C# checked与unchecked用法

checked 关键字用于对整型算术运算和转换显式启用溢出检查。

默认情况下，如果表达式仅包含常数值，且产生的值在目标类型范围之外，则它会导致编译器错误。 如果表达式包含一个或多个非常数值，则编译器不检测溢出。 在下面的示例中，计算赋给 i2 的表达式不会导致编译器错误。

// 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);

默认情况下，在运行时也不检查这些非常数表达式是否溢出，这些表达式不引发溢出异常。 上面的示例显示 -2,147,483,639 作为两个正整数之和。

可以通过编译器选项、环境配置或使用 checked 关键字来启用溢出检查。 下面的示例演示如何使用 checked 表达式或 checked 块，在运行时检测由前面的求和计算导致的溢出。 两个示例都引发溢出异常。

// 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);
}

可以使用unchecked取消溢出检查

此示例演示如何使用 checked 启用运行时溢出检查。

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
 */
}