How do you test validation rules?

Validation rules must fire when expected and not fire when not.

Test cases:

• Should fire: invalid record save attempt; expect error.
• Should not fire: valid record save; expect success.
• Edge cases: boundary values, null handling.

Apex test pattern: `apex @isTest static void testRuleFiresOnInvalid() { Account a = new Account(Name=null); // missing required try { insert a; System.assert(false, 'Should have thrown'); } catch (DmlException e) { System.assert(e.getMessage().contains('Required')); } }

@isTest static void testRuleAllowsValid() { Account a = new Account(Name='Valid'); insert a; System.assertNotEquals(null, a.Id); } `

Manual / UI testing:

• Try invalid input in UI.
• Confirm error message shown.
• Try valid input; confirm save.

Common pitfalls:

• Only testing happy path.
• Not testing the error message text.
• Validation rules with complex logic insufficiently tested.

Senior insight: validation rules are easy to break. Tests catch regressions.