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.