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The checked and unchecked statements (C# reference)

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The checked and unchecked statements specify the overflow-checking context for integral-type arithmetic operations and conversions. When integer arithmetic overflow occurs, the overflow-checking context defines what happens. In a checked context, a System.OverflowException is thrown; if overflow happens in a constant expression, a compile-time error occurs. In an unchecked context, the operation result is truncated by discarding any high-order bits that don't fit in the destination type. For example, addition wraps from the maximum value to the minimum value. The following example shows the same operation in both a checked and unchecked context:

uint a = uint.MaxValue;

unchecked
{
    Console.WriteLine(a + 3);  // output: 2
}

try
{
    checked
    {
        Console.WriteLine(a + 3);
    }
}
catch (OverflowException e)
{
    Console.WriteLine(e.Message);  // output: Arithmetic operation resulted in an overflow.
}

Note

The overflow behavior of user-defined operators and conversions can differ from the one described in the preceding paragraph. In particular, user-defined checked operators might not throw an exception in a checked context.

For more information, see the Arithmetic overflow and division by zero and User-defined checked operators sections of the Arithmetic operators article.

To specify the overflow-checking context for an expression, you can also use the checked and unchecked operators, as the following example shows:

double a = double.MaxValue;

int b = unchecked((int)a);
Console.WriteLine(b);  // output: -2147483648

try
{
    b = checked((int)a);
}
catch (OverflowException e)
{
    Console.WriteLine(e.Message);  // output: Arithmetic operation resulted in an overflow.
}

The checked and unchecked statements and operators only affect the overflow-checking context for those operations that are textually inside the statement block or operator's parentheses, as the following example shows:

int Multiply(int a, int b) => a * b;

int factor = 2;

try
{
    checked
    {
        Console.WriteLine(Multiply(factor, int.MaxValue));  // output: -2
    }
}
catch (OverflowException e)
{
    Console.WriteLine(e.Message);
}

try
{
    checked
    {
        Console.WriteLine(Multiply(factor, factor * int.MaxValue));
    }
}
catch (OverflowException e)
{
    Console.WriteLine(e.Message);  // output: Arithmetic operation resulted in an overflow.
}

At the preceding example, the first invocation of the Multiply local function shows that the checked statement doesn't affect the overflow-checking context within the Multiply function as no exception is thrown. At the second invocation of the Multiply function, the expression that calculates the second argument of the function is evaluated in a checked context and results in an exception as it's textually inside the block of the checked statement.

The behavior of checked and unchecked depends on the type and the operation. Even for integers, operations like unchecked(x / 0) always throw because there's no sensible behavior. Check the behavior for the type and the operation to understand how the checked and unchecked keywords affect your code.

Numeric types and overflow-checking context

The checked and unchecked keywords primarily apply to integral types where there's a sensible overflow behavior. The wraparound behavior where T.MaxValue + 1 becomes T.MinValue is sensible in a two's complement value. The represented value isn't correct since it can't fit in the storage for the type. Therefore, the bits are representative of the lower n-bits of the full result.

For types like decimal, float, double, and Half that represent a more complex value or a one's complement value, wraparound isn't sensible. It can't be used to compute larger or more accurate results, so unchecked isn't beneficial.

float, double, and Half have sensible saturating values for PositiveInfinity and NegativeInfinity, so you can detect overflow in an unchecked context. For decimal, no such limits exist, and saturating at MaxValue can lead to errors or confusion. Operations that use decimal throw in both a checked and unchecked context.

Operations affected by the overflow-checking context

The overflow-checking context affects the following operations:

Default overflow-checking context

If you don't specify the overflow-checking context, the value of the CheckForOverflowUnderflow compiler option defines the default context for nonconstant expressions. By default the value of that option is unset and integral-type arithmetic operations and conversions are executed in an unchecked context.

Constant expressions are evaluated by default in a checked context and overflow causes a compile-time error. You can explicitly specify an unchecked context for a constant expression with the unchecked statement or operator.

C# language specification

For more information, see the following sections of the C# language specification:

See also