Powershell - What does if($variable) test for?

Question

In Powershell, what does if($variable) test for? Is this testing if the variable is set, is null, is true, or something else?

Answer 1

Powershell uses loose typing, and as a part of this any value can be used as a conditional expression. The logic of evaluating values to true or false (such values are often called "truthy" and "falsey") is in the System.Management.Automation.LanguagePrimitives.IsTrue function. The logic is the following:

• $null evaluates to false.
• $true and $false have their usual meaning.
• A numeric value (int, double, decimal etc.) is truthy, if it is non-zero.
• A string is truthy, if it is non-empty.
• A switch parameter evaluates to true, if it has been supplied by the caller.

function Test 
{
   param ( [switch]$Option )
}

Test # $Option evaluates to false
Test -Option # $Option evaluates to true

# (Technically, $Option is not bool; it is of type SwitchParameter which, 
# for almost all intents and purposes, can be used interchangeably with bool.)

• A collection (implementing IList) uses the following logic: An empty collection is falsey; a collection with only one element evaluates to the truth value of its member; a collection with two or more elements is truthy.
• Anything else is truthy.

There are a couple of gotchas:

• The char type is not a numeric type. Indeed, [System.Management.Automation.LanguagePrimitives]::IsTrue([char]0) returns True (per the final clause); yet if([char]0) { "Truthy" } else { "Falsey" } returns "Falsey". I wasn't able to find the reason why this happens.
• As mentioned above, a collection with exactly one element is evaluated by evaluating its member. So what happens if you try to evaluate a collection which only contains itself (or a recursive or very deeply nested chain of collections, each with one element)? For science, you can test it:

$array = New-Object -TypeName System.Object[] 1 # $array = new object[1]
$array[0] = $array
if($array) { "Truthy" } else { "Falsey" }

(The answer is: it crashes the process with StackOverflowException.)