I have Interface IPageObject

 I also have class PageObject implements IPageObject

Here

Function GetById(ByVal pageObjectId As String) As IPageObject

or

Function GetById(ByVal pageObjectId As String) As PageObject

Which is better and why

:(

Why would you want to pass the concrete implementation of the Interface rather than the Interface?

Why is there an Interface there if you aren't going to use it?

Returning Interface can be handy as it implements OOPS concepts.

One more advantage I found over returning interface is that, if a person wants a property

to be exposed as public (to be available in other class) He is force to write it in the interface

Not just in the class

ie he also want it in interface ie our return type.

so he write all properties  in the interface that are going to be public  in class.

this gives good control, every thing public in class will be there in interface and this is good