{"id":3603,"date":"2013-08-06T07:00:00","date_gmt":"2013-08-06T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2013\/08\/06\/the-mysterious-ways-of-the-params-keyword-in-c\/"},"modified":"2013-08-06T07:00:00","modified_gmt":"2013-08-06T07:00:00","slug":"the-mysterious-ways-of-the-params-keyword-in-c","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20130806-00\/?p=3603","title":{"rendered":"The mysterious ways of the params keyword in C#"},"content":{"rendered":"

\nIf a parameter to a C# method is declared with the\nparams<\/code> keyword,\nthen it can match either itself or a comma-separated list of\num itselves(?).\nConsider:\n<\/p>\n

\nclass Program {\n  static void Sample(params int[] ints) {\n   for (int i = 0; i < ints.Length; i++) {\n    System.Console.WriteLine("{0}: {1}", i, ints[i]);\n   }\n   System.Console.WriteLine("-----");\n  }\n  public static void Main() {\n   Sample(new int[] { 1, 2, 3 });\n   Sample(9, 10);\n  }\n}\n<\/pre>\n
\nThis program prints\n<\/p>\n
\n0: 1\n1: 2\n2: 3\n-----\n0: 9\n1: 10\n-----\n<\/pre>\n
\nThe first call to Sample<\/code>\ndoes not take advantage of the params<\/code> keyword\nand passes the array explicitly (formally known as\nnormal form<\/i>).\nThe second call, however, specifies the integers directly\nas if they were separate parameters.\nThe compiler generates a call to the function in what the language\nspecification calls expanded form<\/i>.\n<\/p>\n
\nNormally, there is no conflict between these two styles of\ncalling a function with a params<\/code> parameter\nbecause only one form actually makes sense.\n<\/p>\n
\nSample(new int[] { 0 }); \/\/ normal form\nSample(0); \/\/ expanded form\n<\/pre>\n
\nThe first case must be called in normal form because you cannot\nconvert an int[]<\/code> to an int<\/code>;\nconversely, the second case must be called in expanded form because\nyou cannot convert an int<\/code> to an int[]<\/code>.\n<\/p>\n
\nThere is no real problem in choosing between the two cases\nbecause T<\/code> and T[]<\/code> are not implicitly\nconvertible to each other.\n<\/p>\n
\nOh wait.\n<\/p>\n
\nUnless T<\/code> is object<\/code>!\n<\/p>\n
\nclass Program {\n  static void Sample(params object[] objects) {\n   for (int i = 0; i < objects.Length; i++) {\n    System.Console.WriteLine("{0}: {1}", i, objects[i]);\n   }\n   System.Console.WriteLine("-----");\n  }\n  public static void Main() {\n   Sample(new object[] { "hello", "there" });\n  }\n}\n<\/pre>\n
\nThere are two possible interpretations for that call to\nSample<\/code>:\n<\/p>\n