Constants and readonly fields in C Sharp (C#) - Mini Project: Build & Apply

Constants and readonly fields

📖 Scenario: You are creating a simple program to store information about a car. Some details about the car never change, like the number of wheels, while others can be set once when the car is created and then stay the same, like the car's model year.

🎯 Goal: Build a C# program that uses a const field for the number of wheels and a readonly field for the model year. You will see how to set these values and print them.

📋 What You'll Learn

Create a class called Car.

Add a const int field named NumberOfWheels with the value 4.

Add a readonly int field named ModelYear.

Create a constructor for Car that takes an int parameter to set ModelYear.

Create a method DisplayInfo that prints the number of wheels and model year.

In Main, create a Car object with model year 2023 and call DisplayInfo.

💡 Why This Matters

🌍 Real World

Constants and readonly fields are used in programs to keep important values safe from accidental changes, like fixed settings or initial data.

💼 Career

Understanding constants and readonly fields is important for writing reliable and maintainable code in many software development jobs.

1

Create the Car class with a const field

Create a class called Car and inside it, declare a const int field named NumberOfWheels and set it to 4.

C Sharp (C#)

public class Car
{
    // Your code here
}

Hint

Use public const int NumberOfWheels = 4; inside the Car class.

2

Add a readonly field and constructor

Inside the Car class, add a readonly int field named ModelYear. Then create a constructor for Car that takes an int modelYear parameter and sets the ModelYear field.

C Sharp (C#)

public class Car
{
    public const int NumberOfWheels = 4;

    // Your code here
}

Hint

Use public readonly int ModelYear; and set it inside the constructor.

3

Add a method to display car info

Inside the Car class, add a method called DisplayInfo that prints the number of wheels and the model year using Console.WriteLine. Use the fields NumberOfWheels and ModelYear.

C Sharp (C#)

public class Car
{
    public const int NumberOfWheels = 4;
    public readonly int ModelYear;

    public Car(int modelYear)
    {
        ModelYear = modelYear;
    }

    // Your code here
}

Hint

Use Console.WriteLine with the fields inside DisplayInfo.

4

Create a Car object and show info

In the Main method, create a Car object named myCar with model year 2023. Then call myCar.DisplayInfo() to print the details.

C Sharp (C#)

using System;

public class Car
{
    public const int NumberOfWheels = 4;
    public readonly int ModelYear;

    public Car(int modelYear)
    {
        ModelYear = modelYear;
    }

    public void DisplayInfo()
    {
        Console.WriteLine($"Number of wheels: {NumberOfWheels}");
        Console.WriteLine($"Model year: {ModelYear}");
    }
}

class Program
{
    static void Main()
    {
        // Create a Car object with model year 2023 and call DisplayInfo
        // Your code here
    }
}

Hint

Create the object with new Car(2023) and call DisplayInfo().

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