【转载】C# checked和unchecked关键字

--转自微软 checked 关键字 - C# 参考 | Microsoft Docs /  unchecked 关键字 - C# 参考 | Microsoft Docs

--本文版权归微软所有

 一、checked关键字

checked 关键字用于对整型类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量值，且产生的值在目标类型范围之外，则会导致编译器错误。 如果表达式包含一个或多个非常量值，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量表达式是否溢出，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 关键字阻止溢出检查。

示例

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
 */
}

 

二、unchecked关键字

unchecked 关键字用于取消整型类型的算术运算和转换的溢出检查。

在未经检查的上下文中，如果表达式生成的值超出目标类型的范围，则不会标记溢出。 例如，由于以下示例中的计算在 unchecked 块或表达式中执行，因此将忽略计算结果对于整数而言过大的事实，并且向 int1 赋予值 -2,147,483,639。

unchecked
{
    int1 = 2147483647 + 10;
}
int1 = unchecked(ConstantMax + 10);

如果删除 unchecked 环境，会发生编译错误。 由于表达式的所有项都是常量，因此可在编译时检测到溢出。

在编译时和运行时，默认不检查包含非常数项的表达式。 请参阅启用检查，获取有关使用启用了检查的环境的信息。

由于检查溢出需要时间，因此在没有溢出风险的情况下使用取消检查的代码可能会提高性能。 但是，如果存在溢出的可能，则应使用启用了检查的环境。

示例

此示例演示如何使用 unchecked关键字。

class UncheckedDemo
{
    static void Main(string[] args)
    {
        // int.MaxValue is 2,147,483,647.
        const int ConstantMax = int.MaxValue;
        int int1;
        int int2;
        int variableMax = 2147483647;

        // The following statements are checked by default at compile time. They do not
        // compile.
        //int1 = 2147483647 + 10;
        //int1 = ConstantMax + 10;

        // To enable the assignments to int1 to compile and run, place them inside
        // an unchecked block or expression. The following statements compile and
        // run.
        unchecked
        {
            int1 = 2147483647 + 10;
        }
        int1 = unchecked(ConstantMax + 10);

        // The sum of 2,147,483,647 and 10 is displayed as -2,147,483,639.
        Console.WriteLine(int1);

        // The following statement is unchecked by default at compile time and run
        // time because the expression contains the variable variableMax. It causes
        // overflow but the overflow is not detected. The statement compiles and runs.
        int2 = variableMax + 10;

        // Again, the sum of 2,147,483,647 and 10 is displayed as -2,147,483,639.
        Console.WriteLine(int2);

        // To catch the overflow in the assignment to int2 at run time, put the
        // declaration in a checked block or expression. The following
        // statements compile but raise an overflow exception at run time.
        checked
        {
            //int2 = variableMax + 10;
        }
        //int2 = checked(variableMax + 10);

        // Unchecked sections frequently are used to break out of a checked
        // environment in order to improve performance in a portion of code
        // that is not expected to raise overflow exceptions.
        checked
        {
            // Code that might cause overflow should be executed in a checked
            // environment.
            unchecked
            {
                // This section is appropriate for code that you are confident
                // will not result in overflow, and for which performance is
                // a priority.
            }
            // Additional checked code here.
        }
    }
}

 

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