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.

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.