Difference between constant and read-only in C# dotnet

Hello Kiran,

Please refer below article for more information.

Difference Between ReadOnly and Constant in .NET C#

Thanks

1. Definition

2. When the Value is Assigned

Example:

public class Demo
{
    public const int ConstValue = 10; // compile-time constant

    public readonly int ReadOnlyValue; // runtime constant

    public Demo(int val)
    {
        ReadOnlyValue = val; // can be assigned here
    }
}

3. Changeability

4. Memory Behavior

5. Scope and Usage

Example:

public static readonly MyClass Obj = new MyClass(); // valid
public const MyClass Obj = new MyClass(); // ? invalid

6. Example Comparison

public class Example
{
    public const int ConstValue = 100; // fixed at compile-time
    public readonly int ReadOnlyValue;

    public Example(int val)
    {
        ReadOnlyValue = val; // assigned at runtime
    }

    public void Display()
    {
        Console.WriteLine(ConstValue);   // always 100
        Console.WriteLine(ReadOnlyValue); // depends on constructor input
    }
}

Usage:

Example e1 = new Example(50);
Example e2 = new Example(200);
e1.Display(); // Const=100, ReadOnly=50
e2.Display(); // Const=100, ReadOnly=200

Summary Table

Example Use Case

• Use const for values that never change, like PI, DaysInWeek, etc.

• Use readonly for values that depend on runtime input, like a config ID, database connection string, or environment setting.

https://www.c-sharpcorner.com/blogs/difference-between-constant-and-readonly

Const are Constants, and its value is assigned at the time of compilation while readonly is  runtime variable which is assign on runtime.
Both cannot be changed once assigned

Const is a compile-time constant, while readonly is assigned at runtime and cannot be changed afterward.