Why do we need an interface if we can create functions?

Question

OOP

What's the real difference? (Applies to abstract classes as well)
Like you write:
with an interface:

class Dog : IAnimal
    {
        void IAnimal.Say()
        {
            Console.WriteLine("Gav gav!");
        }
    }

Without interface:

class Dog
    {
        void Say()
        {
            Console.WriteLine("Gav gav!");
        }
    }

Answer 1

An interface guarantees that the methods described in it are implemented in the class.
smoke the topic of interfaces in general, and the OOP tag on the toaster in particular - this question has been asked 100 times already.

Answer 2

You can't store a cat, a dog, and other talking robots in one variable.

Answer 3

An interface is a tool for abstraction. There is no abstraction in your example, moreover, it is bad simply because no one builds an animal-cat/dog hierarchy. This is meaningless, and very quickly you will be buried in a fish, which, as it were, is also an animal, but say is not applicable to it.
Make the interface ISpeakeable - that is, able to speak. Make a method that accepts this interface, and you will understand why it is good.

Answer 4

As far as animals are concerned.
With interfaces, you can do this:

interface ISpeaking {
  public void Speak();
}

public class Dog: ISpeaking {
  public void Speak() => Console.WriteLine("Гав!");
}

public class Cat: ISpeaking {
  public void Speak() => Console.WriteLine("Мяу!");
}

public static void Main() {
 ISpeaking[] speakers = new ISpeaking[] {new Cat(), new Dog()};
 foreach(ISpeaking speaker in speakers)
   speaker.Speak();
}

A more applicable example is the IEnumerable interface.
IEnumerable implement different collections: arrays, List, HashSet, Dictionary.
With IEnumerable, you can iterate through all the elements of a collection. This allows you to get rid of code duplication:
Without interfaces:

static void PrintAll(string[] array) {
  foreach(var e in array)
    Console.WriteLine(e);
}

static void PrintAll(List<string> list) {
  foreach(var e in list) { // Дублирование!
    Console.WriteLine(e);
}

With interface:

// Можно использовать и для string[], и для List<string>, и даже для того, о чём мы ещё не знаем!
static void PrintAll(IEnumerable<string> collection) {
  foreach(var e in collection) {
    Console.WriteLine(e);
  }
}

Interfaces are one way to achieve polymorphism.