How will I know when to create an interface?

Question

I'm at a point in my development learning where I feel like I must learn more about interfaces.

I frequently read about them but it just seems like I cannot grasp them.

I've read examples like: Animal base class, with IAnimal interface for things like 'Walk', 'Run', 'GetLegs', etc - but I've never been working on something and felt like "Hey I should use an interface here!"

What am I missing? Why is it such a hard concept for me to grasp! I am just intimidated by the fact that I might not ever realize a concrete need for one - mostly due to some missing aspect of understanding them! It makes me feel like I'm missing something up top in terms of being a developer! If anyone has had an experience like this and had a breakthrough I would appreciate some tips on how to understand this concept. Thank you.

Answer 1

it solves this concrete problem:

you have a, b, c, d of 4 different types. all over your code you have something like:

a.Process();
b.Process();
c.Process();
d.Process();

why not have them implement IProcessable, and then do

List<IProcessable> list;

foreach(IProcessable p in list)
    p.Process();

this will scale much better when you add, say, 50 types of classes that all do the same thing.

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Another concrete problem:

Have you ever taken a look at System.Linq.Enumerable? It defines a ton of extension methods that operate on any type that implements IEnumerable. Because anything that implements IEnumerable basically says "I support iteration in a unordered foreach-type pattern", you can define complex behaviors (Count, Max, Where, Select, etc.) for any enumerable type.