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C# Constructors

A constructor is a special method that runs when an object is created. It sets the starting values of the new object.

A constructor sets a field

The Car constructor takes one value. It stores that value in the model field.

using System;

class Car
{
    public string model;

    public Car(string modelName)
    {
        model = modelName;
    }
}

class Program
{
    static void Main(string[] args)
    {
        Car myCar = new Car("Ford");
        Console.WriteLine(myCar.model);
    }
}

Two parameters

This constructor takes two values. It stores one value in each field.

using System;

class Car
{
    public string model;
    public int year;

    public Car(string modelName, int carYear)
    {
        model = modelName;
        year = carYear;
    }
}

class Program
{
    static void Main(string[] args)
    {
        Car myCar = new Car("Ford", 2020);
        Console.WriteLine(myCar.model);
        Console.WriteLine(myCar.year);
    }
}

Two constructors, one class

Car has two constructors. The call with no value runs the first one.

using System;

class Car
{
    public string model;

    public Car()
    {
        model = "Unknown";
    }

    public Car(string modelName)
    {
        model = modelName;
    }
}

class Program
{
    static void Main(string[] args)
    {
        Car firstCar = new Car();
        Car secondCar = new Car("BMW");
        Console.WriteLine(firstCar.model);
        Console.WriteLine(secondCar.model);
    }
}

Example

using System;

class Student
{
    public string name;
    public int grade;

    public Student(string studentName, int studentGrade)
    {
        name = studentName;
        grade = studentGrade;
    }
}

class Program
{
    static void Main(string[] args)
    {
        Student student = new Student("Hana", 9);
        Console.WriteLine(student.name + " is in grade " + student.grade + ".");
    }
}

Key points

  • A constructor has the same name as its class.
  • A constructor has no return type.
  • The constructor runs at the moment new creates the object.
  • Parameters pass starting values into the new object.
  • A class can hold more than one constructor.
  • Each constructor in a set needs a different parameter list.

In this example, the Student constructor takes a name and a grade. It stores Hana in the name field. It stores 9 in the grade field. The last line joins the two fields into one sentence.

Note

A constructor runs only once for each object.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Programiz C# Compiler

Exercise

When does a constructor run?