我试图了解使用变体通用接口的可能用途（同时使用 co 和 contra 变体）。有人可以解释一下吗？我分别理解了接口的 co 和 contra 方差示例，
协方差示例
public class Request { }
public class Response { }
public class SuperRequest : Request { }
public class SuperResponse : Response { }
public interface CoVarianceInterface<out O>
{
O Get();
}
public class CoVarianceInterfaceInstance1 : CoVarianceInterface<Response>
{
public Response Get()
{
Console.WriteLine("Co Variance");
return null;
}
}
public class CoVarianceInterfaceInstance2 : CoVarianceInterface<SuperResponse>
{
public SuperResponse Get()
{
Console.WriteLine("Co Variance 1");
return null;
}
}
static void CallCoVarianceInterface(CoVarianceInterface<Response> task)
{
task.Get();
}
static void Test()
{
CoVarianceInterface<Response> i1 = new CoVarianceInterfaceInstance1();
CoVarianceInterface<SuperResponse> i2 = new CoVarianceInterfaceInstance2();
CallCoVarianceInterface(i1);
CallCoVarianceInterface(i2); **//Co variance here bcoz more derived to less derived type?**
}
反方差示例
public interface ContraVarianceInterface<in O>
{
void Set(O input);
}
public class ContraVarianceInterfaceInstance1 : ContraVarianceInterface<Request>
{
public void Set(Request input)
{
Console.WriteLine("Contra Variance");
}
}
public class ContraVarianceInterfaceInstance2 : ContraVarianceInterface<SuperRequest>
{
public void Set(SuperRequest input)
{
Console.WriteLine("Contra Variance 1");
}
}
static void CallContraVarianceInterface(ContraVarianceInterface<SuperRequest> task)
{
task.Set(null);
}
static void Test()
{
ContraVarianceInterface<Request> o1 = new ContraVarianceInterfaceInstance1();
ContraVarianceInterface<SuperRequest> o2 = new ContraVarianceInterfaceInstance2();
CallContraVarianceInterface(o1); **//Contra variance here bcoz less derived to more derived type?**
CallContraVarianceInterface(o2);
}
Co/Contra方差
public interface CoContraInterface<out O, in I>
{
O Execute(I input);
}
public class CoContraInterfaceInstance1 : CoContraInterface<Response, Request>
{
public Response Execute(Request input)
{
Console.WriteLine("Variance");
return new Response();
}
}
public class CoContraInterfaceInstance2 : CoContraInterface<SuperResponse, SuperRequest>
{
public SuperResponse Execute(SuperRequest input)
{
Console.WriteLine("Variance 1");
return new SuperResponse();
}
}
static void CallCoContraInterface(CoContraInterface<Request, Response> task)
{
task.Execute(null);
}
static void Test()
{
CoContraInterface<Response, Request> c1 = new CoContraInterfaceInstance1();
CoContraInterface<SuperResponse, SuperRequest> c2 = new CoContraInterfaceInstance2();
CoContraInterface<Response, SuperRequest> 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?
谢谢