How I applied SOLID principles and Interface patterns in AL to build an AppSource-ready BC extension

Most AL developers have written this at least once: alcase Strategy of Standard: Score := CalculateStandard(...); Weighted: Score := CalculateWeighted(...); end; It works. Until a third strategy. Then a fourth. Then a partner wants their own. Every addition means opening production code, touching the calculator, risking a regression, redeploying. The Cash Flow Risk Engine, a BC extension that scores customers 0–100 on payment behavior and generates risk-adjusted 30/60/90-day forecasts, was built to never have this problem. Here's the core pattern: Interface + Enum = permanently closed calculator alinterface ICFR_RiskStrategy { procedure CalculateScore(...): Decimal; } Each algorithm implements the interface independently. The enum binds each value to its codeunit via AL's Implementation keyword. The calculator becomes: alStrategy := CFRSetup."Default Risk Strategy"; Score := Strategy.CalculateScore(...); It never mentions a specific codeunit by name. A partner ships a new ML-based stra