unchecked（C# 参考）
unchecked 关键字用于取消整型算术运算和转换的溢出检查。
在未检查的上下文中，如果表达式产生的值在目标类型范围之外，并不会标记溢出。例如，下例中的计算在 unchecked 块或表达式中执行，因此将忽略结果对于整数而言过大这一事实，并会对 int1 赋予值 -2,147,483,639。
unchecked
{
int1 = 2147483647 + 10;
}
int1 = unchecked(ConstantMax + 10);
如果移除 unchecked 环境，则发生编译错误。因为表达式的各个项都是常数，所以可以在编译时检测到溢出。
默认情况下，在编译时和运行时不检查包含非常数项的表达式。有关启用检查环境的信息，请参见 checked（C# 参考）。
因为溢出检查比较耗时，所以当无溢出危险时，使用不检查的代码可以提高性能。但是，如果可能发生溢出，则应使用检查环境。
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.
}
}
}
C# 语言规范
有关详细信息，请参阅 C# 语言规范。该语言规范是 C# 语法和用法的权威资料。