8.8 Checked and Unchecked Arithmetic

Let’s go through checked and unchecked arithmetic in C# — a way to control overflow behavior in numeric operations.

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📌 1. What is Overflow?

• Overflow occurs when a calculation produces a value outside the range of the data type.

• Example: int ranges from -2,147,483,648 to 2,147,483,647. Adding 1 to int.MaxValue causes an overflow.

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📌 2. Unchecked Context (Default for C#)

• By default, C# does not throw an exception on overflow for integer arithmetic.

• The value wraps around.

int max = int.MaxValue; // 2147483647
int result = max + 1;
Console.WriteLine(result); // -2147483648 (wrap-around)

• This is called unchecked arithmetic

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📌 3. Checked Context

• Use checked to force an exception when overflow occurs

• Throws System.OverflowException

int max = int.MaxValue;
 
try
{
    int result = checked(max + 1);
}
catch (OverflowException ex)
{
    Console.WriteLine("Overflow detected!"); // Output
}

• You can also apply checked to a block:

checked
{
    int a = int.MaxValue;
    int b = a + 1; // Throws OverflowException
}

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📌 4. Unchecked Keyword

• Explicitly ignore overflow even in a checked context

unchecked
{
    int max = int.MaxValue;
    int result = max + 1; // Wrap-around, no exception
    Console.WriteLine(result); // -2147483648
}

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📌 5. Use Cases

Keyword	Behavior	Use Case
unchecked	Overflow ignored, value wraps around	Performance-critical code where overflow is acceptable
checked	Throws OverflowException on overflow	Safe arithmetic, financial calculations, critical systems

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📌 6. Default Behavior

• Integer literals and arithmetic are unchecked by default

• Can enable checked-by-default in project settings (rarely used)

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📌 7. Summary Table

Concept	Description
Overflow	When value exceeds the data type’s range
checked	Throws exception on overflow
unchecked	Ignores overflow, wraps around
Default behavior	Arithmetic is unchecked unless specified

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✅ Tip:

• Use checked for critical calculations where incorrect results are dangerous

• Use unchecked for performance or low-risk operations