Checked and unchecked arithmetic in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of checked arithmetic overflow

What is the output of this C# code snippet?

C Sharp (C#)

using System;
class Program {
  static void Main() {
    try {
      int max = int.MaxValue;
      int result = checked(max + 1);
      Console.WriteLine(result);
    } catch (OverflowException) {
      Console.WriteLine("Overflow caught");
    }
  }
}

A -2147483648

B 2147483648

C Overflow caught

D Compilation error

💡 Hint

Think about what happens when you add 1 to the maximum int value inside a checked block.

The checked keyword causes the runtime to throw an OverflowException when the arithmetic operation exceeds the range of the int type. Adding 1 to int.MaxValue causes overflow, so the exception is caught and "Overflow caught" is printed.

❓ Predict Output

intermediate

Unchecked arithmetic overflow result

What is the output of this C# code snippet?

C Sharp (C#)

using System;
class Program {
  static void Main() {
    int max = int.MaxValue;
    int result = unchecked(max + 1);
    Console.WriteLine(result);
  }
}

A -2147483648

B 2147483648

C Overflow caught

D Compilation error

💡 Hint

Consider what unchecked does when an integer overflows.

The unchecked keyword disables overflow checking, so the addition wraps around. Adding 1 to int.MaxValue results in int.MinValue due to overflow wrap-around.

🧠 Conceptual

advanced

Behavior difference between checked and unchecked

Which statement best describes the difference between checked and unchecked arithmetic in C#?

A checked disables overflow exceptions; unchecked enables overflow exceptions.

B checked causes overflow exceptions; unchecked ignores overflow and wraps around silently.

C checked and unchecked both throw exceptions on overflow.

D checked and unchecked both ignore overflow and wrap around silently.

💡 Hint

Think about what happens when an integer operation exceeds its limits in each mode.

The checked keyword enables overflow checking and throws exceptions on overflow. The unchecked keyword disables overflow checking and allows wrap-around without exceptions.

❓ Predict Output

advanced

Output of mixed checked and unchecked blocks

What is the output of this C# program?

C Sharp (C#)

using System;
class Program {
  static void Main() {
    int x = int.MaxValue;
    try {
      unchecked {
        x = x + 1;
      }
      checked {
        x = x + 1;
      }
      Console.WriteLine(x);
    } catch (OverflowException) {
      Console.WriteLine("Overflow caught");
    }
  }
}

A 2147483648

B Overflow caught

C -2147483648

D -2147483647

💡 Hint

Consider the effect of unchecked and checked blocks on the variable x.

The first addition in unchecked wraps x from int.MaxValue to int.MinValue (-2147483648). The second addition in checked tries to add 1 to int.MinValue, which is valid and results in -2147483647. However, since the first addition was unchecked, no exception occurs there. The second addition is within checked and does not overflow, so no exception is thrown. The program prints -2147483647.

❓ Predict Output

expert

Output of checked arithmetic in a method with unchecked context

What is the output of this C# program?

C Sharp (C#)

using System;
class Program {
  static int AddOneChecked(int value) {
    checked {
      return value + 1;
    }
  }
  static void Main() {
    unchecked {
      int max = int.MaxValue;
      try {
        int result = AddOneChecked(max);
        Console.WriteLine(result);
      } catch (OverflowException) {
        Console.WriteLine("Overflow caught");
      }
    }
  }
}

A Overflow caught

B -2147483648

C Compilation error

D 2147483648

💡 Hint

Remember that checked and unchecked contexts are scoped and can be nested.

Even though Main is in an unchecked context, the AddOneChecked method uses a checked block. The addition of 1 to int.MaxValue inside checked triggers an OverflowException, which is caught and prints "Overflow caught".

Practice

1.

What does the checked keyword do in C# arithmetic operations?

easy

A. It ignores overflow and wraps the value around.

B. It converts numbers to floating point before calculation.

C. It throws an exception if an arithmetic overflow occurs.

D. It optimizes arithmetic operations for speed.

Solution

1. Step 1: Understand the purpose of checked

   The checked keyword is used to detect overflow in arithmetic operations.

2. Step 2: Behavior on overflow

   When overflow happens inside a checked block, it throws an OverflowException.

3. Final Answer:

   It throws an exception if an arithmetic overflow occurs. -> Option C

4. Quick Check:

   checked = throws exception on overflow [OK]

Hint: Checked throws error on overflow, unchecked ignores it [OK]

Common Mistakes:

• Thinking checked ignores overflow
• Confusing checked with unchecked
• Assuming checked improves speed

2.

Which of the following is the correct syntax to use unchecked for an arithmetic operation in C#?

int result = ?;

easy

A. unchecked(2147483647 + 1);

B. checked 2147483647 + 1;

C. unchecked { 2147483647 + 1 };

D. unchecked 2147483647 + 1;

Solution

1. Step 1: Recall unchecked syntax

   The unchecked keyword can be used as a method-like call: unchecked(expression).

2. Step 2: Identify correct usage

   unchecked(2147483647 + 1); uses unchecked(2147483647 + 1); which is valid syntax for unchecked arithmetic.

3. Final Answer:

   unchecked(2147483647 + 1); -> Option A

4. Quick Check:

   unchecked(expression) is correct syntax [OK]

Hint: Use unchecked(expression) for unchecked arithmetic [OK]

Common Mistakes:

• Using unchecked without parentheses
• Using checked keyword instead
• Missing braces for block syntax

3.

What is the output of the following C# code?

int max = int.MaxValue;
int result = unchecked(max + 1);
Console.WriteLine(result);

medium

A. Throws OverflowException

B. 0

C. 2147483648

D. -2147483648

Solution

1. Step 1: Understand unchecked behavior

   Using unchecked allows overflow to wrap around without exception.

2. Step 2: Calculate overflow wrap

   int.MaxValue is 2147483647; adding 1 wraps to -2147483648 (minimum int value).

3. Final Answer:

   -2147483648 -> Option D

4. Quick Check:

   unchecked wraps overflow to negative min int [OK]

Hint: Unchecked wraps overflow to min int value [OK]

Common Mistakes:

• Expecting exception on unchecked overflow
• Thinking int can hold 2147483648
• Confusing unchecked with checked

4.

Identify the error in this code snippet and fix it:

checked
{
    int x = int.MaxValue;
    int y = x + 10;
    Console.WriteLine(y);
}

medium

A. OverflowException occurs; fix by using unchecked block.

B. No error; code runs fine.

C. Syntax error; missing semicolon after checked.

D. Use long type instead of int to fix overflow.

Solution

1. Step 1: Analyze checked block behavior

   The checked block throws an OverflowException when overflow occurs.

2. Step 2: Identify overflow cause and fix

   Adding 10 to int.MaxValue causes overflow; using unchecked block prevents exception by wrapping.

3. Final Answer:

   OverflowException occurs; fix by using unchecked block. -> Option A

4. Quick Check:

   checked throws on overflow, unchecked ignores [OK]

Hint: Use unchecked to avoid overflow exceptions [OK]

Common Mistakes:

• Ignoring overflow exception
• Trying to fix with syntax changes only
• Assuming checked allows overflow silently

5.

You want to safely add two int values a and b and detect overflow. Which code snippet correctly does this?

int a = int.MaxValue;
int b = 1;
int result;
// What to write here?
Console.WriteLine(result);

hard

A. result = checked(a + b);

B. try { result = checked(a + b); } catch (OverflowException) { result = 0; }

C. result = unchecked(a + b);

D. result = a + b; // no checked or unchecked

Solution

1. Step 1: Understand safe addition with overflow detection

   Using checked detects overflow but throws exception; we must catch it.

2. Step 2: Use try-catch with checked

   Wrap addition in try block with checked and catch OverflowException to handle overflow safely.

3. Final Answer:

   try { result = checked(a + b); } catch (OverflowException) { result = 0; } -> Option B

4. Quick Check:

   Use checked with try-catch for safe overflow handling [OK]

Hint: Use checked inside try-catch to catch overflow [OK]

Common Mistakes:

• Using unchecked and expecting overflow detection
• Not catching OverflowException
• Ignoring overflow and risking wrong results