Mocking a command and getting different results based on the number of times the Mock is called

Question

I'm using Pester to unit test some code I've written. In the tests I mock Test-Path using a parameter filter:

Mock -CommandName 'Test-Path' -MockWith { return $false } `
    -ParameterFilter { $LiteralPath -and $LiteralPath -eq 'c:\dummy.txt' }

The following is psuedo code for what I'm doing:

If ( Test-Path -LiteralPath c:\dummy.txt )
{
    return "Exists"
}
else
{
    Attempt to get file

    If ( Test-Path -LiteralPath c:\dummy.txt )
    {
        return "File obtained"
    }   
}

On the first call to Test-Path I want to return $false and on the second call I want to return $true. I can think of a couple of ways of achieving this, but they don't feel right:

1. On the first call use the Path parameter and on the second use LiteralPath. Have two mocks one with a ParameterFilter for each. I don't like the idea of hacking the code in order to facilitate a test.

2. Create a function with parameters for: Path and InstanceNumber. Create mocks for the function. This is better than the above, but I don't like the idea of having a parameter just for testing purposes.

I've looked and can't find a way to mock based on the nth call. Does Pester facilitate this and I've just missed it? If not is there a cleaner way of achieving what I want?

Answer 1

function Test-File{
    If ( Test-Path -LiteralPath c:\dummy.txt )
            {
                return "Exists"
            }
            else
            {
                If ( Test-Path -LiteralPath c:\dummy.txt )
                {
                    return "File obtained"
                }   
            }
}


Describe "testing files" {
    it "file existence test" {
        #Arrange
        $script:mockCalled = 0
        $mockTestPath = {
                            $script:mockCalled++                                
                            if($script:mockCalled -eq 1)
                            {
                                return $false
                            }
                            else
                            {
                                return $true
                            }
                        }

            Mock -CommandName Test-Path -MockWith $mockTestPath 
            #Act
            $callResult = Test-File 

            #Assert
            $script:mockCalled | Should Be 2
            $callResult | Should Be "File obtained"
            Assert-MockCalled Test-Path -Times $script:mockCalled -ParameterFilter { $LiteralPath -and $LiteralPath -eq 'c:\dummy.txt' }


        }
}

I think you are after this?! let me know if not!