Defining an interface with restrictions as to what classes can implement it

Question

How do I limit an Interface when defining it? What is the correct technical term for what I am describing? For example, I want MyInterface to only be implemented for objects that implement IList(Of T) and ICollection(Of T).

Answer 1

Be aware that an interface is nothing more than a contract too what a class minimal should fulfil.

Nobody is obliged to use the same member names in a class as long as is told what member of the contract (interface) is meant by a member in a class.

The only things an Interface describes are the members (and their attributes) which should exist to fulfil that contract.

In fact it is not even needed that those members do the same.

 

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Answer 2

If you do it this way, It will be impossible to implement your interface without implementing IList and ICollection

 

Interface MyInterface(Of T) : Inherits IList, ICollection
  Sub MyMethod()
  Sub MyOtherMethod()
End Interface

Answer 3

I opened this thread with exactly same thought ;-)

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Answer 4

I was thinking you could do something like

Interface MyInterface(Of T As IList(Of T), ICollection(Of T))

which would restrict the interface to only classes implementing either IList(Of  T) or ICollection(Of T). What I actually want to do is restrict the Interface to classes that implement one of the generic collections. Would IEnumerable(Of T) be sufficient in that case?

 

 

Answer 5

You should not use this method to restrict the arguments type.

This does compile without error, but will crash at run time

    Dim a As IList = New Integer() {1, 2, 3}
    a.Add(3)

You will get the same problem if you try to restrict your argument type this way