Pester Test Throw


I am trying to write a pester test for when my function throws an exception and I am unable to get this to work. I modified this some from my original code but the idea is the same. When I run Pester it fails at this assertion with an exception and says the test failed.

I expect and receive the following exception.
Exception Details: "MethodInvocationException: Exception calling “Substring” with 2 arguments: Index and length must refer to a location within the string.

Please advise.

function Select-SubString {
        [Parameter(Mandatory = $true,
            ValueFromPipeline = $true)]
    Write-Verbose "Taking input and creating substring variable"
    $substring = $($input.Substring(0, 25))
    return $substring
Import-Module MyModule

Describe "myModule.psm1" {
    InModuleScope myModule {
      $string = "abc"
      it 'should throw exception' {
       { $string.toString() | Select-SubString } | Should Throw MethodInvocationException

Instead of MethodInvocationException, use a piece of the exception message like “Exception calling “Substring”” The string you’re asserting is actually the type of exception. Pester is looking at the message.

Thanks for the details Adam. That did work!

I wrote a set of assertions that handle this correctly. Here they are used in their own tests Assert-Throw