Mocking with Pester

Pester provides a set of Mocking functions making it easy to fake dependencies and also to verify behavior. Using these mocking functions can allow you to "shim" a data layer or mock other complex functions that already have their own tests.

DESCRIPTION

With the set of Mocking functions that Pester exposes, one can:

• Mock the behavior of ANY powershell command.
• Verify that specific commands were (or were not) called.
• Verify the number of times a command was called with a set of specified parameters.

MOCKING FUNCTIONS

###Mock Mocks the behavior of an existing command with an alternate implementation.

###Assert-VerifiableMocks Checks if any Verifiable Mock has not been invoked. If so, this will throw an exception.

###Assert-MockCalled Checks if a Mocked command has been called a certain number of times and throws an exception if it has not.

EXAMPLE

function Build ($version) {
    Write-Host "a build was run for version: $version"
}

function BuildIfChanged {
	$thisVersion = Get-Version
	$nextVersion = Get-NextVersion
	if ($thisVersion -ne $nextVersion) { Build $nextVersion }
	return $nextVersion
}

$here = Split-Path -Parent $MyInvocation.MyCommand.Path
$sut = (Split-Path -Leaf $MyInvocation.MyCommand.Path).Replace(".Tests.", ".")
. "$here\$sut"

Describe "BuildIfChanged" {
    Context "When there are Changes" {
    	Mock Get-Version {return 1.1}
    	Mock Get-NextVersion {return 1.2}
    	Mock Build {} -Verifiable -ParameterFilter {$version -eq 1.2}

    	$result = BuildIfChanged

	    It "Builds the next version" {
	        Assert-VerifiableMocks
	    }
	    It "returns the next version number" {
	        $result | Should Be 1.2
	    }
    }
    Context "When there are no Changes" {
    	Mock Get-Version { return 1.1 }
    	Mock Get-NextVersion { return 1.1 }
    	Mock Build {}

    	$result = BuildIfChanged

	    It "Should not build the next version" {
	        Assert-MockCalled Build -Times 0 -ParameterFilter {$version -eq 1.1}
	    }
    }
}

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If you need to mock calls to commands which are made from inside a Script Module, additional code is required. For details, refer to Unit Testing within Modules