Checked and unchecked arithmetic in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the checked keyword do in C#?

The checked keyword forces the runtime to throw an OverflowException if an arithmetic operation exceeds the limits of the data type.

beginner

What happens when arithmetic overflow occurs inside an unchecked block?

The overflow is ignored and the result wraps around without throwing an exception.

intermediate

Give an example of when you might want to use checked arithmetic.

Use checked when you want to catch errors caused by values exceeding the limits, such as in financial calculations where accuracy is critical.

beginner

What is the default behavior of arithmetic overflow in C# if neither checked nor unchecked is specified?

By default, arithmetic overflow in C# does not throw an exception and behaves like unchecked, wrapping around silently.

intermediate

How can you apply checked or unchecked to a single expression?

You can wrap the expression inside checked(expression) or unchecked(expression) to control overflow checking for just that expression.

What exception does checked arithmetic throw on overflow?

A OverflowException

B NullReferenceException

C IndexOutOfRangeException

D DivideByZeroException

checked arithmetic throws OverflowException when the result exceeds the data type limits.

What is the default overflow behavior in C# if no keyword is used?

A Throws OverflowException

B Program crashes

C Throws NullReferenceException

D Wraps around silently

By default, overflow wraps around silently, like unchecked.

Which keyword forces overflow checking only for a specific expression?

A checked(expression)

B unchecked(expression)

C overflow(expression)

D safe(expression)

You can use checked(expression) to check overflow for just that expression.

When would you prefer unchecked arithmetic?

A When debugging errors

B When you want exceptions on overflow

C When performance is critical and overflow is acceptable

D When working with strings

unchecked avoids exceptions and can be faster when overflow is expected or acceptable.

Which of these is true about checked and unchecked blocks?

A <code>checked</code> disables overflow checking

B <code>checked</code> enables overflow exceptions

C <code>unchecked</code> enables overflow exceptions

D Both behave the same

checked enables overflow exceptions; unchecked disables them.

Explain the difference between checked and unchecked arithmetic in C#.

Think about what happens when numbers get too big.

Describe how to use checked and unchecked for controlling overflow in expressions and blocks.

Consider both whole blocks of code and single calculations.

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