C# - Constructor Q & A

Constructor Questions & Answers in C#

1. What is the difference between a constructor and a method in C#?

Answer: 

Constructor: A constructor is used to initialize an object when it is created. It does not have a return type and must have the same name as the class.

Method: A method performs an operation or computation and can return a value (or void if no return is needed). A method can be called multiple times on an object, while a constructor is called only once when the object is instantiated.

2. What is a default constructor in C#?

Answer: A default constructor is a constructor that takes no parameters. If no constructor is explicitly defined, C# provides an implicit default constructor that initializes the object with default values (e.g., null for reference types, 0 for numeric types).

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

    public Person() // Default constructor
    {
        Name = "Unknown";
        Age = 0;
    }
}

3. What is a parameterized constructor in C#?

Answer: A parameterized constructor is a constructor that accepts one or more arguments to initialize the object with specific values when it is created.

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

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

4. Can we have multiple constructors in C#?

Answer: Yes, C# allows constructor overloading, meaning you can have multiple constructors with different parameter lists in the same class. The appropriate constructor is chosen based on the arguments passed when the object is instantiated.

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

    // Default constructor
    public Person()
    {
        Name = "Unknown";
        Age = 0;
    }

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

5. What is a static constructor in C#?

Answer: A static constructor is used to initialize static members of a class. It is automatically called once, before any instance of the class is created or any static members are accessed. It cannot take parameters and does not have an access modifier.

public class Database
{
    public static string ConnectionString;
    static Database() // Static constructor
    {
        ConnectionString = "Server=localhost;Database=myDB;";
    }
}

6. What is a private constructor in C#?

Answer: A private constructor is a constructor that can only be called from within the class itself. It is typically used in design patterns like the Singleton pattern, where you want to restrict the instantiation of the class to a single object.

public class Singleton {

    private static Singleton _instance;   
    private Singleton() {} // Private constructor

    public static Singleton GetInstance()
    {
        if (_instance == null)
            _instance = new Singleton();
        return _instance;
    }
}

7. What is a copy constructor in C#?

Answer: A copy constructor is a constructor that creates a new object as a copy of an existing object. It takes an object of the same type as a parameter and copies its data to the new object. C# does not provide a built-in copy constructor, but you can define one manually.

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


    // Copy constructor
    public Person(Person other)
    {
        Name = other.Name;
        Age = other.Age;
    }
}

8. What is constructor chaining in C#?

Answer:

Constructor chaining refers to calling one constructor from another constructor within the same class using the this keyword. It helps avoid code duplication when you need to initialize the object in different ways.

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

    public Person() : this("Unknown", 0) {} // Chaining to the parameterized constructor
    public Person(string name, int age)
    {
        Name = name;
        Age = age;
    }
}

9. What is the purpose of the this keyword in constructors in C#?

Answer: The this keyword is used to refer to the current instance of the class. In constructors, it can be used for constructor chaining (calling another constructor in the same class) or to distinguish between parameters and class fields when they have the same name.

public class Person
{
    public string Name;
    public int Age;
    public Person(string name, int age)
    {
        this.Name = name; // 'this' differentiates class field from parameter
        this.Age = age;
    }
}

10. What happens if you don’t define a constructor in a class?

Answer:

If no constructor is defined, C# provides a default constructor with no parameters. For reference types, it will initialize fields to their default values (like null), and for value types, it will initialize them to their default values (like 0 for integers).