- I would like this blog post to serve as a quick reference that recalls the basic concept of covariant and contravariant type parameters of generic interfaces in the C# language.
- I tried to keep the example as simple as possible. Included comments explain the key points. No long stories and no dissertations.
- The code does nothing, but compiles on C# 4.0 or newer compiler.
- Try deleting the first slash character in the first line to kind of switch between the snippets (BTW this is cool trick
).
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| //* | |
| // T is covariant | |
| // Implicit conversion from interface instance I<B> which has a method with RETURN type B | |
| // that is MORE derived than specified in target interface I<A> | |
| public interface I<out T> { T foo(); } | |
| public class IImp<T> : I<T> { public T foo() { return default(T); } } | |
| public class A { } | |
| public class B : A { } | |
| class Program | |
| { | |
| static void Main(string[] args) | |
| { | |
| I<A> ia = new IImp<B>(); | |
| } | |
| } | |
| /*/ | |
| // T is contravariant | |
| // Implicit conversion from interface instance I<A> which has a method with ARGUMENT type A | |
| // that is LESS derived than specified in target interface I<B> | |
| public interface I<in T> { void foo(T t); } | |
| public class IImp<T> : I<T> { public void foo(T t) { } } | |
| public class A { } | |
| public class B : A { } | |
| class Program | |
| { | |
| static void Main(string[] args) | |
| { | |
| I<B> ia = new IImp<A>(); | |
| } | |
| } | |
| //*/ |
by 
Interface type parameter covariance and contravariance in C# | PJSen Blog…
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