Migrating Pester v3 → v4

This is the smallest jump — mostly an assertion-syntax rename. Many suites need only minor changes, some need none. It is largely script-automatable, but always review the diff and re-run the suite.

Official guide: https://pester.dev/docs/migrations/v3-to-v4

Heads-up: if your goal is a modern Pester (5 or 6), v3→v4 is only the first step. Do it, get green, then continue with v4-to-v5.md and v5-to-v6.md.

Change 1 — Dashed Should assertion syntax

v4 introduced the parameter-style Should syntax. The bareword form still ran in v4 but is removed in v5, so converting now saves a later migration.

# v3 (bareword)
It 'checks something' { 10 | Should Be 10 }

# v4+ (dashed)
It 'checks something' { 10 | Should -Be 10 }

The rename applies to every operator: Be, BeExactly, Match, Throw, BeNullOrEmpty, Contain, etc. → -Be, -BeExactly, -Match, -Throw, -BeNullOrEmpty, …

There is a well-known AST-based converter, Update-PesterTest (Chris Dent / Wojciech Sciesinski), that inserts the dashes safely by parsing the file rather than regexing it: https://gist.github.com/indented-automation/aeb14825e39dd8849beee44f681fbab3 — it's also reproduced in the official v3→v4 guide. Review its output, especially for non-UTF-8/ASCII files, where it can change encoding.

Change 2 — Contain → FileContentMatch

The Contain assertion was renamed to FileContentMatch (it tests file contents, which the old name made ambiguous against collection containment).

# Should Contain      -> Should -FileContentMatch
# Should Not Contain  -> Should -Not -FileContentMatch
'app.config' | Should -FileContentMatch 'setting'
'app.config' | Should -Not -FileContentMatch 'secret'

A simple regex-based migration script from the official guide (verify results — it can produce false positives):

$content = Get-Content -Path $file -Encoding $encoding
$content = $content -replace 'Should\s+\-?Contain',        'Should -FileContentMatch'
$content = $content -replace 'Should\s+\-?Not\s*-?Contain', 'Should -Not -FileContentMatch'
$content = $content -replace 'Assert-VerifiableMocks',      'Assert-VerifiableMock'
$content | Set-Content -Path $file -Encoding $encoding

Change 3 — Assert-VerifiableMocks → Assert-VerifiableMock

The cmdlet was renamed (dropped the trailing s). Rename all occurrences.

In Pester 5 this is deprecated and in Pester 6 it is removed — when you continue past v4, switch to Should -InvokeVerifiable. See v5-to-v6.md.

Change 4 — Array assertions (watch for edge cases)

Should gained array assertions in v4. This is transparent for most tests, but there are edge cases where an array test that passed under v3 fails under v4. If an array-related test changes result after the rename, inspect it manually rather than forcing it to pass. Background: https://github.com/pester/Pester/issues/873.

Mocking also shifted subtly when Pester moved from functions to aliases; there are no required changes, but if mocked-command behavior looks off, see https://github.com/pester/Pester/issues/810 and https://github.com/pester/Pester/issues/812.

v3 → v4 checklist

• Suite runs on v3 first (baseline).
• All Should <Operator> converted to Should -<Operator> (prefer the AST converter).
• Should Contain → Should -FileContentMatch (and the -Not form).
• Assert-VerifiableMocks → Assert-VerifiableMock.
• Array-assertion behavior changes reviewed manually.
• File encoding preserved by any scripted replacement.
• Suite green on v4; diff reviewed; committed.