{"id":13171,"date":"2022-10-03T05:47:44","date_gmt":"2022-10-03T00:17:44","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-c-generic-inheritance-and-covariance-part-2\/"},"modified":"2022-10-03T05:47:44","modified_gmt":"2022-10-03T00:17:44","slug":"solved-c-generic-inheritance-and-covariance-part-2","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-c-generic-inheritance-and-covariance-part-2\/","title":{"rendered":"[Solved] C# generic inheritance and covariance part 2"},"content":{"rendered":"

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There does not appear to be any question in this question, so I’ll make up a few questions to answer.<\/p>\n

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What is a covariant conversion?<\/p>\n<\/blockquote>\n

Let’s suppose we have some types Fruit<\/code> and Apple<\/code> and Banana<\/code> with the obvious relationships; Apple<\/code> is a kind of Fruit<\/code>, and so on.<\/p>\n

A covariant conversion is one where the convertability of the type argument implies the convertibility of the generic type. If Apple<\/code> is convertible to Fruit<\/code>, and Bowl<Apple><\/code> is convertible to Bowl<Fruit><\/code>, then Bowl<T><\/code> is covariant in T.<\/p>\n

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What is a contravariant conversion?<\/p>\n<\/blockquote>\n

A contravariant conversion is a covariant conversion that reverses<\/em> the direction instead of preserving<\/em> it. If Eater<Fruit><\/code> is convertible to Eater<Apple><\/code> then Eater<T><\/code> is contravariant in T.<\/p>\n

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How do I mark an interface or delegate as being covariant or contravariant in its type parameters?<\/p>\n<\/blockquote>\n

Covariant type parameters are marked out<\/code> and contravariant type parameters are marked in<\/code>. <\/p>\n

This is intended to be mnemonic: covariant interfaces typically have the type parameter appear in output positions and contravariant interfaces typeically have the type parameter appear in input positions.<\/p>\n

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String<\/code> is convertible to Object<\/code>. How can I make IReadOnlyCollection<String><\/code> convertible to IReadOnlyCollection<Object><\/code>?<\/p>\n<\/blockquote>\n

Make IReadOnlyCollection<T><\/code> covariant in T. Mark it out<\/code>.<\/p>\n

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Consider the following code:<\/p>\n<\/blockquote>\n

delegate void Action<in T>(T t);\ninterface IFoo<in X>\n{\n  void M(Action<X> action);\n}\n<\/code><\/pre>\n
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Why does the compiler say that this is not valid?<\/p>\n<\/blockquote>\n
Because it is not valid. Let’s see why.<\/p>\n
class Foo : IFoo<Fruit>\n{\n  public void M(Action<Fruit> action)\n  {\n     action(new Apple()); \/\/ An apple is a fruit.\n  }\n}\n...\nIFoo<Fruit> iff = new Foo();\nIFoo<Banana> ifb = iff; \/\/ Contravariant!\nifb.M(banana => { banana.Peel(); });\n<\/code><\/pre>\n
Follow the logic. This program passes an apple as the “this” of Banana.Peel()<\/code>, which is clearly wrong.<\/p>\n
The compiler knows that this can happen, and so disallows the interface to be declared in the first place.<\/p>\n
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What should I do if I have more questions about variance?<\/p>\n<\/blockquote>\n
You should start by reading my articles on the design and implementation of the feature. Start from the bottom; they are listed in reverse chronological order:<\/p>\n
http:\/\/blogs.msdn.com\/b\/ericlippert\/archive\/tags\/covariance+and+contravariance\/<\/a><\/p>\n
If you still have a question then you should post questions here that actually contain questions, instead of making people guess what the question really is.<\/p>\n<\/p><\/div>\n
<\/div>\n<\/div>\n
            2<\/span> <\/p><\/div>\n<\/div>\n
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solved C# generic inheritance and covariance part 2 <\/p>\n","protected":false},"excerpt":{"rendered":"
[ad_1] There does not appear to be any question in this question, so I’ll make up a few questions to answer. What is a covariant conversion? Let’s suppose we have some types Fruit and Apple and Banana with the obvious relationships; Apple is a kind of Fruit, and so on. A covariant conversion is one … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[324,559],"class_list":["post-13171","post","type-post","status-publish","format-standard","hentry","category-solved","tag-c","tag-generics"],"yoast_head":"\n[Solved] C# generic inheritance and covariance part 2 - JassWeb<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jassweb.com\/solved\/solved-c-generic-inheritance-and-covariance-part-2\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"[Solved] C# generic inheritance and covariance part 2 - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] There does not appear to be any question in this question, so I’ll make up a few questions to answer. 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