Unit Testing a Class-based DSC resource with Pester

Question

I am having a problem with unit testing a class-based DSC resource. I am trying to Mock a couple of functions in the class and I get a cast error.

PSInvalidCastException: Cannot convert the "bool TestVMExists(string vmPath,     
string vmName)" value of type "System.Management.Automation.PSMethod" to type
"System.Management.Automation.ScriptBlock".

My test code is this:

using module 'C:\Program Files\WindowsPowerShell\Modules\xVMWareVM\xVMWareVM.psm1'

$resource = [xVMWareVM]::new()

   Describe "Set" {

    Context "If the VM does not exist" {

        Mock xVMWareVM $resource.TestVMExists {return $false}
        Mock xVMWareVM $resource.CreateVM

        It "Calls Create VM once" {
            Assert-MockCalled $resource.CreateVM -Times 1
        }
    }
}

Does anyone know how to achieve this?

Thanks in advance

Answer 1

You currently won't be able to mock a class function using Pester. The current workaround is to use Add-Member -MemberType ScriptMethod to replace the function. This means you will not get the mock asserts.

I borrowed this for DockerDsc tests by @bgelens.

Without your class code, I haven't been able to test this, but it should give you the idea along with @bgelens code above.

using module 'C:\Program Files\WindowsPowerShell\Modules\xVMWareVM\xVMWareVM.psm1'

   Describe "Set" {

    Context "If the VM does not exist" {
        $resource = [xVMWareVM]::new() 
        $global:CreateVmCalled = 0
        $resource = $resource | 
            Add-Member -MemberType ScriptMethod -Name TestVMExists -Value {
                    return $false
                } -Force -PassThru
        $resource = $resource | 
            Add-Member -MemberType ScriptMethod -Name CreateVM -Value {
                    $global:CreateVmCalled ++ 
                } -Force -PassThru

        It "Calls Create VM once" {
            $global:CreateVmCalled | should be 1
        }
    }
}