Constructors

Constructors

What it means

A constructor is a special method that runs automatically when an object is created with new, used to set up its initial state. It shares the class’s name and has no return type. A class can have multiple constructors with different parameters (overloading), and one constructor can call another with this(...) to avoid duplicating setup logic.

Examples

public class Person
{
    public string Name { get; }
    public int Age { get; }

    public Person(string name, int age)
    {
        Name = name;
        Age = age;
    }

    public Person(string name) : this(name, 0)   // calls the constructor above
    {
    }
}

var alice = new Person("Alice", 30);
var baby = new Person("Baby");        // Age defaults to 0 via constructor chaining

// If a class defines NO constructor at all, C# provides a free parameterless one
public class Empty { }
var e = new Empty();   // works -- implicit default constructor

Common mistake

Defining a custom constructor with parameters and then being surprised that new MyClass() (parameterless) no longer compiles — once you define any constructor, the compiler-provided implicit parameterless constructor disappears.

public class Person
{
    public string Name;
    public Person(string name) { Name = name; }
}

var p = new Person();   // Error: no argument given that corresponds to the required parameter 'name'

// Fix: explicitly add a parameterless constructor if you still need one
public class Person
{
    public string Name;
    public Person() { }
    public Person(string name) { Name = name; }
}

Quick practice

1. What happens to the implicit parameterless constructor once you define any constructor yourself?

   Answer

   It disappears — you must explicitly add a parameterless constructor if you still want new MyClass() to work.

2. What does : this(name, 0) do in a constructor’s signature?

   Answer

   Constructor chaining — it calls another constructor on the same class first, letting you reuse its setup logic instead of duplicating it.

3. Can a class have more than one constructor?

   Answer

   Yes — as long as each has a different parameter list (constructor overloading), just like regular method overloading.