\n“Why can’t I pass a reference to a derived class\nto a function that takes a reference to a base class by reference?”\nThat’s a confusing question, but it’s phrased that way because\nthe simpler phrasing is wrong!\n<\/p>\n
\nThs misleading simplified phrasing of the question is\n“Why can’t I pass a reference to a derived class to a function that\ntakes a base class by reference?”\nAnd in fact the answer is “You can!”\n<\/p>\n
\nclass Base { }\nclass Derived : Base { }\nclass Program {\n static void f(Base b) { }\n public static void Main()\n {\n Derived d = new Derived();\n f(d);\n }\n}\n<\/pre>\n\nOur call to f<\/code> passes a reference to the\nderived class\nto a function that takes a reference to the base class.\nThis is perfectly fine.\n<\/p>\n\nWhen people ask this question, they are typically wondering\nabout passing a reference to the base class by reference<\/i>.\nThere is a double indirection here.\nYou are passing a reference to a variable,\nand the variable is a reference to the base class.\nAnd it is this double reference that causes the problem.\n<\/p>\n\nclass Base { }\nclass Derived : Base { }\nclass Program {\n static void f(ref<\/font> Base b) { }\n public static void Main()\n {\n Derived d = new Derived();\n f(ref<\/font> d); \/\/ error<\/font>\n }\n}\n<\/pre>\n\nAdding the ref<\/code> keyword to the parameter results\nin a compiler error:\n<\/p>\n\nerror CS1503: Argument '1': cannot convert from 'ref Derived' to 'ref Base'\n<\/pre>\n\nThe reason this is disallowed is that it would allow you to violate\nthe type system.\nConsider:\n<\/p>\n
\n static void f(ref Base b) { b = new Base(); }\n<\/pre>\n\nNow things get interesting.\nYour call to f(ref d)<\/code> passes a reference to a\nDerived<\/code> by reference.\nWhen the f<\/code> function modifies its formal parameter b<\/code>,\nit’s actually modifying your variable d<\/code>.\nWhat’s worse, it’s putting a Base<\/code> in it!\nWhen f<\/code> returns, your variable d<\/code>,\nwhich is declared as being a reference to a Derived<\/code>\nis actually a reference to the base class Base<\/code>.\n<\/p>\n\nAt this point everything falls apart.\nYour program calls some method like d.OnlyInDerived()<\/code>,\nand the CLR ends up executing a method on an object that doesn’t\neven support that method.\n<\/p>\n\nYou actually knew this; you just didn’t know it.\nLet’s start from the easier cases and work up.\nFirst, passing a reference into a function:\n<\/p>\n
\nvoid f(SomeClass s);\n...\n T t = new T();\n f(t);\n<\/pre>\n\nThe function f<\/code> expects to receive a reference to a\nSomeClass<\/code>, but you’re passing a reference to a T<\/code>.\nWhen is this legal?\n<\/p>\n