Pester How do I mock up an object to be used as a paramater

PowerShell Help
1

How do I mock up an object so that I can pass it to the method that I want to test. The mocked object needs to have methods of its own that I would want to mock the output of. A simple example can be found below.

Function Method-ToTest ($obj, $value)
{
  $obj.Method($value)
}

Describe “Method-ToTest” {
  Mock $obj = 
  It “Calls tMethod to test” {
     Method-ToTest $obj | Should Be $true
  }
}
2

You create new objects by using New-Object. You’d create an object of the type Object or PSObject, and then add members - including properties and methods. This isn’t necessarily a quick-and-easy process, as PowerShell isn’t presently designed with this use case in mind. What you probably really want is full class support, which we don’t yet have.

3

There’s no functionality for creating mock objects built into Pester; it only helps you with mocking PowerShell commands. The typical use case is you’ve got a PowerShell cmdlet or function which returns an object, and then your code under test does something with that object:

function Get-Thingy
{
    $object =  New-Object psobject -Property @{
        ThingyProperty = 'ThingyValue'
    }

    Add-Member -InputObject $object -MemberType ScriptMethod -Name UpperCaseThingy -Value {
        return $this.ThingyProperty.ToUpper()
    }

    return $object
}

function FunctionUnderTest
{
    $thingy = Get-Thingy
    return $thingy.UpperCaseThingy()
}

In practice, Get-Thingy would probably be returning some sort of .NET object, but here I’m just building up a simple custom object with one property and one method. This happens to be exactly how you would construct your mock of Get-Thingy:

Describe 'FunctionUnderTest' {
    Context 'Without Mocking' {
        It 'Returns the default value' {
            FunctionUnderTest | Should Be 'THINGYVALUE'
        }
    }

    Context 'With Mocking' {
        Mock Get-Thingy {
            $object =  New-Object psobject -Property @{
                ThingyProperty = 'MockedValue'
            }

            Add-Member -InputObject $object -MemberType ScriptMethod -Name UpperCaseThingy -Value {
                return $this.ThingyProperty.ToUpper()
            }

            return $object
        }

        It 'Returns the mocked object' {
            FunctionUnderTest | Should Be 'MOCKEDVALUE'
        }
    }
}
4

If your code under test happens to accept the object as a parameter already (such as with your Method-ToTest example), then you don’t even need to use Pester’s Mock command; just build up the object out in your test script, and pass it in.