What Is the Open-Closed Principle? Extension, Abstraction, Plug-ins and the Expression Problem

Wed, 21 Oct 2026 00:02:00 +0000

A shop adds a new way to ship parcels. In one codebase that means opening the function that prices shipping, adding a branch to the middle of it, and re-testing everything that calls it. In another it means writing one new class in one new file, and nothing that already works is touched. The difference between the two is what the open-closed principle is about: arranging code so that new behaviour arrives as new code, rather than as edits to code that is released, tested and depended on. It is the second of the five SOLID principles, and this is part 2 of a series on them, after the single responsibility principle. This article explains what the principle means, where it came from, what it looks like in running code, why no module can be closed against every change, how the expression problem sets its limits, and when the abstraction it calls for is worth paying for. Every output shown here was produced by running the code on Windows 11, with Python 3.14.6, not written by hand.

What Is Inversion of Control? Frameworks, Callbacks, Dependency Injection and Containers

Tue, 20 Oct 2026 23:00:00 +0000

A web framework calls a function when a request arrives. A test runner finds the tests in a file and runs each one. A browser calls a handler when a button is clicked, and an application server hands a class the database connection it asked for in its constructor. In each case the code a programmer wrote does not decide when it runs or where its collaborators come from: something else does. That reversal has a name, inversion of control, and it is one of the ideas most of modern software is quietly built on. This article explains what it means, where the term came from, the forms it takes, how dependency injection and IoC containers work inside, and when the indirection is worth paying for. It closes a series on SOLID: where the dependency inversion principle says which way dependencies should point, this is the mechanism that makes them point that way at run time. Every output shown here was produced by running the code on Windows 11, with Python 3.14.6 and with the .NET 10 SDK and Microsoft.Extensions.DependencyInjection 10.0.13, not written by hand.

Design Patterns on Artur Rios Lab

What Is the Open-Closed Principle? Extension, Abstraction, Plug-ins and the Expression Problem

What Is Inversion of Control? Frameworks, Callbacks, Dependency Injection and Containers