C# Keywords Tutorial Part 68: readonly

In C#, the readonly keyword is used to declare that a field or variable can only be assigned a value once, either at the time of declaration or in the constructor of the class where it’s defined. Once assigned, the value cannot be changed for the lifetime of the instance.

Here’s a brief explanation and example:

using System;

public class MyClass
{
    // Declaration with initialization
    public readonly int readOnlyField = 10;

    // Constructor
    public MyClass(int value)
    {
        // Assigning value to readonly field in the constructor
        readOnlyField = value;
    }

    public void ChangeValue(int newValue)
    {
        // Error: Cannot assign to 'readOnlyField' because it is read-only
        // readOnlyField = newValue;
    }
}

class Program
{
    static void Main(string[] args)
    {
        MyClass myObj = new MyClass(20);
        Console.WriteLine(myObj.readOnlyField); // Output: 20

        // Error: Cannot assign to 'readOnlyField' because it is read-only
        // myObj.readOnlyField = 30;
    }
}

In this example:

• readOnlyField is declared as readonly, so it can be assigned a value either at the time of declaration or within the constructor.
• Once assigned, its value cannot be modified.
• Any attempt to modify the value of readOnlyField outside of the constructor or without using the constructor will result in a compilation error.

The readonly keyword is useful when you have fields whose values are supposed to be set once and remain constant throughout the lifetime of the object. It ensures the immutability of those fields, adding to the predictability and maintainability of your code.