C# checked與unchecked用法

checked 關鍵字用於對整型算術運算和轉換顯式啟用溢出檢查。

默認情況下，如果表達式僅包含常數值，且產生的值在目標類型范圍之外，則它會導致編譯器錯誤。 如果表達式包含一個或多個非常數值，則編譯器不檢測溢出。 在下面的示例中，計算賦給 i2 的表達式不會導致編譯器錯誤。

// 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);

默認情況下，在運行時也不檢查這些非常數表達式是否溢出，這些表達式不引發溢出異常。 上面的示例顯示 -2,147,483,639 作為兩個正整數之和。

可以通過編譯器選項、環境配置或使用 checked 關鍵字來啟用溢出檢查。 下面的示例演示如何使用 checked 表達式或 checked 塊，在運行時檢測由前面的求和計算導致的溢出。 兩個示例都引發溢出異常。

// 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取消溢出檢查

此示例演示如何使用 checked 啟用運行時溢出檢查。

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
 */
}