泛型类型参数是实现接口的类

Question

Suppose we have a class that implements an interface

class MyParaClass : IMyParaInterface

And another class named MyObject which takes generic T

class MyObject<T> where T: IMyParaInterface

Now I have a method that accepts this parameter

Print(MyObject<IMyParaInterface> parameter)

When I get an object which type is MyObject<MyParaClass> , and try to pass this to Print method,Build fails because it can not convert MyObject<MyParaClass> to MyObject<IMyParaInterface>

I thought there should be not an issue as MyParaClass implements IMyParaInterface .How to solve this or get around?

Answer 1

You should define your Print method as a template method:

void Print<T>(MyObject<T> parameter) where T : IMyParaInterface {}

Which means, that the method would take any MyObject<> instance, whose type argument implements your IMyParaInterface interface.

The problem with your original Print() code is, that it only accepts MyObject< IMyParaInterface > as input. Note, that MyObject< MyParaClass > is not instance of type MyObject< IMyParaInterface >.

There is another solution , where you can use .net's Covariance / contravariance features, by defining an interface for your object, where the type parameter will be marked as covariant ( out T).

public interface IMyObject<out T> where T: IMyParaInterface {}

public class MyObject<T> : IMyObject<T> where T: IMyParaInterface {}


public static void Print(IMyObject<IMyParaInterface> parameter)  {}

In this case, MyObject< MyParaClass > will be type-compatible with IMyObject< IMyParaInterface >.

You can read about covariance / contravariance here: https://docs.microsoft.com/en-us/dotnet/standard/generics/covariance-and-contravariance