unchecked（C# 參考） · MSDN C# 編程指南 & 參考手冊 2015

# unchecked（C# 參考） **unchecked** 關鍵字用于取消整型算術運算和轉換的溢出檢查。在未檢查的上下文中，如果表達式產生的值在目標類型范圍之外，并不會標記溢出。例如，下例中的計算在 **unchecked** 塊或表達式中執行，因此將忽略結果對于整數而言過大這一事實，并會對 int1 賦予值 -2,147,483,639。 ``` unchecked { int1 = 2147483647 + 10; } int1 = unchecked(ConstantMax + 10); ``` 如果移除 **unchecked** 環境，則發生編譯錯誤。因為表達式的各個項都是常數，所以可以在編譯時檢測到溢出。默認情況下，在編譯時和運行時不檢查包含非常數項的表達式。有關啟用檢查環境的信息，請參見 [checked（C# 參考）](https://msdn.microsoft.com/zh-cn/library/74b4xzyw.aspx)。因為溢出檢查比較耗時，所以當無溢出危險時，使用不檢查的代碼可以提高性能。但是，如果可能發生溢出，則應使用檢查環境。此示例演示如何使用 **unchecked** 關鍵字。 ``` 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# 語言規范有關詳細信息，請參閱 [C# 語言規范](https://msdn.microsoft.com/zh-cn/library/ms228593.aspx)。該語言規范是 C# 語法和用法的權威資料。 ## 請參閱 [C# 參考](https://msdn.microsoft.com/zh-cn/library/618ayhy6.aspx) [C# 編程指南](https://msdn.microsoft.com/zh-cn/library/67ef8sbd.aspx) [C# 關鍵字](https://msdn.microsoft.com/zh-cn/library/x53a06bb.aspx) [Checked 和 Unchecked（C# 參考）](https://msdn.microsoft.com/zh-cn/library/khy08726.aspx) [checked（C# 參考）](https://msdn.microsoft.com/zh-cn/library/74b4xzyw.aspx)