OOP And Interfaces In C#: How To Use Them Effectively
Tags: beginners, learning, csharp, programming
Published: 2023-09-20 16:00:00

Object-Oriented Programming (OOP) is a common programming paradigm used in software development. [C# is one of the languages that supports OOP](https://www.devleader.ca/2023/03/01/c-basics-a-guide-for-new-programmers/), providing developers with tools to design and structure their applications effectively. One of the essential tools in OOP within C# is the interface. This article will explore the concept of OOP and interfaces in C# so that you can use them effectively.

---

### Basics of Object-Oriented Programming (OOP)

OOP is a programming approach based on the concept of “objects.” These objects can contain data, in the form of fields, and code, in the form of methods. The primary goals of OOP are to increase the modularity and reusability of code. When I’m writing OOP code, I can more easily visualize systems as building blocks.

The main concepts of OOP include:

* **Classes and Objects:** A class is a blueprint for creating objects. Objects are instances of classes.
    
* **Inheritance:** It allows a class to inherit properties and methods from another class.
    
* **Polymorphism:** It enables one interface to be used for a general class of actions.
    
* **Abstraction:** It hides the complex implementation details and shows only the necessary features of an object.
    
* **Encapsulation:** It restricts direct access to some of an object’s components and can prevent unintended interference.
    

[C# provides support for all these OOP concepts](https://www.devleader.ca/2023/03/01/c-basics-a-guide-for-new-programmers/), allowing developers to create well-structured and efficient applications.

---

### Dive into Interfaces in CSharp

### What are Interfaces?

In the realm of Object-Oriented Programming (OOP), an interface is a contract or a blueprint that defines a group of related functionalities. It’s a way to ensure that a class adheres to a certain standard or set of operations.

Unlike classes, interfaces do not contain any implementation details. They only declare method signatures, properties, events, or indexers. The main distinction between classes and interfaces is that while classes define both methods and their implementations, interfaces only define method signatures.

In C#, this has held true until C# 8 where there have been some changes:

> Beginning with C# 8.0, an interface may define default implementations for some or all of its members. A class or struct that implements the interface doesn’t have to implement members that have default implementations. For more information, see [default interface methods](https://learn.microsoft.com/en-us/dotnet/csharp/advanced-topics/interface-implementation/default-interface-methods-versions).

> [learn.microsoft.com](http://learn.microsoft.com)

### Declaring and Implementing Interfaces in CSharp

Interfaces in C# are declared using the `interface` keyword. Here’s a basic example:

```csharp
public interface IDisplay
{
    void DisplayMessage(string message);
}
```

To implement an interface in a class, you use the `:` symbol followed by the interface name:

```csharp
public class Screen : IDisplay
{
    public void DisplayMessage(string message)
    {
        Console.WriteLine(message);
    }
}
```

In this example, the `Screen` class is now contractually obligated to provide an implementation for the `DisplayMessage` method defined in the `IDisplay` interface.

### Multiple Inheritance with Interfaces

One of the challenges in OOP is the issue of multiple inheritance. Some languages allow a class to inherit from multiple classes, which can lead to ambiguity. C# does not support multiple inheritance with classes. However, it provides a solution through interfaces. A single class in C# can implement multiple interfaces, allowing it to inherit functionalities from several sources without the complications of multiple inheritance.

For instance:

```csharp
public interface IDisplay
{
    void DisplayMessage(string message);
}
```

```csharp
public interface ILog
{
    void LogMessage(string message);
}
```

```csharp
public class Screen : IDisplay, ILog
{
    public void DisplayMessage(string message)
    {
        Console.WriteLine(message);
    }
    
    public void LogMessage(string message)
    {
        // Log the message to a file or database
    }
}
```

In this example, the `Screen` class implements both `IDisplay` and `ILog` interfaces, demonstrating how C# uses interfaces to achieve multiple inheritance.

---

### Practical Benefits of Using Interfaces in CSharp

### Code Reusability

Interfaces play a pivotal role in promoting code reusability. By defining a standard set of operations in an interface, different classes can implement the same interface, ensuring a consistent set of functionalities across them. This means that any functionality built around an interface can be reused with any class that implements that interface.

### Flexibility and Scalability

Interfaces provide a structure that makes software designs more flexible. When classes are designed to work with interface references rather than concrete implementations, it becomes easier to introduce new classes or change existing ones without disrupting the overall system. This adaptability is crucial for scaling applications and accommodating future changes.

When you hear about coupling in code, this can take many forms. Technically, depending on an interface is coupling code to the API of that interface. But when we depend on concrete classes we actually end up coupling our code to the implementation of the concrete class. This is rarely what we actually want but it’s the side effect of using concrete classes as arguments and return types.

### Enhancing Testability

Interfaces are instrumental in the realm of [unit testing](https://www.devleader.ca/?p=2035). When classes are designed to depend on interfaces rather than concrete implementations, it becomes possible to provide mock implementations of these interfaces for testing purposes. This ensures that [units of code can be tested](https://www.devleader.ca/?p=2035) in isolation, without relying on external systems or services.

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### Common Scenarios and Patterns with Interfaces

Want to [keep reading about common scenarios like dependency injection and the strategy pattern](https://www.devleader.ca/2023/09/11/oop-and-interfaces-in-c-how-to-use-them-effectively/)? Head over to my website for the [full article where we dive into those and some best practices for interfaces in C#](https://www.devleader.ca/2023/09/11/oop-and-interfaces-in-c-how-to-use-them-effectively/)!

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