\nLast time we took a look at how anonymous methods are implemented.\nToday we’ll look at a puzzle that can be solved with what we’ve learned.\nConsider the following program fragment:\n<\/p>\n
\nusing System;\nclass MyClass {\n delegate void DelegateA();\n delegate void DelegateB();\n static DelegateB ConvertDelegate(DelegateA d)\n {\n return (DelegateB)\n Delegate.CreateDelegate(typeof(DelegateB), d.Method);\n }\n static public void Main()\n {\n int i = 0;\n ConvertDelegate(delegate { Console.WriteLine(0); });\n }\n}\n<\/pre>\n\nThe ConvertDelegate<\/code> method merely converts\na DelegateA<\/code> to a DelegateB<\/code>\nby creating a DelegateB<\/code> with the same underlying\nmethod.\nSince the two delegate types use the same signature,\nthis conversion goes off without a hitch.\n<\/p>\n\nBut now let’s make a small change to that Main<\/code> function:\n<\/p>\n\n static public void Main()\n {\n int i = 0;\n \/\/ one character change - 0 becomes i\n ConvertDelegate(delegate { Console.WriteLine(i<\/font>); });\n }\n<\/pre>\n\nNow the program crashes with a\nSystem.ArgumentException<\/code> at the point where\nwe try to create the DelegateB<\/code>.\nWhat’s going on?\n<\/p>\n\nFirst,\nobserve that the overload of Delegate.CreateDelegate<\/code>\nthat was used is one that can only be used to create delegates\nfrom static methods.\nNext, note that in Test1<\/code>,\nthe anonymous method references neither its own members\nnor any local variables from its lexically-enclosing method.\nTherefore, the resulting anonymous method is\na “static anonymous method of the easy type”.\nSince the anonymous method is a static member,\nthe use of the “static members only” overload of\nDelegate.CreateDelegate<\/code> succeeds.\n<\/p>\n\nHowever, in Test2<\/code>, the anonymous method dereferences the\ni<\/code> variable from its lexically-enclosing method.\nThis forces the anonymous method to be a “anonymous method of the hard type”,\nand those anonymous methods use an anonymous instance member function\nof an anonymous helper class.\nAs a result,\nd.Method<\/code> is an instance method, and the chosen overload of\nDelegate.CreateDelegate<\/code> throws an invalid parameter\nexception since it works only with static methods.\n<\/p>\n\nThe solution is to use a different overload of\nDelegate.CreateDelegate<\/code>,\none that work with either static or instance member functions.\n<\/p>\n\n DelegateB ConvertDelegate(DelegateA d)\n {\n return (DelegateB)\n Delegate.CreateDelegate(typeof(DelegateB), d.Target,<\/font> d.Method);\n }\n<\/pre>\n\nThe Delegate.CreateDelegate(Type, Object, MethodInfo)<\/code>\noverload creates a delegate for a static method if the\nObject<\/code> parameter is null<\/code> or\na delegate for an instance method if the Object<\/code>\nparameter is non-null<\/code>.\nHardly by coincidence, that is exactly what d.Target<\/code> produces.\nIf the original delegate is for a static method, then\nd.Target<\/code> is null<\/code>; otherwise, it is\nthe object for which the instance method is to be invoked on.\n<\/p>\n\nThis fix, therefore, makes the ConvertDelegate<\/code>\nfunction handle conversion of delegates for either static or\ninstance methods.\nWhich is a good thing, because it may now be called upon to\nconvert delegates for instance methods as well as static ones.\n<\/p>\n\nOkay, this time we were lucky that the hidden gotcha of anonymous\nmethods resulted in an exception.\nNext time, we’ll see a gotcha that merely results in incorrect\nbehavior that will probably take you forever to track down.\n<\/p>\n