C# checked and unchecked usage

checked keyword is used to explicitly enable overflow checking for integer arithmetic operations and conversions.

By default, if an expression contains only constant values, it causes a compiler error if the resulting value is outside the range of the target type. If the expression contains one or more non-constant values, the compiler does not detect overflow. In the following example, evaluating the expression assigned to i2 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 runtime, and these expressions do not throw an overflow exception. The above example shows -2,147,483,639 as the sum of two positive integers.

Overflow checking can be enabled through compiler options, environment configuration, or using the checked keyword. The following example demonstrates how to use a checked expression or a checked block to detect at runtime an overflow caused by a previous sum calculation. Both examples throw overflow exceptions.

// 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);
}

Overflow checking can be canceled using unchecked

This example demonstrates how to use checked to enable runtime overflow checking.

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
 */
}

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