How can inheritance be reasonable without covariance? (C#)

Question

I'm quite new to C# but I have quite good C++ knowledge. From C++ I'm used that something like this works:

class Base1 {};
class Derived1 : Base1 {};
class Base2 {
   Base1 foo();
};
class Derived2 {
   Derived1 foo();
};

which I know under the term Covariance. The piece of code might miss something but I hope you get what I want to say. As I tried to produce something like that in C# with override and stuff, the Compiler complained that the type had to be the same. like this:

class Base1 {};
class Derived1 : Base1 {};
class Base2 {
   Base1 foo();
};
class Derived2 {
   Base1 foo();
};

Might that lead to any problems? Of course as I am the programmer of function foo I can assure there will always be a Derived1 returned. But whoever calls my function doesn't know that.

I have a project in which I have some Manager classes, for which I wanted to use an Interface IManager (it's my own class, not any from .NET if there exists any with that name), because they all have to implement a method Load() for example. Load() always returns a an Object, that is managed by that certain Manager. If every Manager implements IManager, all of them would get the Interface

ManagableObject Load();

Is that good, bad? Or even better, is there any workaround?

I also read that it worked with templates, but only like this I guess:

List<Base1> myList;
myList.push_back(new Derived1());

Or can I use List as return type in Base1 and List in class Derived2? That would be ok too.

Thanks and best regards, Expecto

There is nothing wrong with your first set of class definitions (apart from requiring method implementations) - so I think you might be missing something in your explanation. What compiler error do you get? — RJ Lohan, Jul 23 '12 at 05:08
C# actually supports covariance in various places; however _covariant return values_ are not supported, which I believe is what you're after here. Your samples are also currently broken with regards to showing the actual problem. — rjnilsson, Jul 23 '12 at 05:55
@RJLohan in English it would be sth like: "The return value has to be ManagableObject to match the overwritten method IManager.Load()" — Verena Haunschmid, Jul 23 '12 at 09:28
@cwan yes, I meant the return value, but I thought I made that clear, sorry — Verena Haunschmid, Jul 23 '12 at 09:30

score 0 · Answer 1 · answered Jul 23 '12 at 05:16

0

You need to declare your method using the new keyword.

 class Derived2 {
  new Derived1 foo();
};

answered Jul 23 '12 at 05:16

test1

11
1

Use the new modifier to explicitly hide a member inherited from a base class. To hide an inherited member, declare it in the derived class using the same name, and modify it with the new modifier. (http://msdn.microsoft.com/en-us/library/51y09td4%28v=vs.71%29.aspx#vclrfnew_newmodifier) it used when class base { void test(){}} class derived:base{ new void test(){}} (new is used only to hide base method, without new keyword compiler produces only warning message but code is compilable and executable) – ASpirin Jul 23 '12 at 06:01

score 0 · Accepted Answer · edited May 23 '17 at 12:21

0

That's what I meant: Does C# support return type covariance? but I didn't know to look for the term return type covariance

edited May 23 '17 at 12:21

Community

1
1

answered Jul 25 '12 at 16:48

Verena Haunschmid

1,252
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How can inheritance be reasonable without covariance? (C#)

2 Answers2