{"id":233,"date":"2014-08-15T07:00:00","date_gmt":"2014-08-15T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2014\/08\/15\/the-scope-of-the-c-checkedunchecked-keyword-is-static-not-dynamic\/"},"modified":"2014-08-15T07:00:00","modified_gmt":"2014-08-15T07:00:00","slug":"the-scope-of-the-c-checkedunchecked-keyword-is-static-not-dynamic","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20140815-00\/?p=233","title":{"rendered":"The scope of the C# checked\/unchecked keyword is static, not dynamic"},"content":{"rendered":"

\nC# has operators checked<\/code> and unchecked<\/code>\nto control the behavior of the language in the face of integer overflow.\nThere are also checked<\/code> and unchecked<\/code> statements\nwhich apply the behavior to blocks of statements rather than single\nexpressions.\n<\/p>\n

\nint x;\nx = checked(a + b); \/\/ evaluate with overflow checking\ny = unchecked(a + b); \/\/ evaluate without overflow checking\nchecked {\n x = a + b; \/\/ evaluate with overflow checking\n}\nunchecked {\n x = a + b; \/\/ evaluate without overflow checking\n}\n<\/pre>\n
\nWhy, then, doesn’t this code below raise an overflow exception?\n<\/p>\n
\nclass Program {\n static int Multiply(int a, int b) { return a * b; }\n static int Overflow() { return Multiply(int.MaxValue, 2); }\n public static void Main() {\n  System.Console.WriteLine(checked(Overflow()));\n  checked {\n    System.Console.WriteLine(Overflow());\n  }\n }\n}\n<\/pre>\n
\n(Mini-exercise: Why couldn’t I have just written\nstatic int Overflow() { return int.MaxValue * 2; }<\/code>?)\n<\/p>\n
\nThe answer is that the scope of the checked<\/code> or\nunchecked<\/code> keyword is static, not dynamic.\nWhether a particular arithmetic is checked or unchecked is\ndetermined at compile time, not at run time.\nSince the multiplication in the Multiply<\/code> function\nis not explicitly marked checked or unchecked,\nuses the overflow context implied by your compiler options.\nAssuming you’ve left it at the default of\nunchecked<\/code>,\nthis means that\nthere is no overflow checking in the Multiply<\/code> function,\neven if you call it from a checked context.\nBecause once you call the Multiply<\/code> function,\nyou have left the checked context.\n<\/p>\n
\nThe C# language specification addresses this issue not once,\nnot twice, but three times!\n(But it seems that some people miss it,\npossibly because there is\n\ntoo much documentation<\/a>.)\n<\/p>\n
\nFirst, there is an\n\nexplicit list<\/a> of\noperations which\nare controlled by the checked<\/code> or unchecked<\/code>\nkeyword:\n<\/p>\n
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