OOP (object oriented programming)What is the class?What do you mean by object?What are the differences between class and object?Can you create an object without using new operator in C#?What is constructor and how many constructors can have one class?Differences between constructor and method of the class? What is default constructor?What is parameterized Constructor in C#?What is private constructor: In what instances you will declare a constructor to be private?What is static constructor, Is it possible to have a static constructor in class. If yes why we need to have a static constructor?Does C# provide copy constructor for an object? How do you call the multiple constructors of a class with single object creation?What is constructor chaining in C#?Can a constructor be called directly from a method?What is constructor overloading and how it’s different than method overloading?What is the difference between constructor overloading and method overloading?Is it possible to overload copy constructor in C#?Can we overload static constructors in C#?Can we overload private constructors in C#?Can we give return type of the constructor in C#?What is the destructor and when it’s called?Is it possible to call constructor and destructor explicitly?What is the Structure and why we need it although we have a class?What are the similarities between Class and Structure?What is the difference between Class and Structure?What is copy structure?What is nested structure?Is it always necessary to create an object of the class?How many different ways to create an object of the class?What are the pros and cons of creating object by new() keyword?What are the pros and cons of delegate object creation to DI container?What are the pros and cons of creating an object by reflection?What are the pros and cons of getting an object from an object pool?What are the pros and cons of creating an object by deserialization?Is it possible to create an object without a class in C#?What is constant?What is static modifier? What are the Static fields and methodsWhat is Static ReadOnly?What are the limitations of static?What is readonly? What’s the difference between constant and read-only?What is this keyword?What is base keyword?What is the difference between this and base keyword?Can “this” keyword be used within static method?What are the accessors?What is the static class? Why we need of static class?If someone wants to create static class then what are the rules for the static class?What are the limitations of using static keyword?What are finalizers in c#?How to create N number of instances of C# class?What are the Nested Classes and why we use them?What are the basic four pillars of OOP?What is the Inheritance and why we need of inheritance?How do you inherit a class into other class in C#?What is the concept of base and derive class?What are the different types of inheritance?We have two classes’ base class and child class. A is the base class and B is the child class, If we create an instance of child class then which class’s constructor called first?Does a derived class can inherit the constructors of its base class?What should we do that if we create an object of child class then the parameterized constructor of base class must be invoked?As we know that base constructor invoked first when we create instance of child class but if we create an instance of child class by parameterized constructor and base class has both default and parameterized constructor then which constructor of the base will be invoked?Can you assign an object of derived class to the variable of base class and if both have the same method name then which will be invoked?Can we create instance of base class and store it to derive class?Can we create derive class object inside base class, and if create instance of child class then what will happen?Can we inherit child class from 2 base classes? if yes then how? If not then why?Does C# support Multiple Inheritance?Why multiple inheritance is not supported in C# and why it’s supported in C++?How is multiple inheritance achieved in C#?What are Access Modifiers? Explain private, public, protected, internal, protected internal access modifiersWhat are the default access modifiers of the class?Why classes cannot be declared as protected?Can we declare private class in namespace?What are the valid access specifier used for the declaration of class at namespace level? If we inherit a class, do the private variables also get inherited?Can you prevent your class from being inherited?Can you prevent your class from being inherited without using sealed keyword?What is abstraction?What is encapsulation?What is the difference between abstraction and encapsulation?What is polymorphism?What is static or compile time polymorphism?What is runtime polymorphism or late binding or dynamic binding?What is method overloading?When and why we should use overload methods?What is inheritance based overloading?What are the advantages of using overloading?Can we overload the method in the same class?What is the execution control flow in overloaded methods?What is method overriding?What s virtual keyword?What are the key points to make the method as overridden?When it is must to override the method?When a derived class can overrides the base class member?Can we declare fields inside the class as virtual?When we treat sub-class method as an overriding method?Can we override private virtual method in c#?Can we override method in the same class?Can we execute parent class method if it is overridden in the child class?If we have virtual in base class and the same method is overridden in child class, by creating instance of child class and assign it to base class, then which of the method will be invoked first.What is the difference between method overloading and method overriding?What is method hiding?Can you access a hidden method in the derived which is declared in the base class?What is the difference between method overriding and method hiding?You have a component with 2 parameters and deployed to client side, now you have changed your method with 3 parameters, how can you deploy this without affecting the client code?What is operator overloading?What is abstract class and why we need of it?What are the rules of abstract classes?What is an abstract method?What is concrete method?When do you use abstract class in C#?When to use the abstract method in C#?

What is readonly? What’s the difference between constant and read-only?

In C#, 'readonly' is a keyword used to declare fields whose values can only be assigned at the time of declaration or within the constructor of the class. Once the 'readonly' field is assigned a value, it cannot be modified or changed afterward. The readonly modifier is often used for fields that need to be set during construction and remain constant throughout the lifetime of the object.

Here's an example of a 'readonly' field in use:

public class Car
    public readonly string Make;
    public readonly string Model;
    public readonly int ProductionYear;

    public Car(string make, string model, int productionYear)
        Make = make;
        Model = model;
        ProductionYear = productionYear;

    public void DisplayCarDetails()
        Console.WriteLine($"Make: {Make}, Model: {Model}, Production Year: {ProductionYear}");

class Program
    static void Main()
        Car car1 = new Car("Toyota", "Camry", 2022);
        Car car2 = new Car("Honda", "Civic", 2023);

        car1.DisplayCarDetails(); // Output: Make: Toyota, Model: Camry, Production Year: 2022
        car2.DisplayCarDetails(); // Output: Make: Honda, Model: Civic, Production Year: 2023

        // Error: Cannot assign to 'ProductionYear' because it is read-only
        // car1.ProductionYear = 2024;

In this example, we have a 'Car' class with three readonly fields: 'Make', 'Model', and 'ProductionYear'. These fields are initialized only once, either at the time of declaration or within the constructor of the class. Once assigned, their values cannot be changed.

In the 'Main' method, we create two 'Car' objects 'car1' and 'car2', and their properties 'Make', 'Model', and 'ProductionYear' are set during object creation. After initialization, we cannot modify these fields for each car instance, ensuring that their values remain constant throughout the object's lifetime.

Now, let's highlight the key differences between 'constant' and 'readonly':

  1. Value Assignment:
    • 'constant' : The value of a 'constant' must be assigned at the time of declaration, and it cannot be changed at any point during the program's execution. It is determined at compile-time and remains constant throughout the program.
    • 'readonly' : The value of a 'readonly' field can be assigned either at the time of declaration or within the constructor of the class. Once assigned, its value cannot be modified. The value is determined at runtime during object creation and can vary between different objects of the same class.
  2. Usage:
    • 'constant' : Constants are typically used for values that remain the same for all instances of the class and are known at compile time. They are often used for settings, fixed values, or mathematical constants.
    • 'readonly' : Read-only fields are useful for values that are instance-specific and need to be set at runtime during object creation. They are suitable for values that are known only at runtime or need to be initialized based on constructor parameters.
  3. Scope:
    • 'constant' : Constants are static by default and are accessed through the class name, not through object instances.
    • 'readonly' : Read-only fields can be static or instance-specific. Static 'readonly' fields are shared among all instances of the class, while instance-specific 'readonly' fields are specific to each object instance.

In summary, 'constant' is used for fixed, compile-time known values that remain the same throughout the program's execution, while 'readonly' is used for values that can be set at runtime and remain constant for each object instance. Both provide a way to enforce constant behavior, but they are used in different scenarios depending on the nature of the values they represent.