Const vs Readonly vs Init in C# - Complete Guide

Const vs Readonly vs Init in C# - Complete Guide

Understanding the difference between const, readonly, and init is extremely important for mastering C# fundamentals and clearing .NET interviews.

🎥 Watch the full explanation video here: Const vs Readonly vs Init in C#

1️⃣ What is const in C#?

A const field is a compile-time constant. Its value must be known at compile time and must be assigned at declaration. 

public const double PI = 3.14159;
  • Implicitly static
  • Must be assigned at declaration
  • Cannot be changed
  • Value is embedded at compile time

⚠ Important Interview Point

If you change a const value in a library, all referencing assemblies must be recompiled.

2️⃣ What is readonly in C#?

A readonly field is a runtime constant. It can be assigned either at declaration or inside the constructor. 

public class Student
{
    public readonly DateTime AdmissionDate;

    public Student()
    {
        AdmissionDate = DateTime.Now;
    }
}
  • Evaluated at runtime
  • Can use runtime values
  • Can only be assigned in constructor or declaration

3️⃣ What is init in C#?

The init keyword (introduced in C# 9) allows properties to be set only during object initialization. 

public class Employee
{
    public string Name { get; init; }
}

var emp = new Employee { Name = "Sudipto" };
  • Works with properties
  • Allows object initializer syntax
  • Becomes immutable after initialization

🔎 Constructor Behavior

When using object initializer syntax: 

var emp = new Employee { Name = "Sudipto" };

The constructor runs first, and then the property is assigned.

🧠 Key Differences Summary

Feature const readonly init
Evaluated At Compile Time Runtime Runtime
Works On Fields Fields Properties
Assigned in Constructor No Yes No
Immutable After Creation Yes Yes Yes

🎯 Final Thoughts

If you want strong fundamentals in C#, you must clearly understand when values are determined — compile time vs runtime — and how immutability works in .NET.

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🚀 Keep Learning. Stay Fundamental.

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