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Version: v5

Custom Assertions

Pester allows users to create their own Should-operators for more advanced assertions. This is done by defining a test-function and registering it with Pester using the guidelines below.

Create the function#

Requirements / Interface#

Should will automatically pass on values from pipeline to your function as ActualValue. This can be an array or a single object depending on whether you register the function with -SupportsArrayInput.

When Should calls your custom assertion, it will invoke it with the following parameters:

  • ActualValue: The value passed to Should to assert.
  • Negate: Internal switch/bool parameter used to delcare if the user called Should -Not ....
  • CallerSessionState: Used by some internal assertions to invoke code in the user's state. This can usually be ignored.
  • Remaining bound parameters, typically matching the parameters your accept in your assertion, like ExpectedValue, Because, MyCustomParameter etc.

The function should return a object with the following properties:

  • Succeeded: Bool result of assert
  • FailureMessage: Message to the user explaining why the assert failed, typically in the format Expected <expected value>, but got <actual value>. If the function supports the commonly used -Because parameter, this property should include that message.

If a problem occurs in your function, ex. it was called with invalid parameters, throw an exception. This will also fail the Should assertion, while making it clear to the user that it's not normal behavior.

ActualValue is provided as a single parameter

Should accepts the value to assert by pipeline but passes it to the assertion using -ActualValue $ActualValue. This means ValueFromPipelineByPropertyName is not available inside your function, but you can write your own logic to work with complex types provided to ActualValue.

Using advanced functions as assertions

Advanced functions, typically enabled by using [CmdletBinding()] or [Parameter()], disallows arguments by default. In this scenario, make sure to accept all mandatory parameters in your param-block to avoid errors even if you don't use them. They are ActualValue, Negate and CallerSessionState.

Good practices#

  • Support -Not for negative assertions. If not supported, throw an exception when used.
    • Note that this is passed as $Negate to the function.
  • Support -Because to let users customize the error message on failure.
  • Write tests for you custom assertions to make sure they work the way you intended.
    • Should will create error records for failed assertions with the FullyQualifiedErrorId set to PesterAssertionFailed.
  • Provide comment-based help with synopsis and examples so users can find it using Get-ShouldOperator.
    • If you register an operator with a different name than the internal function, specify -InternalName YourFunctionName in Add-ShouldOperator. This is required for Get-ShouldOperator to retrieve your comment-bases help.

Register function as operator#

When your function is ready you need to register it with Pester. See Add-ShouldOperator for help or check out the examples below. Remember:

  • If -Name is different from the actual function name, make sure to specify -InternalName with your function name. This is used to retrieve help in Get-ShouldOperator.
  • Specify -SupportsArrayInput if your function should work with array input, ex. 1,2,3 | Should -BeNumber

Example registering a assertion function named Should-BeNumber:

Add-ShouldOperator -Name BeNumber `    -InternalName 'Should-BeNumber' `    -Test ${function:Should-BeNumber} `    -Alias 'BeNum' `    -SupportsArrayInput
Maxmimum number of Should operators

Due to a limitation in PowerShell and the current design on Should, Pester is limited to a maximum of 32 Should operators. Pester currently includes 25 built-in operators, which limits how many custom operators you can register in a single session. The limitation is tracked in this issue


Simple example#


#Requires -Module Pester
function Should-BeUpperCaseOnly ([string] $ActualValue, [switch] $Negate, [string] $Because) {    <#    .SYNOPSIS        Checks provided string for only uppercase letters    .EXAMPLE        "ABC" | Should -BeUpperCaseOnly
        Checks if "ABC" only contains uppercase letters. This should pass.    .EXAMPLE        "aBc" | Should -Not -BeUpperCaseOnly
        Checks if "aBc" does NOT contain only uppercase. Since there are lowercase letters in the string, this should pass.    #>    $arr = $ActualValue.ToCharArray()    [bool] $succeeded = @($arr | Where-Object { [char]::IsLetter($_) -and [char]::IsUpper($_) }).Count -eq $arr.Count    if ($Negate) { $succeeded = -not $succeeded }
    if (-not $succeeded) {        if ($Negate) {            $failureMessage = "Expected '$ActualValue' to not only contain uppercase letters$(if($Because) { " because $Because"})."        }        else {            $failureMessage = "Expected '$ActualValue' to only contain uppercase letters$(if($Because) { " because $Because"})."        }    }
    return [pscustomobject]@{        Succeeded      = $succeeded        FailureMessage = $failureMessage    }}
Add-ShouldOperator -Name BeUpperCaseOnly `    -InternalName 'Should-BeUpperCaseOnly' `    -Test ${function:Should-BeUpperCaseOnly} `    -Alias 'BU'


BeforeDiscovery {    # Loads and registers my custom assertion. Ignores usage of unapproved verb with -DisableNameChecking    Import-Module "$PSScriptRoot/MyCustomAssertions.psm1" -DisableNameChecking}
Describe 'Testing Should -BeUpperCaseOnly' {    It 'Success' {        "HELLO" | Should -BeUpperCaseOnly    }
    It 'Failure' {        "HeLLo" | Should -BeUpperCaseOnly -Because "it looks cooler"    }
    It 'Not all characters are uppercase' {        "hELLO" | Should -Not -BeUpperCaseOnly    }
    It 'We can test failed assertions' {        { "fails" | Should -BeUpperCaseOnly } | Should -Throw -ErrorId 'PesterAssertionFailed'    }}


Invoke-Pester ./demo.tests.ps1
Starting discovery in 1 files.Discovery found 4 tests in 181ms.Running tests.[-] Testing custom assertion.Failure 22ms (20ms|3ms) Expected 'HeLLo' to only contain uppercase letters because it looks cooler. at "HeLLo" | Should -BeUpperCaseOnly -Because "it looks cooler", /workspaces/Pester/Samples/demo.tests.ps1:11 at <ScriptBlock>, /workspaces/Pester/Samples/demo.tests.ps1:11Tests completed in 651msTests Passed: 3, Failed: 1, Skipped: 0 NotRun: 0

SupportsArrayInput example#


#Requires -Module Pester
function Should-ContainOnlyValuesOf ($ActualValue, [int]$ExpectedValue, [switch] $Negate, [string] $Because) {    <#    .SYNOPSIS        Asserts if collection contains only the expected value    .EXAMPLE        3,3,3 | Should -ContainOnlyValuesOf 3
        Checks if the array contains only values of 3. This should pass.
    .EXAMPLE        1,2,3 | Should -Not -ContainOnlyValuesOf 3
        Checks if the array contains other values than 3. This should pass.    #>
    $notEqual = foreach ($val in $ActualValue) {        if ($val -ne $ExpectedValue) { $val }    }    [bool] $succeeded = @($notEqual).Count -eq 0    if ($Negate) { $succeeded = -not $succeeded }
    if (-not $succeeded) {        if ($Negate) {            $failureMessage = "Expected values not equal to $ExpectedValue to be in collection @($($ActualValue -join ', '))$(if($Because) { " because $Because"})."        }        else {            $failureMessage = "Expected only values equal to $ExpectedValue in collection @($($ActualValue -join ', '))$(if($Because) { " because $Because"})."        }    }
    return [pscustomobject]@{        Succeeded      = $succeeded        FailureMessage = $failureMessage    }}
Add-ShouldOperator -Name ContainOnlyValuesOf `    -InternalName 'Should-ContainOnlyValuesOf' `    -Test ${function:Should-ContainOnlyValuesOf} `    -SupportsArrayInput


BeforeDiscovery {    # Loads and registers my custom assertion. Ignores usage of unapproved verb with -DisableNameChecking    Import-Module "$PSScriptRoot/MyCustomAssertions.psm1" -DisableNameChecking}
Describe 'Testing Should -ContainOnlyValuesOf' {    It 'All values are as expected' {        3, 3, 3 | Should -ContainOnlyValuesOf 3    }
    It 'Failure' {        3, 3, 3 | Should -Not -ContainOnlyValuesOf 3    }}


Invoke-Pester ./demo.tests.ps1
Starting discovery in 1 files.Discovery found 2 tests in 25ms.Running tests.[-] Testing Should -ContainOnlyValuesOf.Failure 8ms (6ms|2ms) Expected values not equal to 3 to be in collection @(3, 3, 3). at 3, 3, 3 | Should -Not -ContainOnlyValuesOf 3, /workspaces/Pester/Samples/demo.tests.ps1:11 at <ScriptBlock>, /workspaces/Pester/Samples/demo.tests.ps1:11Tests completed in 83msTests Passed: 1, Failed: 1, Skipped: 0 NotRun: 0