C# - Difference Between Const and ReadOnly

There are many different keywords available in C#. This tutorial will look at the differences between the readonly and const keywords in C# and when to use each.

Const in C#

By default, a const is static. In C#, a constant is a value that cannot be changed during the execution or run time of a program. That means value of Const will not change during whole program. It's mandatory to assign a value to it when we declared it. 

Readonly in C#

A Readonly field can be initialized either at the time of declaration or within the constructor of the same class. We can also change the value of a Readonly at runtime or assign a value to it at runtime (but in a non-static constructor only).

Difference

• Initialization

  • const - Must be initialized at the time of declaration and cannot be changed afterwards.
    

    public const int MaxValue = 100; 

  • readonly - Can be initialized either at the time of declaration or within a constructor. This allows for different values to be assigned to different instances of a class.
    

    public readonly int MaxValue;
    public MyClass(int maxValue)
    {
        MaxValue = maxValue; // Allowed in constructor
    } 

• Value Type

  • const - The value must be a compile-time constant, such as a number, string, or other primitive type.
    

    public const double Pi = 3.14159;

  • readonly -The value can be any type, including objects, and can be determined at runtime.
    

    public readonly DateTime CreationTime = DateTime.Now;

• Scope

  • const - Implicitly static, meaning it belongs to the type itself and not to any instance. It is shared across all instances.
    

    public const int MaxValue = 100;

  • readonly - Can be either instance-specific or static. If declared as static, it is shared across all instances; if not, each instance can have its own value.
    

    public readonly int MaxValue; // Instance-specific
    public static readonly int GlobalValue = 100; // Shared across all instances

    
    

• Mutability

  • const - Completely immutable after declaration.
  • readonly - Immutable after initialization (either at declaration or in the constructor).

When to use Const and Readonly?

• const

  • Suitable for fixed values known at compile time.
  • Implicitly static, meaning they belong to the type itself and are shared among all instances.
  • Must be initialized at declaration and cannot change afterwards.
    

• readonly

  • Suitable for values that should not change after initialization but are not known until runtime.
  • Can be instance-specific or static.
  • Can be initialized at declaration or in a constructor.

Difference between Const and ReadOnly