Constant vs Readonly in C#: Full Explanation for Beginners

Constant vs Readonly in C#: Full Explanation for Beginners



If you're learning C# programming, you've probably come across the keywords const and readonly. While both are used to define unchangeable values, they behave differently under the hood — and knowing when to use which one can make your code cleaner, safer, and easier to maintain.

In this beginner-friendly guide, we'll break down constant vs readonly in C# with simple examples, practical tips, and easy-to-understand explanations.


What is a Constant in C#?

A constant represents a value that is set at compile-time and never changes during the life of the program.

Here’s what you need to know:

  • Declared with the const keyword.
  • Must be initialized at the time of declaration — you can’t assign it later.
  • Implicitly static, meaning you access it through the class name.
  • Great for values that will never change, like mathematical constants.

Example of a Constant

class MathUtils
{
    public const double Pi = 3.14159;
    public const int MaxValue = 100;
}

Using Constants:

double circumference = 2 * MathUtils.Pi * radius;

In this example:

  • Pi and MaxValue are constants inside the MathUtils class.
  • They are fixed values known at compile time.

What is a Readonly Field in C#?

A readonly field allows a value to be assigned at runtime — either directly where it’s declared or inside a constructor. Once assigned, it cannot be changed.

Key points about readonly fields:

  • Declared with the readonly keyword.
  • Initialized at runtime, giving you more flexibility.
  • Each instance of a class can have different readonly values.
  • Useful when the value needs to be calculated, generated, or passed into a constructor.

Example of a Readonly Field

class Person
{
    public readonly string Name;

    public Person(string name)
    {
        Name = name;
    }
}

Using Readonly Fields:

Person person1 = new Person("Alice");
Person person2 = new Person("Bob");

In this example:

  • Name is a readonly field set during object creation.
  • person1 and person2 can have different names, assigned once at runtime.

Key Differences Between Constant and Readonly in C#

Here’s a simple way to remember the difference:

  • Static or Instance:

    • Constants are implicitly static.
    • Readonly fields are tied to specific instances unless declared static.
  • Assignment Timing:

    • Constants must be assigned at declaration.
    • Readonly fields can be assigned at declaration or in the constructor.
  • Flexibility:

    • Constants are best for known fixed values (e.g., Pi, gravity).
    • Readonly fields are better for values determined at runtime (e.g., a user’s ID).
  • Compilation:

    • Constant values are hard-coded into your compiled program.
    • Readonly fields are set dynamically when your program runs.

When to Use Constant vs Readonly in C#?

Use const when:

  • You know the value will never change (like days of the week, conversion factors).
  • You want a lightweight, compile-time constant.

Use readonly when:

  • The value is only known at runtime.
  • You need different values for different instances.
  • You want the immutability of a field but with flexibility during initialization.

Pro Tip:
Avoid using const for anything that could potentially change in the future, even if it seems unlikely. If the value changes, you’ll have to recompile every assembly that uses it!


Final Thoughts

Understanding the difference between constant and readonly in C# is essential for writing reliable and maintainable code.

Whenever you need a fixed value at compile-time, go for const. If the value is determined at runtime but should never change once set, readonly is your best friend.

Need more help mastering C#? Check out Microsoft’s detailed guides on Constants and Readonly Fields.

Now that you know the difference, make sure to choose the right one in your next C# project!


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