I don't have a direct answer to your questions. I myself have similiar questions. I personally have a little background in foxpro and found the following website http://www.vfpconversion.com/Articles.aspx very useful in explaining c# oop principles. One article deals with interfaces,maybe you can benefit from it

>Hi All, I have been doing some reading to clear a few things up on my mind.
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>I have been struggling for some reason with Interfaces.
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>From what I understand is that an Interface is a design which MUST not contain any Implementation. Okay I got this.
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>But what is the purpose of having a Interface it all it contains is empty methods, why would I not create a class?
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>Because I inherit from a interface but there is no implementation so, I have to re-code all my code for that type of implementation.
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>Can someone help explain this interface question, and maybe a code example, that applies to a real world scenario, the one in the book is not helping, me it uses IToken, etc...and I just don't get that. My background lies in personnel, timekeeping and budgeting, so if you have an example for that , this would be awesome.
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>Thanks In Advance.