Why does a PowerShell array containing zero evaluate as false?

PowerShell applies truthiness rules when it converts a collection to a Boolean. An empty collection is false. A collection with one element takes its truth value from that element, so @(0) is false because 0 is false. A collection with more than one element is true.

If you want to know whether an argument was supplied, check the number of elements instead of the truthiness of the collection. For an actual array, if ($argList.Length -gt 0) distinguishes @(0) from an empty array.

There's a wrinkle with nested collections. [bool] @(0) is false, but [bool] (, @(0)) is true. The outer array contains an inner array, and nested collections don't always behave like a simple recursive truthiness check would suggest.

If you're accepting arbitrary argument shapes, test the cases you care about rather than assuming extra array wrapping leaves the result unchanged.

Also watch the shape of values coming back from a PowerShell pipeline. A single result may arrive as a scalar rather than an array, so code that expects an array can behave differently with zero, one, or several results.

If a collection is what you need, wrap the output in @() and then check its Count. Decide separately how you want to handle a null value, since a present null and no result aren't necessarily the same thing.

This is PowerShell's conversion behavior, not necessarily a bug in the array. It can be handy when you really do want a truthiness test. For argument processing, though, presence and value are different questions, so use an explicit count check for presence.