Toper Turor

PowerShell testing empty parameters with if and false

PowerShell testing empty parameters with if and false

I just ran into the most interesting behavior.  I say that because I never ran into it before.

In many of my scripts I evaluate to see if a value has been passed in order to decide if I am going to process something.

Within my scripts it simply looks like this:

if ($VirtualMachineMigration) { }

If this parameter contains something, then act upon that something.  The reverse is to use the not:

if (!$VirtualMachineMigration) { }

If this parameter contains nothing, then do something.

Today I learned that this works all well and fine with strings, and objects, and integers, etc.  But not so well with Booleans.

Lets take a look at the following:

function EnableVMMigration
{
    [CmdletBinding()]
    param
    (
        [System.Boolean]
        $VirtualMachineMigration
    )

    if ($VirtualMachineMigration) {
        "in my If, making my next decision"
    }
    "Out of my IF"
}

That is simple enough, it accepts a Boolean parameter of VirtualMachineMigration.  Lets give it a go:

First give it a $true:

PS C:UsersAdministratorDocuments> EnableVMMigration -VirtualMachineMigration $true
in my If, making my next decision
Out of my IF

Exactly as I expected.

Now, give it a $false

PS C:UsersAdministratorDocuments> EnableVMMigration -VirtualMachineMigration $false
Out of my IF

It never even entered my if statement.  Because the If is $false, because I am sending it one � although the condition of the evaluation of �is $VirtualMachineMigration� empty is actually no different than it was when it was $true.

So, what is happening?  PowerShell is simply evaluating the parameter itself, not the question of �is this empty�.

Tricky little shell.

So, what if I really want to check if this is empty and the possibility is either $true or $false?  (this is my first idea to work around this, I am sure that there are others)

So, lets update the function so that I truly see if the parameter is empty before deciding how to act on whether it is $true or $false

function EnableVMMigration
{
    [CmdletBinding()]
    param
    (
        [System.Boolean]
        $VirtualMachineMigration
    )

    if ($VirtualMachineMigration) {
        "making a positive choice"
    }
    elseif (!$VirtualMachineMigration) {
        "Making a negative choice"
    }
    "Out of my IF"
}

PS C:UsersAdministratorDocuments> EnableVMMigration -VirtualMachineMigration $true
making a positive choice
Out of my IF

PS C:UsersAdministratorDocuments> EnableVMMigration -VirtualMachineMigration $false
Making a negative choice
Out of my IF

Now, that works the way I wanted it to.

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