Testing with Pester 5.3.3

Question

I have written a function to write the errors that occur in my functions in a csv file, this function is called in the catch block of my functions. I would like to write a Test in Pester to check that my function works correctly, but to be honest I don't know where to start, I have tried some things but they don't work for me, I have also been reading in the documentation but I am still not clear, I would appreciate any help/comments.

Here is the function for which I want to write a Test in Pester:

function Write-Logs {
    param (
        [ValidateSet("Error")]$MessageType,
        [string][Parameter(Mandatory=$true)]$Message,
        $LogFilePath,
        $Source
    )
    $CSVLogPath = Join-Path -Path $PSScriptRoot -ChildPath ".\errorslog.csv"
    $CSVLogObject = [PSCustomObject] @{
        Date = Get-Date
        Message = $Message
        MessageType = $MessageType
        Source = $Source
    }
    $CSVLogObject | Export-Csv -Path $CSVLogPath -NoTypeInformation -Encoding UTF8 -Append
}

and so I´m calling the function in the catch block:

    catch {
        Write-Logs -LogFilePath:$CSVLogPath -Message:$Error[0].Exception.Message `
        -Source:"FunctionName()" -MessageType:"Error"
        return
    }

Structure your tests as [described by the docs](https://pester.dev/docs/usage/test-file-structure). Call `Write-Logs` in an `It` block, but without `try/catch`. Read CSV file and compare with expected result, using `Compare-Object $expectedCsv $actualCsv -Property Date, Message, MessageType, Source | Should -BeNullOrEmpty` (meaning there is no difference). — zett42, Jun 28 '22 at 10:50
@zett42 thank you very much for your comment, but I can't check that the file is empty, because the errorslog.csv file exists and has information about the errors that have occurred. — Thomas Steso, Jun 28 '22 at 11:03
The command I suggested doesn't check whether the file is empty. Create another CSV that contains the expected content of the CSV that is written during the test. Use `Compare-Object` to test if they are equal. When they are equal, `Compare-Object` returns `$null`, which is what `| Should -BeNullOrEmpty` tests for. — zett42, Jun 28 '22 at 11:08
@zett42 I think I should do a Mock to the Write-Log function, otherwise the test writes the error to the errorslog.csv file when it is executed, regardless if I have another file created with the expected content. — Thomas Steso, Jun 28 '22 at 11:24
Actually you should make `$CSVLogPath` a parameter, so you can direct it to a temporary file during testing. You even have parameter `-LogFilePath` already, but currently you don't use it. — zett42, Jun 28 '22 at 11:40
@zett42 could you show me an example? I have been working with Pester and Powershell for a very short time and I am still not very clear, thank you very much for your time. — Thomas Steso, Jun 28 '22 at 11:46

zett42 · Accepted Answer · 2022-06-28T12:14:34.800

Continuing from my comments, here is some code.

First make the function testable, by actually using the -LogFilePath parameter. This way you can write the log to a temporay file during testing. Due to the default value you can still use it without -LogFilePath when calling it from normal code.

function Write-Logs {
    param (
        [ValidateSet("Error")]$MessageType,
        [string][Parameter(Mandatory=$true)]$Message,
        $LogFilePath = (Join-Path -Path $PSScriptRoot -ChildPath ".\errorslog.csv"),
        $Source
    )
    $CSVLogObject = [PSCustomObject] @{
        Date = Get-Date
        Message = $Message
        MessageType = $MessageType
        Source = $Source
    }
    $CSVLogObject | Export-Csv -Path $LogFilePath -NoTypeInformation -Encoding UTF8 -Append
}

Test code:

BeforeAll {
    . $PSCommandPath.Replace('.Tests.ps1','.ps1')
}

Describe "Write-Logs" {
    BeforeEach{
        # Mock Get-Date so it returns a constant value suitable for testing
        $expectedDate = [DateTime]::new( 2022, 06, 28, 12, 36, 21 )
        Mock Get-Date { return $expectedDate }
    }   

    It "writes the expected CSV" {
    
        # You might read this from a file using Import-Csv
        $expectedCsv = [PSCustomObject]@{
            Date = $expectedDate
            Message = 'test message'
            MessageType = 'Error'
            Source = 'test source'
        }

        # Write log to temp file (Pester cleans it automatically, when It block ends)
        $testLogPath = "TestDrive:\test.log"
        Write-Logs -LogFilePath $testLogPath -MessageType $expectedCsv.MessageType -Message $expectedCsv.Message -Source $expectedCsv.Source

        $actualCsv = Import-Csv $testLogPath

        # Test if $expectedCsv equals $actualCsv
        Compare-Object $expectedCsv $actualCsv -Property Date, Message, MessageType, Source | Should -BeNullOrEmpty
    }
}

TestDrive: is a temporary drive created by Pester for each script block. It is very convenient for writing temporary files because Pester cleans it automatically when the script block ends. See Pester docs.
Once you got some basic tests working, you might want to improve your test code by using data-driven tests. This avoids duplication as you only need a single test, that can be fed from different data sets. See Pester docs, especially section "Providing external data to tests".

Testing with Pester 5.3.3

1 Answers1