Checked and unchecked arithmetic in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable overflow checking for the addition.

C Sharp (C#)

checked {
    int result = 1000000 [1] 3000;
}

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication instead of addition.

Forgetting to use the checked block for overflow detection.

The checked block ensures that the addition operation throws an exception if overflow occurs. The operator for addition is +.

2fill in blank

medium

Complete the code to disable overflow checking for the multiplication.

C Sharp (C#)

unchecked {
    int result = 20000 [1] 2000;
}

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

A-

B/

C+

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition or division instead of multiplication.

Expecting overflow exceptions inside unchecked blocks.

The unchecked block disables overflow checking. The operator for multiplication is *.

3fill in blank

hard

Fix the error in the code to correctly check for overflow during subtraction.

C Sharp (C#)

checked {
    int result = 1000 [1] 2000;
}

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

A+

B/

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition or multiplication instead of subtraction.

Not using checked block for overflow detection.

The subtraction operator is -. Using it inside a checked block will detect overflow during subtraction.

4fill in blank

hard

Fill both blanks to create a dictionary with keys as numbers and values as their squares, using checked arithmetic for multiplication and a condition to include only squares less than 100.

C Sharp (C#)

var squares = new Dictionary<int, int> {
    { [1], checked([1] [2] [1]) }
};

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

A5

B*

C10

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using a number other than 5 for the key.

Using addition or division instead of multiplication.

The key is 5 and the value is its square using multiplication * inside checked to detect overflow.

5fill in blank

hard

Fill all three blanks to create a checked block that divides two numbers and assigns the result to a variable.

C Sharp (C#)

checked {
    int result = [1] [2] [3];
}

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

A100

B/

C5

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using multiplication instead of division.

Swapping the numbers causing wrong division.

The expression divides 100 by 5 using the division operator / inside a checked block to detect overflow or exceptions.

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