Constants and readonly fields in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of constant and readonly fields

What is the output of this C# program?

C Sharp (C#)

using System;
class Test {
    public const int ConstValue = 10;
    public readonly int ReadonlyValue;
    public Test(int val) {
        ReadonlyValue = val;
    }
    public void PrintValues() {
        Console.WriteLine($"ConstValue = {ConstValue}, ReadonlyValue = {ReadonlyValue}");
    }
}
class Program {
    static void Main() {
        Test t = new Test(20);
        t.PrintValues();
    }
}

A ConstValue = 20, ReadonlyValue = 10

B ConstValue = 10, ReadonlyValue = 10

C ConstValue = 0, ReadonlyValue = 20

D ConstValue = 10, ReadonlyValue = 20

💡 Hint

Remember that const values are fixed at compile time and readonly values can be set in the constructor.

ConstValue is a constant set to 10 and cannot change. ReadonlyValue is set in the constructor to 20, so it prints 20.

❓ Predict Output

intermediate

Readonly field modification attempt

What happens when you try to compile and run this code?

C Sharp (C#)

class Sample {
    public readonly int ReadonlyField = 5;
    public void Change() {
        ReadonlyField = 10;
    }
}
class Program {
    static void Main() {
        Sample s = new Sample();
        s.Change();
        System.Console.WriteLine(s.ReadonlyField);
    }
}

A Compilation error: Cannot assign to 'ReadonlyField' because it is readonly

B Output: 10

C Output: 5

D Runtime error: Field is readonly

💡 Hint

Readonly fields can only be assigned in declaration or constructor.

Assigning to a readonly field outside the constructor or declaration causes a compile-time error.

🧠 Conceptual

advanced

Difference between const and readonly

Which statement correctly describes the difference between const and readonly fields in C#?

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

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

C Both const and readonly values can be changed anytime during program execution.

D const and readonly are interchangeable and have no difference.

💡 Hint

Think about when the values are assigned and if they can change after that.

const fields are fixed at compile time and cannot be changed. readonly fields can be assigned once at runtime, typically in the constructor.

❓ Predict Output

advanced

Readonly field with static constructor

What is the output of this program?

C Sharp (C#)

using System;
class Example {
    public static readonly int Value;
    static Example() {
        Value = 42;
    }
}
class Program {
    static void Main() {
        Console.WriteLine(Example.Value);
    }
}

A 42

B 0

C Compilation error: readonly field must be assigned inline or in instance constructor

D Runtime error: static constructor failed

💡 Hint

Static readonly fields can be assigned in static constructors.

Static readonly fields can be assigned in static constructors and keep that value for the program lifetime.

❓ Predict Output

expert

Constant expression evaluation

What is the output of this program?

C Sharp (C#)

using System;
class ConstTest {
    public const int A = 5;
    public const int B = A * 2;
    public const int C = B + 3;
}
class Program {
    static void Main() {
        Console.WriteLine(ConstTest.C);
    }
}

A 8

B 10

C 13

D Compilation error: const expressions cannot use other const fields

💡 Hint

Const expressions can use other const fields in their definitions.

Const fields can be used in expressions to define other const fields. Here C = (5*2)+3 = 13.

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