Difference Between readonly and const in C# with Examples?

Introduction

When working with C#, developers often come across two keywords that look similar but behave quite differently: readonly and const.

At first glance, both seem to represent values that cannot be changed, but their behavior, usage, and purpose are not the same.

Understanding the difference between readonly vs const in C# is important for writing clean, maintainable, and predictable code—especially in real-world applications.

In this article, we will break down both concepts in a clear and practical way, along with examples, use cases, and common mistakes.

What is const in C#?

A const (constant) is a value that is fixed at compile time and cannot be changed anywhere in the program.

Key Characteristics of const

Value must be assigned at the time of declaration
Value is known at compile time
Cannot be modified later
By default, it is static (shared across all instances)

Example of const in C#

public class Example
{
    public const int MaxUsers = 100;
}

Explanation

MaxUsers is assigned a value at compile time
This value is embedded directly into the compiled code
It cannot be changed at runtime or in constructors

Real-world Example

public class AppSettings
{
    public const string AppName = "MyApplication";
}

This is useful for values that never change, like application name, mathematical constants, or fixed configuration values.

What is readonly in C#?

A readonly field is a variable whose value can only be assigned at runtime, but only once—either during declaration or inside a constructor.

Key Characteristics of readonly

Value can be assigned at runtime
Can be assigned in constructor
Cannot be modified after initialization
Not necessarily static

Example of readonly in C#

public class User
{
    public readonly DateTime CreatedAt;

    public User()
    {
        CreatedAt = DateTime.Now;
    }
}

Explanation

CreatedAt is assigned when the object is created
Each object can have a different value
Once set, it cannot be changed

Real-world Example

public class Order
{
    public readonly Guid OrderId;

    public Order()
    {
        OrderId = Guid.NewGuid();
    }
}

Each order gets a unique ID at runtime, which cannot be modified later.

Key Differences Between readonly and const in C#

1. Time of Assignment

const → Assigned at compile time
readonly → Assigned at runtime

2. Where Value is Set

const → Must be set during declaration
readonly → Can be set in constructor

3. Flexibility

const → Completely fixed
readonly → Flexible during object creation

4. Static Behavior

const → Always static
readonly → Can be instance-level or static

5. Use Case

const → Fixed values (PI, limits, strings)
readonly → Values depending on runtime logic

Code Comparison Example

public class Demo
{
    public const int ConstValue = 10;
    public readonly int ReadonlyValue;

    public Demo(int value)
    {
        ReadonlyValue = value;
    }
}

Explanation

ConstValue is fixed and shared across all objects
ReadonlyValue can vary for each instance

When to Use const in C#

Use const when:

Value is known at compile time
Value will never change
Value is universal across application

Examples

Mathematical constants
Fixed strings
Configuration flags

When to Use readonly in C#

Use readonly when:

Value depends on runtime logic
Value should not change after initialization
Each object may have different values

Examples

Database IDs
Timestamps
Configuration loaded at runtime

Common Mistakes Developers Make

Using const for runtime values

This leads to compilation errors because const requires compile-time values.

Overusing readonly for fixed values

If a value never changes, const is more appropriate and efficient.

Forgetting that const is static

Many developers assume const behaves like instance variables, which is incorrect.

Advantages of const

Faster performance (compile-time optimization)
Memory efficient
Easy to use for fixed values

Advantages of readonly

More flexible
Supports runtime initialization
Safer for dynamic scenarios

Summary

The difference between readonly and const in C# lies in when and how their values are assigned. Const is used for values that are completely fixed and known at compile time, while readonly is used for values that are assigned at runtime but should not change afterward. Choosing the right keyword helps improve code clarity, performance, and maintainability in ASP.NET Core and C# applications.