Constants and readonly fields in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a constant integer named MaxValue with value 100.

C Sharp (C#)

public class Config {
    public const int MaxValue = [1];
}

Drag options to blanks, or click blank then click option'

Aint

Breadonly

CMaxValue

D100

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'readonly' instead of a value.

Not assigning a value to the constant.

The keyword const declares a constant value that cannot change. Here, MaxValue is set to 100.

2fill in blank

medium

Complete the code to declare a readonly field named creationTime initialized in the constructor.

C Sharp (C#)

public class Logger {
    public readonly DateTime creationTime;

    public Logger() {
        creationTime = [1];
    }
}

Drag options to blanks, or click blank then click option'

ADateTime

Breadonly

CDateTime.Now

Dnew DateTime()

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to assign a type name instead of a value.

Using 'readonly' keyword as a value.

The readonly field creationTime can be assigned in the constructor. DateTime.Now gives the current time.

3fill in blank

hard

Complete the code to initialize the readonly field named configName in the constructor.

C Sharp (C#)

public class Settings {
    public readonly string configName;

    public Settings(string name) {
        configName = [1];
    }
}

Drag options to blanks, or click blank then click option'

Aname

B"default"

Cthis.name

DconfigName

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string literal instead of the parameter.

Using 'this.name' which does not refer to the parameter.

The readonly field configName can only be assigned at declaration or inside constructors. Assign the constructor parameter name to it.

4fill in blank

hard

Fill both blanks to declare a constant double Pi and a readonly double radius initialized in the constructor.

C Sharp (C#)

public class Circle {
    public const double Pi = [1];
    public readonly double radius;

    public Circle(double r) {
        radius = [2];
    }
}

Drag options to blanks, or click blank then click option'

A3.14159

Br

Cradius

D3.14

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning radius to itself instead of the parameter.

Using an incorrect value for Pi.

Pi is a constant set to 3.14159. The readonly field radius is assigned the constructor parameter r.

5fill in blank

hard

Fill all three blanks to declare a constant int MaxScore, a readonly int currentScore initialized in the constructor, and a method returning their sum.

C Sharp (C#)

public class Game {
    public const int MaxScore = [1];
    public readonly int currentScore;

    public Game(int score) {
        currentScore = [2];
    }

    public int TotalScore() {
        return MaxScore + [3];
    }
}

Drag options to blanks, or click blank then click option'

A100

Bscore

CcurrentScore

D50

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect variable names in the return statement.

Assigning readonly fields outside constructor.

MaxScore is a constant set to 100. The readonly field currentScore is assigned the constructor parameter score. The method TotalScore returns the sum of MaxScore and currentScore.

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