Difference Between const and readonly in C#

Difference Between const and readonly in C#

In C#, both const and readonly are used to declare values that should not change after assignment. However, they serve different purposes and behave differently in key aspects such as initialization timing, usage, and flexibility.

What is const?

• const is short for constant.
• The value must be assigned at compile time.
• It is implicitly static, meaning it belongs to the type, not an instance.
• It must be initialized at the time of declaration.
• Cannot be changed anywhere else in the code.

Example:

public class MyClass

{

    public const double Pi = 3.14159;

}

What is readonly?

• readonly means the value can only be assigned once, either at declaration or in a constructor.
• It can be used for instance-level or static data.
• Its value can be determined at runtime.
• It provides more flexibility than const.

Example:

public class MyClass

{

    public readonly int Id;

 

    public MyClass(int id)

    {

        Id = id; // Can be set here

    }

}

Comparison Between const and readonly

Feature	const	readonly
When value is assigned	At compile time	At runtime (declaration or constructor)
Static	Always	Optional
Can be set in constructor	No	Yes
Can hold runtime data	No	Yes
Allowed types	Primitive, string, enum	Any type
Use case	Fixed values like Pi, max limits	Values that vary per instance or are initialized dynamically

Practical Use Cases

Use const when:

• The value never changes.
• The value is known at compile time.
• You want maximum performance and inlining by the compiler.

Use readonly when:

• The value is only known at runtime.
• You want to assign the value in the constructor.
• You are working with complex types or need more flexibility.

Summary

Use const for compile-time constants, and readonly for runtime-assigned but immutable values. Choosing the right keyword helps ensure clarity, safety, and appropriate usage in the codebase.