Extension methods in C# allow you to "extend" the behavior of existing types (like string, int, List, or even custom classes) without modifying their source code or using inheritance.

These methods are defined as static methods in a static class, and use the this keyword with the first parameter to specify the type they extend.

Benefits of Using Extension Methods

Improved Readability - Extension methods make code cleaner and easier to understand by allowing method calls directly on the object being extended.

No Source Code Modification - You can add new functionality to existing classes without modifying their original code.

Enhances Reusability - Common utility methods can be reused across your entire application by extending the relevant types.

Organized Utility Logic - Helps keep utility logic separate in static classes instead of mixing it with domain logic.

Enables Fluent Syntax - Supports chaining methods, improving the fluency and expressiveness of your code (like LINQ).

Supports Interfaces - Extension methods can be used to add behavior to interfaces (e.g., IEnumerable), enabling rich API design.

Example of an Extension Method

• We extended the built-in int type with a method called IsPrime.
• The method checks if a number is a prime using basic logic.
• Now, instead of calling a static utility method, you can directly use number.IsPrime() like it's part of the int type itself.

Conclusion

Extension methods are a powerful C# feature that bring flexibility, reusability, and readability to your code. They help follow the Open/Closed Principle (OCP) by allowing your types to be extended without being modified.

If used wisely, extension methods can help you write clean, maintainable, and expressive code in a way that integrates naturally into your existing architecture.