checked (C# Reference)

The checked keyword is used to explicitly enable overflow checking for integral-type arithmetic operations and conversions.

By default, an expression that contains only constant values causes a compiler error if the expression produces a value that is outside the range of the destination type. If the expression contains one or more non-constant values, the compiler does not detect the overflow. Evaluating the expression assigned to i2 in the following example does not cause a compiler error.

// The following example causes compiler error CS0220 because 2147483647 
// is the maximum value for integers.  
//int i1 = 2147483647 + 10; 

// The following example, which includes variable ten, does not cause 
// a compiler error. 
int ten = 10;
int i2 = 2147483647 + ten;

// By default, the overflow in the previous statement also does 
// not cause a run-time exception. The following line displays  
// -2,147,483,639 as the sum of 2,147,483,647 and 10.
Console.WriteLine(i2);

By default, these non-constant expressions are not checked for overflow at run time either, and they do not raise overflow exceptions. The previous example displays -2,147,483,639 as the sum of two positive integers.

Overflow checking can be enabled by compiler options, environment configuration, or use of the checked keyword. The following examples demonstrate how to use a checked expression or a checked block to detect the overflow that is produced by the previous sum at run time. Both examples raise an overflow exception.

// If the previous sum is attempted in a checked environment, an  
// OverflowException error is raised. 

// Checked expression.
Console.WriteLine(checked(2147483647 + ten));

// Checked block. 
checked
{
    int i3 = 2147483647 + ten;
    Console.WriteLine(i3);
}

The unchecked keyword can be used to prevent overflow checking.

This sample shows how to use checked to enable overflow checking at run time.

class OverFlowTest
{
    // Set maxIntValue to the maximum value for integers. 
    static int maxIntValue = 2147483647;

    // Using a checked expression. 
    static int CheckedMethod()
    {
        int z = 0;
        try
        {
            // The following line raises an exception because it is checked.
            z = checked(maxIntValue + 10);
        }
        catch (System.OverflowException e)
        {
            // The following line displays information about the error.
            Console.WriteLine("CHECKED and CAUGHT:  " + e.ToString());
        }
        // The value of z is still 0. 
        return z;
    }

    // Using an unchecked expression. 
    static int UncheckedMethod()
    {
        int z = 0;
        try
        {
            // The following calculation is unchecked and will not  
            // raise an exception.
            z = maxIntValue + 10;
        }
        catch (System.OverflowException e)
        {
            // The following line will not be executed.
            Console.WriteLine("UNCHECKED and CAUGHT:  " + e.ToString());
        }
        // Because of the undetected overflow, the sum of 2147483647 + 10 is  
        // returned as -2147483639. 
        return z;
    }

    static void Main()
    {
        Console.WriteLine("\nCHECKED output value is: {0}",
                          CheckedMethod());
        Console.WriteLine("UNCHECKED output value is: {0}",
                          UncheckedMethod());
    }
    /*
   Output:
   CHECKED and CAUGHT:  System.OverflowException: Arithmetic operation resulted
   in an overflow.
      at ConsoleApplication1.OverFlowTest.CheckedMethod() 

   CHECKED output value is: 0
   UNCHECKED output value is: -2147483639
 */
}

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

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