\nconverting constructor<\/a>.\nOr maybe it’s a function that takes two parameters,\nbut the second parameter has a default value.\n<\/p>\n\nNobody knows the answers to these questions, not even the compiler.\nIt’s only when you decide to instantiate the template\n<\/p>\n
\nAbel<Baker> abel;\n<\/pre>\n\nthat these burning questions can be answered,\noverloaded operators can be resolved,\nconversion operators can be hunted down,\nparameters can get pushed on the stack in the correct order,\nand the correct type of call<\/code> instruction can be\ngenerated.\n<\/p>\n\nIn fact, the compiler doesn’t even care whether or not Baker<\/code>\nhas a Bloober<\/code> method, as long as you never call\nAbel<Baker>::DoBloober<\/code>!\n<\/p>\n\nvoid f()\n{\n Abel<int> a; \/\/ no error!\n}\nvoid g()\n{\n Abel<int> a;\n a.DoBloober(0, 1); \/\/ error here\n}\n<\/pre>\nOnly if you actually call the method does the compiler start looking for\nhow it can generate code for the DoBloober<\/code> method.<\/p>\n\nC# generics aren’t like that.\n<\/p>\n
\nUnlike C++, where a non-instantiated template exists only\nin the imaginary world of potential code that could exist but doesn’t,\na C# generic results in code being generated,\nbut with placeholders where the type parameter should be inserted.\n<\/p>\n
\nThis is why you can use generics implemented in another assembly,\neven without the source code to that generic.\nThis is why a generic can be recompiled without having to recompile\nall the assemblies that use that generic.\nThe code for the generic is generated when the generic is compiled<\/i>.\nBy comparison no code is generated for C++ templates\nuntil the template is instantiated.\n<\/p>\n
\nWhat this means for C# generics is that if you want to do something\nwith your type parameter, it has to be something that the compiler\ncan figure out how to do without knowing what T<\/code> is<\/i>.\nLet’s look at the example that generated today’s question.\n<\/p>\n\nclass Foo<T>\n{\n class Bar : T\n { ... }\n}\n<\/pre>\n\nThis is flagged as an error by the compiler:\n<\/p>\n
\nerror CS0689: Cannot derive from 'T' because it is a type parameter\n<\/pre>\n\nDeriving from a generic type parameter is explicitly forbidden by\n25.1.1 of the C# language specification.\nConsider:\n<\/p>\n
\nclass Foo<T>\n{\n class Bar : T\n {\n public void FlubberMe()\n {\n Flubber(0);\n }\n }\n}\n<\/pre>\n\nThe compiler doesn’t have enough information to generate the IL for\nthe FlubberMe<\/code> method.\nOne possibility is\n<\/p>\n\nldarg.0 \/\/ \"this\"\nldc.i4.0 \/\/ integer 0 - is this right?\ncall T.Flubber \/\/ is this the right type of call?\n<\/pre>\n\nThe line ldc.i4.0<\/code> is a guess.\nIf the method T.Flubber<\/code> were actually\nvoid Flubber(long l)<\/code>,\nthen the line would have to be ldc.i4.0; conv.i8<\/code>\nto load an 8-byte integer onto the stack instead of a 4-byte integer.\nOr perhaps it’s\nvoid Flubber(object o)<\/code>,\nin which case the zero needs to be boxed.\n<\/p>\n\nAnd what about that call instruction?\nShould it be a call<\/code> or callvirt<\/code>?\n<\/p>\n\nAnd what if the method returned a value, say,\nstring Flubber(int i)<\/code>?\nNow the compiler also has to generate code to discard the\nreturn value from the top of the stack.\n<\/p>\n\nSince the source code for a generic is not included in the assembly,\nall these questions have to be answered at the time the generic is\ncompiled.\nBesides, you can\nwrite a generic in Managed C++ and use it from VB.NET.\nEven saving the source code won’t be much help if the generic\nwas implemented in a language you don’t have the compiler for!<\/p>\n","protected":false},"excerpt":{"rendered":"
A lot of questions about C# generics come from the starting point that they are just a cutesy C# name for C++ templates. But while the two may look similar in the source code, they are actually quite different. C++ templates are macros on steroids. No code gets generated when a template is “compiled”; the […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-17093","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
A lot of questions about C# generics come from the starting point that they are just a cutesy C# name for C++ templates. But while the two may look similar in the source code, they are actually quite different. C++ templates are macros on steroids. No code gets generated when a template is “compiled”; the […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/17093","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=17093"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/17093\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=17093"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=17093"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=17093"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}