What are and how to use interfaces in C#

An interface in C# is a collection of definitions that a class or a structure can implement (such as methods, properties, events, or indexers).

Unlike classes, interfaces do not contain method implementations, they only define the members that must be implemented by the classes or structures that implement them.

Interfaces are fundamental in object-oriented programming because they allow defining functionalities and behaviors that can be implemented by different classes, and they promote maintainability and code reuse.

Interfaces in C#

The declaration of an interface is done using the keyword interface.

[modifier] interface InterfaceName
{
    // Method definition
    dataType MethodName(parameters);

    // Property definition
    dataType PropertyName { get; set; }
}

Implementation of interfaces

To implement an interface in a class the colon syntax (:) is used followed by the name of the interface. The class must provide implementations for all members defined in the interface.

Here is an example of how to define and implement an interface in C#:

public interface IDriveable
{
    string LicensePlate { get; set; }

    // Methods
    void Drive();
    void Brake();
}

public class Car : IDriveable
{
    // Implementation of properties    
    public string LicensePlate { get; set; }

    // Implementation of methods
    public void Drive()
    {
        Console.WriteLine("The car is driving.");
    }

    public void Brake()
    {
        Console.WriteLine("The car has stopped.");
    }
}

Polymorphism with interfaces

One of the most important features of interfaces is their ability to support polymorphism. This allows an instance of a class that implements an interface to be treated as an instance of that interface.

Suppose we have another class that implements IDriveable called Bicycle

public class Bicycle : IDriveable
{
	// implementation
}

Now we can define a variable of type IDriveable, and assign variables of type Car or Bicycle.

IDriveable vehicle;

vehicle = new Car();
vehicle.Start(); // Output: The car has started.
vehicle.Stop();  // Output: The car has stopped.

vehicle = new Bicycle();
vehicle.Start(); // Output: The bicycle has started moving.
vehicle.Stop();  // Output: The bicycle has stopped.

Implementing multiple interfaces

C# does not support direct multiple inheritance (a class derived from multiple base classes). However, a class can implement multiple interfaces, which provides a way to achieve behavior similar to multiple inheritance.

For example, in this example, the class Duck implements the interfaces IFlyable and ISwimmable.

public interface IFlyable
{
    void Fly();
}

public interface ISwimmable
{
    void Swim();
}

public class Duck : IFlyable, ISwimmable
{
    public void Fly()
    {
        Console.WriteLine("The duck is flying.");
    }

    public void Swim()
    {
        Console.WriteLine("The duck is swimming.");
    }
}

Explicit implementation

C# allows explicit implementation of interface members. This is useful when a class implements multiple interfaces that may have methods with the same name, or when you want to provide specific implementations that are not directly accessible through the class.

In this example, the class Multifunctional implements the interfaces IPrintable and IScannable. But both interfaces declare a method Print(). We can explicitly declare the interfaces to resolve the naming conflict.

public interface IPrintable
{
    void Print();
}

public interface IScannable
{
    void Print();
}

public class Multifunctional : IPrintable, IScannable
{
    void IPrintable.Print()
    {
        Console.WriteLine("Printing...");
    }

    void IScannable.Print()
    {
        Console.WriteLine("Scanning...");
    }
}

// Usage
IPrintable printer = new Multifunctional();
printer.Print(); // Output: Printing...

IScannable scanner = new Multifunctional();
scanner.Print(); // Output: Scanning...

Practical examples

public interface IPowerable
{
    // Methods
    void PowerOn();
    void PowerOff();
    void Restart();
}

public class Phone : IPowerable
{
    // Implementation of methods
    public void PowerOn()
    {
        Console.WriteLine("The phone is powering on.");
    }

    public void PowerOff()
    {
        Console.WriteLine("The phone is powering off.");
    }

    public void Restart()
    {
        Console.WriteLine("The phone is restarting.");
    }
}

public interface ICommunicator
{
    // Methods
    void Call(string number);
    void SendMessage(string number, string message);
}

public class Phone : ICommunicator, IPowerable
{
	// ... IPowerable code from previous example ... //
	
    // Implementation of ICommunicator methods
    public void Call(string number)
    {
        Console.WriteLine($"Calling number {number}.");
    }

    public void SendMessage(string number, string message)
    {
        Console.WriteLine($"Sending message to number {number}: {message}");
    }
}

public interface IPayments
{
    // Methods
    void ProcessPayment(decimal amount);
    void RefundPayment(decimal amount);
}

public class ElectronicPayment : IPayments
{
    // Implementation of methods
    public void ProcessPayment(decimal amount)
    {
        Console.WriteLine($"Processing payment of {amount}.");
    }

    public void RefundPayment(decimal amount)
    {
        Console.WriteLine($"Refunding payment of {amount}.");
    }
}