Casting generic type with interface constraint

问题

I have the following classes and interfaces

public interface IFoo {}

public class Foo : IFoo {}

public interface IWrapper<T> where T : IFoo {}

public class Wrapper<Foo> : IWrapper<Foo> {}

How can I cast Wrapper<Foo> to IWrapper<IFoo>? An exception is raised when using Cast (InvalidCastException) as I get null when using as.

Thanks for the help!

UPDATE

Here is a more concrete example:

public interface IUser {}

public class User : IUser {}

public interface IUserRepository<T> where T : IUser {}

public class UserRepository : IUserRepository<User> {}

Now I need to be able to do something like this:

 UserRepository up =  new UserRepository();
 IUserRepository<IUser> iup = up as IUserRepository<IUser>;

I'm using .net 4.5. Hope this helps.

回答1:

From your edit, you actually want:

public interface IUserRepository<out T> where T : IUser {}
public class UserRepository : IUserRepository<User> {}

then you can do:

IUserRepository<IUser> iup = new UserRepository();

note you can only add add the out modifier to the type parameter T if it appears in the output position everywhere in the definition of IUserRepository e.g.

public interface IUserRepository<out T> where T : IUser
{
    List<T> GetAll();
    T FindById(int userId);
}

if it appears anywhere in the input position, such as a method parameter or property setter it will fail to compile:

public interface IUserRepository<out T> where T : IUser
{
    void Add(T user);       //fails to compile
}

回答2:

Wrapper<Foo> needs to be Wrapper<IFoo>. Then you should be able to cast it. And it needs to implement the interface too.

The cast below works... I don't think you can cast an objects generic type parameter to a different type (i.e. IWrapper<Foo> to IWrapper<IFoo>).

void Main()
{
    var foo = new Wrapper();
    var t = foo as IWrapper<IFoo>;
    t.Dump();       
}


public interface IFoo {}

public class Foo : IFoo {}

public interface IWrapper<T> where T : IFoo {}

public class Wrapper : IWrapper<IFoo> {}

来源：https://stackoverflow.com/questions/17956407/casting-generic-type-with-interface-constraint

标签

generics

interface

casting