📦 C# Constants – Define Fixed Values with const in C#

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🧲 Introduction – Why Use Constants in C#?

In C#, constants are used to define immutable values that remain unchanged throughout the life of a program. Declaring constants helps prevent accidental changes and makes your code more reliable, readable, and maintainable.

🎯 In this guide, you’ll learn:

• How to declare and use constants in C#
• Differences between const and readonly
• When and why constants are preferred
• Common best practices and examples

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🔍 Core Concept – What Are Constants?

A constant in C# is a value that is assigned at compile time and cannot be changed later. It must be declared with the const keyword and initialized when defined.

🔹 Syntax:

const data_type constant_name = value;

Example:

const double Pi = 3.14159;

🧠 Note: Constants are implicitly static and accessible without object instantiation.

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💻 Code Example – Declaring and Using Constants

using System;

class Circle
{
    const double Pi = 3.14159;

    static void Main()
    {
        double radius = 5;
        double area = Pi * radius * radius;

        Console.WriteLine($"Radius: {radius}");
        Console.WriteLine($"Area: {area}");
    }
}

📤 Output:

Radius: 5
Area: 78.53975

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⚙️ const vs readonly

Feature	const	readonly
Assigned when?	At compile time	At runtime (in constructor)
Mutability	Immutable, cannot change	Immutable after constructor
Use Case	Fixed, unchanging values (e.g., Pi)	Runtime-initialized fixed values
Static?	Implicitly static	Must be explicitly marked if static

📘 Best Practice: Use const for universal constants (e.g., Pi, MaxSize) and readonly for configuration or runtime constants.

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💡 Tips, Pitfalls & Best Practices

💡 Tip: Use all-uppercase naming convention for constants (MAX_USERS, DEFAULT_RATE) in some enterprise teams.

⚠️ Pitfall: Avoid using const for values that might change in future versions — re-compilation is needed in every referencing assembly.

📘 Best Practice: Place constants in a dedicated Constants class or static class for global access and organization.

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🛠️ Use Cases – Where to Use Constants

• Mathematical constants like Pi, E
• Limits like MAX_RETRIES, TIMEOUT
• Configuration settings that never change
• Message templates or version strings

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📌 Summary – Recap & Next Steps

C# constants are perfect for fixed values that never change. They’re compile-time safe, help prevent bugs, and improve code clarity.

🔍 Key Takeaways:

• Use const for compile-time fixed values
• Constants are implicitly static
• Use readonly when the value is known only at runtime

⚙️ Coming next: Dive into 📦 C# Data Types to understand how different values are categorized and stored.

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❓ FAQ – C# Constants

❓ What is a constant in C#?
✅ A const is a fixed value known at compile time and cannot be modified later.

❓ Can I assign a constant inside a method?
✅ Yes, but the value must still be fixed at compile time.

❓ What’s the difference between const and readonly?
✅ const is for compile-time constants, readonly for values set at runtime.

❓ Are constants implicitly static?
✅ Yes. Constants are treated as static by the compiler.

❓ Can constants be public?
✅ Yes. You can specify access modifiers like public const int Max = 100;.

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