I've learned from this Stack Overflow question, that PowerShell return semantics are different, let's say, from C#'s return semantics. Quote from the aforementioned question:
PowerShell has really wacky return semantics - at least when viewed from a more traditional programming perspective. There are two main ideas to wrap your head around: All output is captured, and returned. The return keyword really just indicates a logical exit point.
Let's look at this example:
function Calculate
{
echo "Calculate"
return 11
}
$result = Calculate
If you echo $result you will realise that something is not right. You'd expect this:
11
But the reality, what you actually see, is different:
Calculate
11
So instead of getting back only the intended return value, you actually get back an array.
You could get rid of the echo statements and not polluting the return value, but if you call another function from your function, which echoes something, then you're in trouble again.
How can I write a PowerShell function that only returns one thing? If the built-in PowerShell functions do return only one value, why can't I do that?
The workaround that I'm using now and I'd love to get rid of:
function Calculate
{
# Every function that returns has to echo something
echo ""
return 11
}
# The return values is the last value from the returning array
$result = (Calculate)[-1]
Several answers already given do not fully answer your question because they do not account for other cmdlets you might call--as you point out--that might "pollute" your output. The answer from @Laezylion is essentially correct but I believe bears further elaboration.
It is clear that you appreciate that your current workaround--forcing every function to return an array then taking the last element--is not a very robust solution. (In fact, I think it is safe to call that a kludge. :-) The correct approach--and a direct answer to your question--is clear... though at first glance it sounds like a tautology:
Question: How do you ensure a function returns only one thing?
Answer: By ensuring that it returns only one thing.
Let me elucidate. There are really two kinds of functions/cmdlets in PowerShell: (A) those that return data and (B) those that do not return data but may instead report progress or diagnostic messages. Problems arise, as you have seen, when you try to mix the two. And this may easily happen inadvertently. Thus, it is your responsibility, as the function author, to understand each function/cmdlet call within your function: specifically, does it return anything, be it a true return value or just diagnostic output? You are expecting output from type A functions, but you need to be wary of any type B functions. For any one that does return something you must either assign it to a variable, feed it into a pipeline, or... make it go away. (There are several ways to make it go away: pipe it to Out-Null, cast it to void, redirect it to $null, or assign it to $null. See Jason Archer’s Stack Overflow post that evaluates the performance of each of these flavors, suggesting you shy away from Out-Null.)
Yes, this approach takes more effort, but you get a more robust solution by doing so. For a bit more discussion on this very issue, these A/B functions, etc. see Question 1 on A Plethora of PowerShell Pitfalls recently published on Simple-Talk.com.
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