68 lines
1.9 KiB
Markdown
68 lines
1.9 KiB
Markdown
---
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Title: Covariance and Contravariance in C#3
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Date: 2009-03-09 21:19:51
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---
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A short introduction to Covariance and Contravariance in *C# 3* preparing
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you to an article about that in *C# 4*. So what is covariance?
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Covariance is basically using a method which returns something derived
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from the expected type. An exemple? It's safe to have a method
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returning a cat when you expect it to return an animal. In C sharp it's
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:::csharp
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public class Animal
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{
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}
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public class Cat : Animal
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{
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}
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public class Dog : Animal
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{
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}
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// It's safe to say that something returns a Animal when in fact this thing returns a Cat
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class Covariance
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{
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void test()
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{
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Func a = Method; // OK
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Func b = delegate { return new Cat(); }; // OK
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Func c = () => new Cat(); // OK
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}
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Cat Method()
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{
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return new Cat();
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}
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}
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So Funcs `a`, `b`, and `c` are returning animals which in fact are
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cats, which is true. And, what is contravariance? Contravariance is
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basically using a method which takes something which is a parent of
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the expected type. An exemple? It's safe to give a method an animal
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when it expects to receive a cat.
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:::csharp
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// It's safe to say that something can take a Cat if in fact this thing can take any Animal
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class Contravariance
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{
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static void test()
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{
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Action a = Method; // OK
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Action b = delegate(Animal value) { }; // ERROR
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// From C#3 Specification :
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// $7.14.1 Anonymous function signatures :
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// [...] contra-variance of anonymous function parameter types is not supported.
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Action d = (Animal value) => { }; // idem... anonymous... not supported.
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}
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public static void Method(Animal value)
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{
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}
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}
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So `Action` `a` take `Cats`, but in fact can take any `Animals`, so it's safe.
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