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// SOLID & Principles · 06

DRY

Same rule in two placesleads toOne source of truth

Don't Repeat Yourself: each piece of knowledge lives in one place.

How it fits together

DRY structureBeforeAfter«external»VAT rate changeOrderServicenet * 0.19InvoiceServicenet * 0.19«client»CartTotalsnet * 0.19OrderServiceTotal(lines)InvoiceServiceTotal(lines)«client»CartTotalsTotal(lines)VatCalculatorApply(net)Rate = 0.19edit 1edit 2edit 3Apply(net)Apply(net)Apply(net)
publishes / notifiesuses / calls
  • VAT rate change publishes / notifies OrderService (edit 1)
  • VAT rate change publishes / notifies InvoiceService (edit 2)
  • VAT rate change publishes / notifies CartTotals (edit 3)
  • OrderService uses / calls VatCalculator (Apply(net))
  • InvoiceService uses / calls VatCalculator (Apply(net))
  • CartTotals uses / calls VatCalculator (Apply(net))

Spot it

  • A rule change needs edits in several files
  • Validation differs between API and UI
  • A bug fixed in one copy survives in another

It costs

Merging look-alike code couples things that should change independently.

In code

public sealed class VatCalculator{    public const decimal Rate = 0.19m;​    public decimal Apply(decimal net) => net * Rate;}​public sealed class OrderService(VatCalculator vat){    public decimal Total(IEnumerable<OrderLine> lines)    {        var net = lines.Sum(line => line.Net);        return net + vat.Apply(net);    }}
Go deeperThe full explanation and what to ask for · 1 min readExpand

Hunt and Thomas defined it in The Pragmatic Programmer: every piece of knowledge must have a single, unambiguous, authoritative representation within a system. It is about knowledge, not text.

Two blocks that look the same but answer to different rules are incidental duplication. Merging them produces a shared helper full of flags and branches that every caller is afraid to touch.

A useful boundary: deduplicate freely within a bounded context, reluctantly across them. The rule of three helps: wait for the third copy before abstracting.

Ask for it

  • single source of truth
  • knowledge duplication
  • incidental duplication
  • rule of three
  • shared kernel

Find where the VAT calculation rule is duplicated across services and the frontend. Consolidate it into one source of truth, a single tested class or shared contract, and leave alone code that merely looks similar but changes for different reasons.

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