When to use Interface variable = new class variable OOP

Question

i saw often people use this code like

interface IAnimal 
{
    void die();
}

class Cat : IAnimal 
{
    void die() { ... }
    void meow() { ... }
}

IAnimal anAnimal = new Cat();
Cat aCat= new Cat();

C# knows for sure anAnimal.die() works, because die() is defined in IAnimal. But it won't let you do anAnimal.meow() even though it's a Cat, whereas aCat can invoke both methods.

so tell me why & when we should write this kind of code

IAnimal anAnimal = new Cat();

what is the advantage ??

thanks

This question appears to be off-topic because it is about broad, general programming concepts. This question belongs on http://programmers.stackexchange.com/ — Greg D, Feb 20 '14 at 08:57
Kind of explained [here](http://stackoverflow.com/a/7211580/122005) — chridam, Feb 20 '14 at 08:58
Downvote because the question is just copied and a little bit changed from the answer here http://stackoverflow.com/a/7211580/2655508 — Heslacher, Feb 20 '14 at 09:02
@Heslacher, yes, there should be a reference to it in the question. As there is no reference, it looks more like an attempt to ask *popular* question. His last [question](http://stackoverflow.com/q/21902878/1997232) is also following same pattern. — Sinatr, Feb 20 '14 at 09:30
possible duplicate of [Interface instantiation vs class instantiation](http://stackoverflow.com/questions/7211571/interface-instantiation-vs-class-instantiation) — t1w, Feb 20 '14 at 09:46

score 0 · Accepted Answer · answered Feb 20 '14 at 09:42

0

If anAnimal could also be assigned a different implementation of IAnimal, this is necessary.
If the code that uses anAnimal isn't Cat-specific, this is probably a good idea to make the code more generic and reusable.

answered Feb 20 '14 at 09:42

Sam

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When to use Interface variable = new class variable OOP

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