Mocking Missing Cmdlet Pipelines with Pester

Following on from the previous the Mocking Missing Cmdlets with Pester and Mocking Missing Cmdlet ErrorAction with Pester posts, I also encountered (yet) another interesting problem when attempting to mock cmdlets that were not present on the test system. This one is pretty similar to the -ErrorAction edge-case but involves the pipeline. Here’s a pseudo test that was failing:

In the above example, we wish to mock both the Get-VM and Remove-VM cmdlets and assert that the Remove-VM cmdlet is called. If we run this on a system that does not have the Hyper-V cmdlets installed we receive the following error:

The error message is fairly self-explanatory. Just like your standard Advanced Function definition, we need to indicate that a parameter needs to be able to accept input via the pipeline using the ValueFromPipeline attribute. To fix this we just need to add the Parameter attribute to our stub function definition:

Whilst it’s not rocket-science – just in case someone else runs into it – I thought it would be worth quickly documenting! Here’s the working example:

 

Mocking Missing Cmdlet ErrorAction with Pester

Following on from the previous Mocking Missing Cmdlets with Pester post, I also encountered another interesting problem when attempting to mock cmdlets that were not present on the test system. This one is more of an edge-case, hence its own post. Just like last time, the tests worked when I had the Hyper-V cmdlets installed, but failed when running within an Appveyor VM.

It’s probably not uncommon that you will need to ensure that the code under test should throw an error here-and-there. Here is a pseudo-example that tests that Get-VM writes an error when passed with a non-existent VM name:

When this test is run it fails:

The problem here is that the stub function is not an advanced function and the –ErrorAction preference switch is ignored! This is easily resolved by adding the [CmdletBinding()] attribute to the stub function definition:

Running the test now results in the expected output:

This is not a Pester issue and it’s not a Powershell issue either. It’s just the way the normal Powershell functions work. But, just in case someone else runs into it I thought it would be worth quickly documenting!

Mocking Missing Cmdlets with Pester

When writing Pester unit tests for your Powershell code you will probably have a need to mock calls to external functions before too long. This process works as you would expect when Pester can locate a defined function/cmdlet with a matching name. However, if Pester cannot find a definition, it will fail.

This problem will normally surface in a Continuous Integration (CI) environment. For me, it was writing the first suite of tests for the xHyper-V DSC resource module. The Hyper-V cmdlets where present on my authoring machine but were not present on the Appveyor build VM.

Here is an overly simplified Pester test that I’ll use for demonstration purposes:

If we run the test and Pester cannot locate a defined function then it will report an error. Note: if you run this on a machine with the Hyper-V module installed (and you have a VM called ‘TestVM’) then it will pass – but you knew that already ;).

This is easily overcome by defining an empty function within the test file. Note: this will need to be defined within the ‘InModuleScope’ script block if you’re testing a module’s internals.

The test now passes as we would expect. Yay \o/

Now, what you really need to know is that for Pester to enumerate and mock parameter filters, those parameters need to be defined on the stub function. If we were to update the test to check for the passing of a particular –Name parameter like so:

When we run the test it will now fail again.

For Pester to enumerate the dynamic parameters on the function, it needs to have the parameters (only the one’s you’re interested in) defined. This can easily be fixed like so:

The tests will now once again pass successfully!

Hopefully this helps someone and saves some time. It took me a while to work out what was going on as I had the cmdlets available on my development machine but the tests were failing when running in an Appveyor VM. Perhaps I should submit a pull request to get this put into the Pester help documentation?!

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