Interface Segregation Principle in C#: The I of SOLID

The Interface Segregation Principle (ISP) is one of the five principles of SOLID, a set of guidelines for writing maintainable and scalable software in object-oriented programming. In this blog post, we will take a look at the Interface Segregation Principle in depth, specifically in the context of C#.

What is the Interface Segregation Principle?

The Interface Segregation Principle says that a class should not be forced to implement interfaces it does not use. To put simply, it’s better to have multiple small, specialized interfaces rather than a single large interface containing unrelated methods. The ISP helps create modular, decoupled code, making it easier to maintain and extend.

The Problem with Large Interfaces

When a class implements a large interface with many unrelated methods, it can lead to several issues:

1. Violation of the Single Responsibility Principle: Large interfaces often combine unrelated responsibilities, making it harder to create cohesive classes.
2. Increased coupling: Implementing a large interface forces a class to depend on methods it does not use, leading to tighter coupling between classes.
3. Reduced maintainability: Making changes to a large interface can impact many classes, making the codebase harder to maintain.

Applying the Interface Segregation Principle in C#

Let’s take a look at an example of how to apply the ISP in a C# codebase.

The Problem

Consider an e-commerce application with a Product class and an IProduct interface:

public interface IProduct
{
    string Name { get; }
    decimal Price { get; }
    void Save();
    void Load(int productId);
    void ExportToJson(string filePath);
}

public class Product : IProduct
{
    // Implementation of the IProduct interface
}

The IProduct interface has methods related to product data persistence (Save and Load), as well as exporting the product to a JSON file (ExportToJson). Although these methods may be useful, they are not all related to the core responsibility of the Product class. A class implementing this interface is forced to implement all these methods, even if it only requires a subset of them.

The Solution

To adhere to the ISP, we can break the IProduct interface into smaller, focused interfaces:

public interface IProductData
{
    string Name { get; }
    decimal Price { get; }
}

public interface IPersistable
{
    void Save();
    void Load(int productId);
}

public interface IExportable
{
    void ExportToJson(string filePath);
}

public class Product : IProductData, IPersistable
{
    // Implementation of the IProductData and IPersistable interfaces
}

Now we have three small interfaces, each with a single responsibility. The Product class can implement only the interfaces it requires, reducing coupling and making the code more maintainable.

Conclusion

The Interface Segregation Principle is an important guideline for creating maintainable and scalable C# code. By breaking down large interfaces into smaller, more specialized ones, you can reduce coupling, and create a more maintainable codebase.