Mocking functions with Pester not working

Question

I have the following code in my Test:

It "Set mock partitions"{
    Mock Get-Disk {
        return [PSCustomObject]@{
            SerialNumber = "SERIAL"
        }
    }
    Mock Get-Partition {
        param([PSCustomObject] $InputObject)
        return @([PSCustomObject]@{
            PartitionNumber = 1
        },[PSCustomObject]@{
            PartitionNumber = 2
        })
    }
    Custom-Function
}

In my main script, i have the following function:

function Custom-Function(){
  $Disk = Get-Disk
  $Partition = Get-Partition -InputObject $Disk
}

When i run the test, i get the following error message: The error happens on the line where Get-Partition -InputObject $Disk is called. Is there something wrong with my mock-definition of the InputObject parameter, or something else i am missing?

Answer 1

The error is correct in that the Get-Partition cmdlet doesn't have an -InputObject parameter and despite you adding one to your Mock, it isn't accessible in Pester. Pester Mocks the parameters automatically.

I managed to get a working example by using the CimInstance constructor to create the expected object that Get-Disk returns and then sending this as input to the -Disk parameter:

Describe 'Tests' {

    BeforeAll {
        function Custom-Function() {
            $Disk = Get-Disk
            $Partition = Get-Partition -Disk $Disk
        }
    }

    It "Set mock partitions" {
        Mock Get-Disk {
            $Result = [Microsoft.Management.Infrastructure.CimInstance]::new('MSFT_Disk','root/Microsoft/Windows/Storage')
            $Result | Add-Member -Name SerialNumber -Value 'SERIAL' -MemberType NoteProperty
            Return $Result
        }

        Mock Get-Partition { 
            return @([PSCustomObject]@{
                    PartitionNumber = 1
                }, [PSCustomObject]@{
                    PartitionNumber = 2
                })
        }
        
        Custom-Function

        Assert-MockCalled Get-Disk
        Assert-MockCalled Get-Partition
    }
}

This issue was helpful: https://github.com/pester/Pester/issues/1944