Difference between readonly and const keyword in C#

In C#, both readonly and const are used to create fields whose values cannot be modified after being set. The key difference is when the value is determined − const is resolved at compile time, while readonly is resolved at runtime (in the constructor).

const Keyword

The const keyword creates a compile-time constant. The value must be assigned at the time of declaration and cannot be changed afterward. const fields are implicitly static and can be declared inside methods.

readonly Keyword

The readonly keyword creates a runtime constant. The value can be assigned either at declaration or inside a constructor, allowing different instances to have different values. readonly fields cannot be declared inside methods.

Key Differences

Example

The following example demonstrates both keywords ?

using System;

public class Program {
    // const: must be assigned here, implicitly static
    public const int MAX_VALUE = 10;

    // readonly: can be assigned in constructor
    public readonly int instanceValue;

    Program(int value) {
        instanceValue = value;  // OK: assigned in constructor
    }

    public static void Main() {
        // const accessed via class name (static)
        Console.WriteLine("const MAX_VALUE: " + MAX_VALUE);

        // readonly can differ per instance
        Program p1 = new Program(11);
        Program p2 = new Program(22);
        Console.WriteLine("readonly p1: " + p1.instanceValue);
        Console.WriteLine("readonly p2: " + p2.instanceValue);
    }
}

The output of the above code is ?

const MAX_VALUE: 10
readonly p1: 11
readonly p2: 22

Notice how const has a single fixed value, while readonly allows each instance to have a different value assigned through the constructor.

Conclusion

Use const for values that are truly constant and known at compile time (like mathematical constants or fixed configuration). Use readonly for values that are determined at runtime and may vary between instances (like configuration loaded from a file or values passed to a constructor).