unchecked（C# 参考） · MSDN C# 编程指南 & 参考手册 2015

# 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）](https://msdn.microsoft.com/zh-cn/library/74b4xzyw.aspx)。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](https://msdn.microsoft.com/zh-cn/library/ms228593.aspx)。该语言规范是 C# 语法和用法的权威资料。 ## 请参阅 [C# 参考](https://msdn.microsoft.com/zh-cn/library/618ayhy6.aspx) [C# 编程指南](https://msdn.microsoft.com/zh-cn/library/67ef8sbd.aspx) [C# 关键字](https://msdn.microsoft.com/zh-cn/library/x53a06bb.aspx) [Checked 和 Unchecked（C# 参考）](https://msdn.microsoft.com/zh-cn/library/khy08726.aspx) [checked（C# 参考）](https://msdn.microsoft.com/zh-cn/library/74b4xzyw.aspx)