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POImplementing Co and Contra variance generic interface & its practical uses
 

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  1. POImplementing Co and Contra variance generic interface & its practical uses
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    <p>I am trying to understand the possible use of using a variant generic interface(using both co and contra variant). Can someone please explain? I understood co and contra variance examples for interfaces individually, </p> <p><strong>Co variance example</strong></p> <pre><code>public class Request { } public class Response { } public class SuperRequest : Request { } public class SuperResponse : Response { } public interface CoVarianceInterface&lt;out O&gt; { O Get(); } public class CoVarianceInterfaceInstance1 : CoVarianceInterface&lt;Response&gt; { public Response Get() { Console.WriteLine("Co Variance"); return null; } } public class CoVarianceInterfaceInstance2 : CoVarianceInterface&lt;SuperResponse&gt; { public SuperResponse Get() { Console.WriteLine("Co Variance 1"); return null; } } static void CallCoVarianceInterface(CoVarianceInterface&lt;Response&gt; task) { task.Get(); } static void Test() { CoVarianceInterface&lt;Response&gt; i1 = new CoVarianceInterfaceInstance1(); CoVarianceInterface&lt;SuperResponse&gt; i2 = new CoVarianceInterfaceInstance2(); CallCoVarianceInterface(i1); CallCoVarianceInterface(i2); **//Co variance here bcoz more derived to less derived type?** } </code></pre> <p><strong>Contra variance example</strong></p> <pre><code>public interface ContraVarianceInterface&lt;in O&gt; { void Set(O input); } public class ContraVarianceInterfaceInstance1 : ContraVarianceInterface&lt;Request&gt; { public void Set(Request input) { Console.WriteLine("Contra Variance"); } } public class ContraVarianceInterfaceInstance2 : ContraVarianceInterface&lt;SuperRequest&gt; { public void Set(SuperRequest input) { Console.WriteLine("Contra Variance 1"); } } static void CallContraVarianceInterface(ContraVarianceInterface&lt;SuperRequest&gt; task) { task.Set(null); } static void Test() { ContraVarianceInterface&lt;Request&gt; o1 = new ContraVarianceInterfaceInstance1(); ContraVarianceInterface&lt;SuperRequest&gt; o2 = new ContraVarianceInterfaceInstance2(); CallContraVarianceInterface(o1); **//Contra variance here bcoz less derived to more derived type?** CallContraVarianceInterface(o2); } </code></pre> <p><strong>Co/Contra variance</strong></p> <pre><code>public interface CoContraInterface&lt;out O, in I&gt; { O Execute(I input); } public class CoContraInterfaceInstance1 : CoContraInterface&lt;Response, Request&gt; { public Response Execute(Request input) { Console.WriteLine("Variance"); return new Response(); } } public class CoContraInterfaceInstance2 : CoContraInterface&lt;SuperResponse, SuperRequest&gt; { public SuperResponse Execute(SuperRequest input) { Console.WriteLine("Variance 1"); return new SuperResponse(); } } static void CallCoContraInterface(CoContraInterface&lt;Request, Response&gt; task) { task.Execute(null); } static void Test() { CoContraInterface&lt;Response, Request&gt; c1 = new CoContraInterfaceInstance1(); CoContraInterface&lt;SuperResponse, SuperRequest&gt; c2 = new CoContraInterfaceInstance2(); CoContraInterface&lt;Response, SuperRequest&gt; c3 = new CoContraInterfaceInstance1(); c3 = c1; **//here Request is typed to SuperRequest, so is it contra variance?** c3 = c2; **//here Request is typed to SuperRequest, so is it contra variance? also SuperResponse to Response, so is it Co variance as well?** CreateSample(c1); CreateSample(c2); CreateSample(c3); } is the above code right? if so where we can practically use an interface that have both Co and Contra variance? </code></pre> <p>Thanks</p>
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    1. USSuresh
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    1. COIt's non-idiomatic and slightly confusing when you don't name your interfaces with a leading `I`.
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      1. USDrew Noakes
 

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