> For the complete documentation index, see [llms.txt](https://raviram.gitbook.io/c-programing/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://raviram.gitbook.io/c-programing/oop-animals/stage-4-interface-introduced.md).

# Stage 4: Interface introduced

## Stage 4: Interface introduced

### Highlights:

* The question is where to define or implement a behavior only for certain birds.
* The solution is to use interfaces, which are prefixed with "I" and denote objects that support a certain behavior.
* Interfaces are like pure abstract classes, meaning they only define a contract and not implementation.
* The implementation will be provided by the specific child that adheres to the interface.
* The "ISingable" interface is used for birds that can sing, and if a class doesn't implement it, there will be an error.
* An example of a bird that doesn't implement the interface is the eagle, which cannot sing.
* By having well-defined contracts, developers are prevented from using objects in ways they're not supposed to.
* This provides an additional layer of robustness while keeping everything neat and understandable.

### Animal.cs

No change!

### Bird.cs

No change!

### Cat.cs

No change!

### Dog.cs

No change!

### Eagle.cs

```csharp
namespace MyApplication.Animals;

public class Eagle(string name) : Bird(name)
{
    public override void Climb()
    {
        Console.WriteLine($"{Name} is climbing a tower...");
    }
}
```

### ISingable.cs

```csharp
namespace MyApplication.Animals;

public interface ISingable //pure abstract class
{
    void Sing();
}
```

### Parrot.cs

```csharp
namespace MyApplication.Animals;

public class Parrot(string name) : Bird(name), ISingable
{    
    public override void Climb()
    {
        Console.WriteLine($"{Name} is climbing on a tree!");
    }

    public void Sing()
    {
        Console.WriteLine($"{Name} is singing...");
    }
}
```

### Program.cs

```csharp

using MyApplication.Animals;

var cat = new Cat("Tom");
Console.WriteLine($"{cat.Name} created!");
cat.Eat();
cat.Climb();
Console.WriteLine();

var dog = new Dog("Mike");
Console.WriteLine($"{dog.Name} created!");
dog.Eat();
dog.Climb();
Console.WriteLine();

// parrot can sing now!
var parrot = new Parrot("Glory");
Console.WriteLine($"{parrot.Name} created!");
parrot.Eat();
parrot.Climb();
parrot.Fly();
parrot.Sing();  
Console.WriteLine();

//introducing eagle (cannot sing!)
var eagle = new Eagle("Jack");
Console.WriteLine($"{eagle.Name} created!");
eagle.Eat();
eagle.Climb();
eagle.Fly();
Console.WriteLine();
```