bcquality/custom/knowledge/testing/test-one-when-per-test.md

bc-version	domain	keywords	technologies	countries	application-area
all	testing	test when scenario single-action bdd atdd given-when-then	al	w1	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

// 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

// 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;

Flow tests — the deliberate exception

A flow test verifies the accumulated outcome of a multi-round business flow — partial receipt then invoicing, multiple posting rounds against one document. Here the sequence IS the scenario: the claim under test is "after round 1, 2 and 3, the balances are correct", and it cannot be expressed as three independent tests without losing the very interaction being verified.

Flow tests are permitted with multiple [WHEN] blocks when ALL of these hold:

1. The name declares the flow: the codeunit or procedure name contains the sequence (SVPartialFlowTests, ReceiveThenInvoice_QuantitiesAreCorrect).
2. Each [WHEN] is labelled as a round of ONE scenario ("Runde 1: kun modtagelse"), not as an unrelated action.
3. The [THEN] asserts the accumulated end-state — not one unrelated claim per WHEN. If the assertions decompose cleanly per action, it is two tests in disguise: split.

Unit-level tests keep the strict one-WHEN rule without exception. (Edison eval 2026-07-02, Jernpladsen @ b7656b1: five multi-WHEN procedures in SVPartialFlowTests, all deliberate round-labelled flows — the rule previously gave no verdict on them.)

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
• ReceiveThenInvoice_QuantitiesAreCorrect → legitimate flow test IF the flow-test conditions above are met