Vb.net接口，make方法只接受与参数相同类型的实例

Question

I'm trying to define a method in my vb.net interface that will only accept objects that inherit from the same interface and are also of the same class as the instance which receives the method call. Is this possible in vb.net?

Below I try to illustrate with a (invalid) generic type constraint what I wish to do:

Public Interface IFoo

   CompareStuff(Of T as sametype)(obj as T) as Boolean

End Interface

Answer 1

You are looking for the Curiously recurring template pattern .

I'm a little rusty on my VB, so here it is in C#

public interface IFoo<T>
    where T : IFoo<T>
{
    bool CompareStuff(T obj);
}

Then you implement it like this

public class Foo : IFoo<Foo>
{
    bool CompareStuff(Foo obj);
}

Check out this article about it from Eric Lippert. Pay special attention to the end where he says:

My advice is to think very hard before you implement this sort of curious pattern in C#; do the benefits to the customer really outweigh the costs associated with the mental burden you're placing on the code maintainers?

Answer 2

Public Interface IFoo(Of T)
    CompareStuff(obj As T) As Boolean

Use like this:

Public Class Foo Implements IFoo(Of Foo)
    Function CompareStuff(obj As Foo) As Boolean

but unfortunately also allows this to compile:

Public Class Bar Implements IFoo(Of Foo)
    Function CompareStuff(obj As Foo) As Boolean

Can't think of anything properly constraining right now.

Answer 3

If type X derives from type Y , and a Y can perform some operation upon another Y , then the Liskov Substitution Principle requires that a Y must be able to perform that same action on an X , and also that an X must be able to perform that same action on a Y . Putting those requirements together, an X must be able to perform that operation upon another X .

Note that it would be possible for an X to be able to perform some operation on another X , without such action being performable by, or upon, a Y .

Incidentally, while it's useful to implement IEquatable<T> for structures and sealed classes, the question of whether any object is equal to any other can be answered meaningfully for any two objects of any types. For example, if one has a collection of Cat , one may reasonably try to compare the objects in that collection against a parameter of type SiameseCat (while the collection may contain instances of PersianCat , it might also contain some of type SiameseCat ) or type Animal (some passed-in objects might be of type Dog . but others might be of type Cat ). While querying a collection of Cat to see if it contains some particular Dog might be "silly", the collection should have no problem answering the query ("Zero instances found").