What Is the Interface Segregation Principle? Fat Interfaces, Role Interfaces and the Clients That Shape Them

Wed, 21 Oct 2026 00:04:00 +0000

An invoice printer needs to read one order. In many codebases it is handed an object that can also save orders, cancel them, total the month’s revenue and export everything to a spreadsheet, because that is the interface the order store happens to have. The printer never calls those methods, but it depends on them all the same: when one of them changes, the printer is rebuilt, retested and redeployed, and anyone writing a stand-in for it in a test has to implement all five. The interface segregation principle is the rule against that arrangement, and the fourth of the five SOLID principles this series covers. This article explains what it says, where it came from, what a fat interface costs, how to split one into interfaces shaped by their clients, and where splitting stops paying. Every output shown here was produced by running the code on Windows 11, with Python 3.14.6 and with the .NET 10 SDK, not written by hand.

Interfaces on Artur Rios Lab

What Is the Interface Segregation Principle? Fat Interfaces, Role Interfaces and the Clients That Shape Them