Difference Between const and readonly in C#

🔍 Introduction

In C#, both const and readonly are used to declare unchangeable values. However, their use cases, behavior, and limitations differ significantly.

📌 What is const?

• Short for constant
• Value must be assigned at compile time
• Implicitly static — shared across instances
• Must be initialized at declaration
• Cannot be modified elsewhere

public class MyClass
{
    public const double Pi = 3.14159;
}
  

📌 What is readonly?

• Value can be assigned at declaration or in constructor
• Can be instance-level or static
• Initialized at runtime
• More flexible than const

public class MyClass
{
    public readonly int Id;

    public MyClass(int id)
    {
        Id = id; // Set only once
    }
}
  

⚖️ Comparison: const vs readonly

Feature	const	readonly
When value is assigned	Compile time	Runtime (declaration or constructor)
Static	Always	Optional
Set in constructor?	No	Yes
Supports runtime data?	No	Yes
Allowed types	Primitive, string, enum	Any type
Use case	Compile-time constants	Instance or dynamic values

💼 Practical Use Cases

✅ Use const when:

• The value never changes
• It’s known at compile time
• Performance and inlining are important

✅ Use readonly when:

• The value is assigned at runtime
• You need constructor-based initialization
• You use complex types or objects

📌 Summary

Use const for fixed, compile-time values. Use readonly for flexible, runtime-assigned values that shouldn't change after initialization. Choosing the right one improves maintainability and safety in your C# code.