{
    static void Main()
    {
        // We can access an instance of this object that was created.
        // ... The private constructor was used.
        Test test = Test.Instance;

        // Use the class instance.
        Console.WriteLine(test.A);
        test.A++;
        Console.WriteLine(test.A);
    }
}
5
6

Expression-bodied constructor

A shorter syntax form can be used to specify a constructor. We can use an expression for the constructor's body.

using System;

class Dog
{
    int paws;
    // Expression-bodied constructor.
    // ... Do not use parentheses around the right side.
    public Dog() => paws = 4;
    // Expression-bodied property.
    public int Paws { get => paws; }
}

class Program
{
    static void Main()
    {
        Dog dog = new Dog();
        // Use property to get value of paws.
        Console.WriteLine(dog.Paws);
    }
}
4

Primary constructor

It is possible to specify the fields of a class as an argument list to the class definition. This is called primary constructor syntax.

using System;

class Bird(string color, int feathers)
{
    public void Display()
    {
        // Access fields from primary constructor syntax.
        var colorUppercase = color.ToUpper();
        Console.WriteLine($"Color = {colorUppercase}, Feathers = {feathers}");
    }
}

class Program
{
    static void Main()
    {
        // Use primary constructor class.
        var bird = new Bird("blue", 1000);
        bird.Display();
    }
}
Color = BLUE, Feathers = 1000

Review

There are many kinds of constructors in the C# language. We use constructors to make code clearer—with them, we formalize how objects are created.