Why Constants and readonly fields in C Sharp (C#)? - Purpose & Use Cases

The Big Idea

What if one small number change could break your whole program? Constants and readonly fields save you from that nightmare.

The Scenario

Imagine you have a program where you use the same value, like the number of days in a week, in many places. You write the number 7 everywhere by hand.

Later, you realize you need to change it or you accidentally type 8 somewhere. This causes bugs and confusion.

The Problem

Manually typing the same value repeatedly is slow and risky. You might mistype it or forget to update all places if the value changes.

This leads to errors that are hard to find and fix.

The Solution

Constants and readonly fields let you store a value once and use it everywhere safely.

Constants are fixed forever, while readonly fields can be set once when the program runs.

This keeps your code clean, easy to update, and less error-prone.

Before vs After

✗ Before

int daysInWeek = 7; // repeated everywhere
int totalDays = 7 * 4; // what if 7 changes?

✓ After

const int DaysInWeek = 7;
int totalDays = DaysInWeek * 4; // safe and clear

What It Enables

You can write safer, clearer code that is easy to maintain and less likely to break when values need to change.

Real Life Example

Think of a game where the maximum player health is always 100. Using a constant for this value means you never accidentally use the wrong number, and if you want to change max health later, you do it in one place.

Key Takeaways

Manually repeating values causes bugs and wastes time.

Constants and readonly fields store values safely and clearly.

This makes your code easier to read, update, and trust.

Practice

1. What is the main difference between const and readonly fields in C#?

easy

A. const and readonly are exactly the same and can be used interchangeably.

B. readonly values are set at compile time and cannot change; const values can be set at runtime but only once.

C. const values are set at compile time and cannot change; readonly values can be set at runtime but only once.

D. const values can be changed anytime; readonly values cannot be changed.

Solution

1. Step 1: Understand const behavior

   const fields are fixed at compile time and cannot be changed later.

2. Step 2: Understand readonly behavior

   readonly fields can be assigned once at runtime, usually in a constructor, and then cannot change.

3. Final Answer:

   const values are set at compile time and cannot change; readonly values can be set at runtime but only once. -> Option C

4. Quick Check:

   const fixed at compile, readonly set once at runtime [OK]

Hint: Remember: const fixed early, readonly set once later [OK]

Common Mistakes:

• Thinking readonly can be changed multiple times
• Confusing when const values are set
• Assuming const can be assigned in constructors

2. Which of the following is the correct way to declare a constant integer named MaxValue with value 100 in C#?

easy

A. readonly int MaxValue = 100;

B. const int MaxValue = 100;

C. int const MaxValue = 100;

D. static readonly MaxValue = 100;

Solution

1. Step 1: Check correct syntax for constant declaration

   In C#, const must be followed by type, then name and value, like const int MaxValue = 100;.

2. Step 2: Identify incorrect options

   readonly requires a type and cannot be declared like a constant; int const is invalid syntax in C#; static readonly requires a type and cannot omit it.

3. Final Answer:

   const int MaxValue = 100; -> Option B

4. Quick Check:

   const syntax = const type name = value; [OK]

Hint: const must include type before name [OK]

Common Mistakes:

• Using 'int const' instead of 'const int'
• Omitting type in readonly declaration
• Confusing const and readonly syntax

3. What will be the output of the following C# code?

class Test {
    public readonly int Value;
    public Test(int val) {
        Value = val;
    }
}

class Program {
    static void Main() {
        var t = new Test(10);
        Console.WriteLine(t.Value);
    }
}

medium

A. 10

B. 0

C. Compilation error

D. Runtime error

Solution

1. Step 1: Understand readonly assignment in constructor

   The readonly field Value is assigned in the constructor with val, which is 10.

2. Step 2: Check output of Console.WriteLine

   The program prints the assigned value of Value, which is 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   readonly set in constructor prints assigned value [OK]

Hint: readonly can be set once in constructor, then used [OK]

Common Mistakes:

• Assuming readonly defaults to 0 if assigned in constructor
• Thinking readonly cannot be assigned in constructor
• Expecting compilation or runtime errors

4. Identify the error in the following C# code:

class Sample {
    public const int Number;
    public Sample() {
        Number = 5;
    }
}

medium

A. Cannot assign a value to a const field outside declaration.

B. Const fields must be assigned in constructor, not declaration.

C. Const fields can be assigned multiple times.

D. No error, code is correct.

Solution

1. Step 1: Check const assignment rules

   const fields must be assigned a value when declared and cannot be assigned later.

2. Step 2: Analyze code assignment

   The code tries to assign Number = 5; inside the constructor, which is invalid for const.

3. Final Answer:

   Cannot assign a value to a const field outside declaration. -> Option A

4. Quick Check:

   const must be assigned only at declaration [OK]

Hint: const value fixed at declaration, not in constructor [OK]

Common Mistakes:

• Trying to assign const in constructor
• Confusing const with readonly assignment rules
• Assuming no error in code

5. You want to create a class that stores a fixed tax rate that never changes and a discount rate that can be set once when the object is created but never changed after. Which declarations are correct in C#?

hard

A. public double TaxRate = 0.15; public readonly double DiscountRate;

B. public readonly double TaxRate = 0.15; public const double DiscountRate;

C. public static double TaxRate = 0.15; public double DiscountRate;

D. public const double TaxRate = 0.15; public readonly double DiscountRate;

Solution

1. Step 1: Assign fixed tax rate with const

   The tax rate never changes, so const is appropriate for TaxRate.

2. Step 2: Assign discount rate once at object creation

   The discount rate can be set once when the object is created, so readonly is suitable for DiscountRate.

3. Step 3: Eliminate incorrect options

   public readonly double TaxRate = 0.15; public const double DiscountRate; swaps const and readonly incorrectly; public static double TaxRate = 0.15; public double DiscountRate; uses mutable fields; public double TaxRate = 0.15; public readonly double DiscountRate; uses non-const for tax rate.

4. Final Answer:

   public const double TaxRate = 0.15; public readonly double DiscountRate; -> Option D

5. Quick Check:

   const fixed forever, readonly set once at creation [OK]

Hint: Use const for fixed, readonly for one-time set [OK]

Common Mistakes:

• Using readonly for fixed values
• Using const for values set at runtime
• Not understanding when to use static