Checked and unchecked arithmetic in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Checked and unchecked arithmetic

Start Arithmetic Operation

↓

Is 'checked' context?

No→Perform unchecked operation

|Yes↓

Perform checked operation

↓

Overflow?

No→Return result

Yes↓

Throw OverflowException

The program decides if arithmetic is checked or unchecked. Checked operations detect overflow and throw an error; unchecked operations ignore overflow.

Execution Sample

C Sharp (C#)

int a = int.MaxValue;
int b = 1;
int c = checked(a + b);
int d = unchecked(a + b);

This code adds 1 to the maximum int value using checked and unchecked contexts to show overflow behavior.

Execution Table

Step	Operation	Context	Calculation	Overflow Detected	Result or Exception
1	a = int.MaxValue	N/A	a = 2147483647	No	a = 2147483647
2	b = 1	N/A	b = 1	No	b = 1
3	c = checked(a + b)	checked	2147483647 + 1	Yes	OverflowException thrown
4	d = unchecked(a + b)	unchecked	2147483647 + 1	Yes but ignored	-2147483648 (wrap around)

💡 Execution stops at step 3 due to OverflowException in checked context; unchecked continues with wrap-around.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
a	undefined	2147483647	2147483647	2147483647	2147483647
b	undefined	undefined	1	1	1
c	undefined	undefined	undefined	Exception	Exception
d	undefined	undefined	undefined	undefined	-2147483648

Key Moments - 2 Insights

Why does step 3 throw an OverflowException but step 4 does not?

Step 3 uses 'checked' context which detects overflow and throws an exception, as shown in execution_table row 3. Step 4 uses 'unchecked' context which ignores overflow and wraps the value, as shown in row 4.

What does 'unchecked' arithmetic do when overflow happens?

'Unchecked' arithmetic ignores overflow and wraps around the value to fit the data type, demonstrated in execution_table row 4 where the result becomes -2147483648.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of variable 'd' after step 4?

A OverflowException

B 2147483648

C -2147483648

D 2147483647

💡 Hint

Check the 'Result or Exception' column in row 4 of the execution_table.

At which step does the program throw an OverflowException?

A Step 3

B Step 2

C Step 4

D No exception thrown

💡 Hint

Look at the 'Overflow Detected' and 'Result or Exception' columns in the execution_table.

If we remove 'checked' from step 3, what would happen to variable 'c'?

A It would throw OverflowException

B It would wrap around like 'd'

C It would be zero

D It would be unchanged

💡 Hint

Compare the behavior of checked vs unchecked contexts in the execution_table rows 3 and 4.

Concept Snapshot

Checked and unchecked arithmetic in C# control overflow behavior.
Use 'checked' to detect overflow and throw exceptions.
Use 'unchecked' to ignore overflow and wrap values.
Default context depends on compiler settings.
Example: checked(a + b) throws if overflow occurs.
Unchecked allows silent wrap-around.

Full Transcript

This visual trace shows how C# handles arithmetic overflow using checked and unchecked contexts. Variables a and b are set to maximum int and 1. Adding them in a checked context causes an OverflowException at step 3. In an unchecked context at step 4, overflow is ignored and the result wraps around to -2147483648. The variable tracker shows values before and after each step. Key moments clarify why exceptions happen only in checked context and how unchecked wraps values. The quiz tests understanding of variable values and overflow behavior. The snapshot summarizes the key rules and syntax for checked and unchecked arithmetic.

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