Mocking CimInstance does not work

[rifftual]

General summary of the issue

A mock object of 'Microsoft.Management.Infrastructure.CimInstance' does not work. When trying to use the created object the following error is thrown: NullReferenceException: Object reference not set to an instance of an object.

Not sure if related, but see #345

Describe your environment

Pester version     : 5.1.1
PowerShell version : 5.1.18362.1474
OS version         : Microsoft Windows NT 10.0.18363.0

Steps to reproduce

$mock =  New-MockObject 'Microsoft.Management.Infrastructure.CimInstance'
Write-Host $mock

Expected Behavior

The mocked object works.

Current Behavior

The mocked object does not work.

[nohwnd]

This does not seem to be related to the linked issue. New-Mock object simply creates uninitialized instance of the given type. There is probably a property getter that relies on internal state that the object does not have. Please show the full error. And ideally stacktrace.

[rifftual]

File contents

#Requires  -Version 5.1

Describe 'Test' {
    Describe 'foo' {
        It 'bar' {
            $fixture = New-MockObject 'Microsoft.Management.Infrastructure.CimInstance'
            Write-Host $fixture
        }
    }
}

Command

Invoke-Pester -Configuration @{ Run = @{ Path = 'test.ps1' };Output = @{ Verbosity = 'Detailed' } }

Starting discovery in 1 files.
Discovering in C:\Users\foo\source\repos\rmd-devops-core\test.ps1.
Found 1 tests. 6ms
Discovery finished in 11ms.

Running tests from 'C:\Users\foo\source\repos\rmd-devops-core\test.ps1'
Describing Test
 Describing foo
   [-] bar 5ms (3ms|2ms)
    at <ScriptBlock>, C:\Users\foo\source\repos\rmd-devops-core\test.ps1:7
    NullReferenceException: Object reference not set to an instance of an object.
Tests completed in 105ms
Tests Passed: 0, Failed: 1, Skipped: 0 NotRun: 0

[nohwnd]

This is not Pester fault, internal state of the object is missing because you use intializer that does not set the internal state:

 Write-Host $fixture
PS> $error[0].exception.stacktrace
   at Microsoft.Management.Infrastructure.Internal.Data.CimPropertiesCollection.get_Item(String propertyName)
   at Microsoft.Management.Infrastructure.CimInstance.ToString()
   at Microsoft.PowerShell.Commands.WriteHostCommand.ProcessObject(Object o)
   at Microsoft.PowerShell.Commands.WriteHostCommand.ProcessObject(Object o)
   at Microsoft.PowerShell.Commands.WriteHostCommand.ProcessRecord()
   at System.Management.Automation.CommandProcessor.ProcessRecord()

Namely it is CimPropertiesCollection.get_Item that fails.

You can use one of the public constructors to get an instance of that type:

 [Microsoft.Management.Infrastructure.CimInstance]::new

OverloadDefinitions
-------------------
ciminstance new(ciminstance cimInstanceToClone)
ciminstance new(string className)
ciminstance new(string className, string namespaceName)
ciminstance new(cimclass cimClass)

PS> $c = [Microsoft.Management.Infrastructure.CimInstance]::new("abc")
PS> Write-Host $c
abc

[fflaten]

The exception is caused by ToString() looking for an internal member, but you'd get the same with GetType() ++ as the object really is incomplete. Mocking a generic CimInstance by itself isn't very useful. You get the shell, but every CimInstance is by itself generic. It's first when you link it to a CIM-class and properties that it gives any value.

New-MockObject creates uninitialized instances of the named class, which won't work well out-of-the-box for types like this that rely on unknown internal state/properties.

You'd be better of creating a real ciminstance-object for the class (without saving it ofc) or you could use the CimInstance-constructor and generate what you need manually.

# Real CIM-object as reference
$realProc = Get-CimInstance Win32_Process | Select -first 1
$realProc

ProcessId Name                HandleCount WorkingSetSize VirtualSize
--------- ----                ----------- -------------- -----------
0         System Idle Process 0           8192           8192


$realProc.CimInstanceProperties.Count
45

# Mock
$procMock = [Microsoft.Management.Infrastructure.CimInstance]::new('Win32_Process','root/cimv2')
$procMock

ProcessId Name HandleCount WorkingSetSize VirtualSize
--------- ---- ----------- -------------- -----------


$procMock.CimInstanceProperties.Count
0

# No properties exists by default, only the extended aliasproperties from ETS in PowerShell
$procMock | Get-Member

   TypeName: Microsoft.Management.Infrastructure.CimInstance#root/cimv2/Win32_Process

Name                      MemberType     Definition
----                      ----------     ----------
Handles                   AliasProperty  Handles = Handlecount
ProcessName               AliasProperty  ProcessName = Name
VM                        AliasProperty  VM = VirtualSize
WS                        AliasProperty  WS = WorkingSetSize
#.... only base class stuff, none of the cim-properties you'd expect from Win32_Process

$name = [Microsoft.Management.Infrastructure.CimProperty]::Create('Name','MyProcess', [cimtype]::String, 'Property, ReadOnly')
$handlecount = [Microsoft.Management.Infrastructure.CimProperty]::Create('Handlecount', 123, [cimtype]::UInt32 , 'Property, ReadOnly')
$procMock.CimInstanceProperties.Add($name)
$procMock.CimInstanceProperties.Add($handlecount)

$procMock

ProcessId Name      HandleCount WorkingSetSize VirtualSize
--------- ----      ----------- -------------- -----------
          MyProcess 123


$procMock.CimInstanceProperties.Count
2