bcquality/custom/knowledge/testing/test-one-when-per-test.md
Michael Dieringer f058095dec Add 3 testing knowledge files from book review
- test-one-when-per-test: one WHEN per test, split if multiple actions
- ui-test-codeunit-naming: _UT suffix for TestPage-based codeunits
- test-feature-scenario-tags: [FEATURE]/[SCENARIO] comment structure

Based on patterns from Automatiserede tests med Business Central (Dieringer).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
2026-06-20 21:38:12 +02:00

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---
bc-version: [all]
domain: testing
keywords: [test, when, scenario, single-action, bdd, atdd, given-when-then]
technologies: [al]
countries: [w1]
application-area: [all]
---
## Description
Each test procedure must contain exactly **one** `[WHEN]` block — one action that
triggers the behaviour under test. A test with multiple WHENs ("do A, then do B,
then check C") is really two or more tests in disguise. Split them.
This constraint serves two purposes:
1. **Failure isolation** — when the test fails you know which action caused it.
2. **Readable specification** — each test reads as a single, falsifiable claim
about the system's behaviour.
A scenario that genuinely requires a precondition action (e.g. "post an order so
that a ledger entry exists") belongs in `[GIVEN]`. Only the action being asserted
belongs in `[WHEN]`.
## Anti Pattern
```al
// WRONG: two actions in one test
[Test]
procedure GetPrice_ThenGetDiscount_ReturnsCorrectValues()
var
UnitPrice, LineDiscPct: Decimal;
begin
// [GIVEN] ...
// [WHEN] first action
FindPriceMgt.GetSalesPrice(CustomerNo, ItemNo, '', UnitPrice, LineDiscPct);
// [WHEN] second action — this is a second test in disguise
FindPriceMgt.GetSalesPriceTiers(CustomerNo, ItemNo, '', TempBuffer);
// [THEN] asserting two unrelated things
Assert.AreEqual(100, UnitPrice, '');
Assert.IsFalse(TempBuffer.IsEmpty(), '');
end;
```
## Best Practice
```al
// CORRECT: split into two focused tests
[Test]
procedure GetPrice_CustomerPrice_ReturnsCorrectUnitPrice()
var
UnitPrice, LineDiscPct: Decimal;
begin
// [GIVEN] a customer with a price list line at 100
WarecoLib.GivenCustomerWithPrice(Customer, Item, '', 100);
// [WHEN]
FindPriceMgt.GetSalesPrice(Customer."No.", Item."No.", '', UnitPrice, LineDiscPct);
// [THEN]
Assert.AreEqual(100, UnitPrice, 'Unit price must match price list');
end;
[Test]
procedure GetPriceTiers_CustomerTier_ReturnsOneTierLine()
var
TempBuffer: Record "Find Price Tier Buffer" temporary;
begin
// [GIVEN] a customer with a tier price at min qty 10
WarecoLib.GivenCustomerWithTierPrice(Customer, Item, '', 10, 90);
// [WHEN]
FindPriceMgt.GetSalesPriceTiers(Customer."No.", Item."No.", '', TempBuffer);
// [THEN]
Assert.AreEqual(1, TempBuffer.Count(), 'Exactly one tier line expected');
end;
```
## Naming implication
The procedure name should make the single WHEN self-evident.
A name with "And" or "Then" in the middle is a strong signal to split:
- `GetPrice_AndDiscount_ReturnsValues` → split
- `GetPrice_CustomerPrice_ReturnsUnitPrice` → good