I came across this weird issue in Powershell (not in other languages). Could anyone please explain to me why this happened?
I tried to return a specified number (number 8), but the function keeps throwing everything at me. Is that a bug or by design?
Function GetNum() {
Return 10
}
Function Main() {
$Number10 = GetNum
$number10 #1 WHY NO OUTPUT HERE ??????? I don't want to use write host
$result = 8 # I WANT THIS NUMBER ONLY
PAUSE
return $result
}
do {
$again = Main
Write-Host "RESULT IS "$again # Weird Result, I only want Number 8
} While ($again -eq 10) # As the result is wrong, it loops forever
Is that a bug or by design?
By design. In PowerShell, cmdlets can return a stream of objects, much like using yield return in C# to return an IEnumerable collection.
The return keyword is not required for output values to be returned, it simply exits (or returns from) the current scope.
From Get-Help about_Return (emphasis added):
The Return keyword exits a function, script, or script block. It can be
used to exit a scope at a specific point, to return a value, or to indicate
that the end of the scope has been reached.
Users who are familiar with languages like C or C# might want to use the
Return keyword to make the logic of leaving a scope explicit.
In Windows PowerShell, the results of each statement are returned as
output, even without a statement that contains the Return keyword.
Languages like C or C# return only the value or values that are specified
by the Return keyword.
Mathias is spot on as usual.
I want to address this comment in your code:
$number10 #1 WHY NO OUTPUT HERE ??????? I don't want to use write host
Why don't you want to use Write-Host? Is it because you may have come across this very popular post from PowerShell's creator with the provocative title Write-Host Considered Harmful?
If so, I encourage you to read what I think is a great follow-up/companion piece by tby, titled Is Write-Host Really Harmful?
With this information, it should be clear that as Mathias said, you are returning objects to the pipeline, but you should also be armed with the information needed to choose an alternative, whether it's Write-Verbose, Write-Debug, or even Write-Host.
If I were going to be opinionated about it, I would go with Write-Verbose, altering your function definition slightly in order to support it:
function Main {
[CmdletBinding()]
param()
$Number10 = GetNum
Write-Verbose -Message $number10
$result = 8 # I WANT THIS NUMBER ONLY
PAUSE
$result
}
When you invoke it by just calling $again = Main you'll see nothing on the screen, and $again will have a value of 8. However if you call it this way:
$again = Main -Verbose
then $again will still have the value of 8, but on the screen you'll see:
VERBOSE: 10
likely in differently colored text.
What that gives is not only a way to show the value, but a way for the caller to control whether they see the value or not, without changing the return value of the function.
To drive some of the points in the articles home further, consider that it's not necessarily necessary to invoke your function with -Verbose to get that.
For example, let's say you stored that whole script in a file called FeelingNum.ps1.
If, in addition to the changes I made above, you also add the following to the very top of your file:
[CmdletBinding()]
param()
Then, you still invoked your function "normally" as $again = Main, you could still get the verbose output by invoking your script with -Verbose:
powershell.exe -File FeelingNum.ps1 -Verbose
What happens there is that using the -Verbose parameter sets a variable called $VerbosePreference, and that gets inherited on each function called down the stack (unless it's overridden). You can also set $VerbosePreference manually.
So what you get by using these built-in features is a lot of flexibility, both for you as the author and for anyone who uses your code, which is a good thing even if the only person using it is you.
Related
I've got very weird behaviour which I suppose is related to dot-sourcing somehow, but I cannot wrap my head around it. Here's what I have:
A script sourced.ps1 which contains the two functions and a class:
class MyData {
[string] $Name
}
function withClass() {
$initialData = #{
Name1 = "1";
Name2 = "2";
}
$list = New-Object Collections.Generic.List[MyData]
foreach ($item in $initialData.Keys) {
$d = [MyData]::new()
$d.Name = $item
$list.Add($d)
}
}
function withString() {
$initialData = #{
Name1 = "1";
Name2 = "2";
}
$list = New-Object Collections.Generic.List[string]
foreach ($item in $initialData.Keys) {
$list.Add($item)
}
}
I also have a script caller.ps1 which dot-sources the one above and calls the function:
$ErrorActionPreference = 'Stop'
. ".\sourced.ps1"
withClass
I then call the caller.ps1 by executing .\caller.ps1 in the shell (Win terminal with PS Core).
Here's the behaviour I cannot explain: if I call .\caller.ps1, then .\sourced.ps1 and then caller.ps1 again, I get the error:
Line |
14 | $list.Add($d)
| ~~~~~~~~~~~~~
| Cannot find an overload for "Add" and the argument count: "1".
However, if I change the caller.ps1 to call withString function instead, everything works fine no matter how many times I call caller.ps1 and sourced.ps1.
Furthermore, if I first call caller.ps1 with withString, then change it to withClass, there is no error whatsoever.
I suppose using modules would be more correct, but I'm interested in the reason for such weird behaviour in the first place.
Written as of PowerShell 7.2.1
A given script file that is both dot-sourced and directly executed (in either order, irrespective of how often) creates successive versions of the class definitions in it - these are distinct .NET types, even though their structure is identical. Arguably, there's no good reason to do this, and the behavior may be a bug.
These versions, which have the same full name (PowerShell class definitions created in the top-level scope of scripts have only a name, no namespace) but are housed in different dynamic (in-memory) assemblies that differ by the last component of their version number, shadow each other, and which one is effect depends on the context:
Other scripts that dot-source such a script consistently see the new version.
Inside the script itself, irrespective of whether it is itself executed directly or dot-sourced:
In PowerShell code, the original version stays in effect.
Inside binary cmdlets, notably New-Object, the new version takes effect.
If you mix these two ways to access the class inside the script, type mismatches can occur, which is what happened in your case - see sample code below.
While you can technically avoid such errors by consistently using ::new() or New-Object to reference the class, it is better to avoid performing both direct execution and dot-sourcing of script files that contain class definitions to begin with.
Sample code:
Save the code to a script file, say, demo.ps1
Execute it twice.
First, by direct execution: .\demo.ps1
Then, via dot-sourcing: . .\demo.ps1
The type-mismatch error that you saw will occur during that second execution.
Note: The error message, Cannot find an overload for "Add" and the argument count: "1", is a bit obscure; what it is trying to express that is that the .Add() method cannot be called with the argument of the given type, because it expects an instance of the new version of [MyData], whereas ::new() created an instance of the original version.
# demo.ps1
# Define a class
class MyData { }
# Use New-Object to instantiate a generic list based on that class.
# This passes the type name as a *string*, and instantiation of the
# type happens *inside the cmdlet*.
# On the second execution, this will use the *new* [MyData] version.
Write-Verbose -Verbose 'Constructing list via New-Object'
$list = New-Object System.Collections.Generic.List[MyData]
# Use ::new() to create an instance of [MyData]
# Even on the second execution this will use the *original* [MyData] version
$myDataInstance = [MyData]::new()
# Try to add the instance to the list.
# On the second execution this will *fail*, because the [MyData] used
# by the list and the one that $myDataInstance is an instance of differ.
$list.Add($myDataInstance)
Note that if you used $myDataInstance = New-Object MyData, the type mismatch would go away.
Similarly, it would also go away if you stuck with ::new() and also used it to instantiate the list: $list = [Collections.Generic.List[MyData]]::new()
I'm using a function that I call from another script. It prompts a user for input until it gets back something that is not empty or null.
function GetUserInputValue($InputValue)
{
do{
$UserValue = Read-Host -Prompt $InputValue
if (!$UserValue) { $InputValue + ' cannot be empty' }
}while(!$UserValue)
$UserValue
return $UserValue
}
The issue is quite strange and likely a result of my lack of powershell experience. When I run the code and provide empty results, the messages from the if statement queue up and only display when I finally provide a valid input. See my console output below.
Console Results
test:
test:
test:
test:
test:
test:
test: 1
test cannot be empty
test cannot be empty
test cannot be empty
test cannot be empty
test cannot be empty
test cannot be empty
1
I can make this work however in the main file with hard coded values.
do{
$Server = Read-Host -Prompt 'Server'
if (!$Server) { 'Server cannot be empty' }
}while(!$Server)
I'm working Visual Studio Code. This is a function I have in another file I've named functions.ps1.
I call this from my main file like this,
$test = GetUserInputValue("test")
$test
When you put a naked value in a script like "here's a message" or 5 or even a variable by itself $PID what you're implicitly doing is calling Write-Output against that value.
That returns the object to the pipeline, and it gets added to the objects that that returns. So in a function, it's the return value of the function, in a ForEach-Object block it's the return value of the block, etc. This bubbles all the back up the stack / pipeline.
When it has nowhere higher to go, the host handles it.
The console host (powershell.exe) or ISE host (powershell_ise.exe) handle this by displaying the object on the console; this just happens to be the way they handle it. Another host (a custom C# application for example can host the powershell runtime) might handle it differently.
So what's happening here is that you are returning the message that you want to display, as part of the return value of your function, which is not what you want.
Instead, you should use Write-Host, as this writes directly to the host, skipping the pipeline. This is the correct command to use when you want to display a message to the user that must be shown (for other information you can use different commands like Write-Verbose, Write-Warning, Write-Error, etc.).
Doing this will give you the correct result, and prevent your informational message from being part of the return value of your function.
Speaking of which, you are returning the value twice. You don't need to do:
$UserValue
return $UserValue
The first one returns the value anyway (see the top of this answer); the second one does the same thing except that it returns immediately. Since it's at the end of the function anyway, you can use wither one, but only use one.
One more note: do not call PowerShell functions with parentheses:
$test = GetUserInputValue("test")
This works only because the function has a single parameter. If it had multiple params and you attempted to call it like a method (with parentheses and commas) it would not work correctly. You should separate arguments with spaces, and you should usually call parameters by name:
$test = GetUserInputValue "test"
# better:
$test = GetUserInputValue -InputValue "test"
I understand from this answer that when you have default parameters and multiple parameter sets in a Powershell function, the default parameter values will be instantiated even if the parameter set in use is not the one in which they are inserted.
Is there a way to avoid this?
For example, in the function below, assuming that there is a really expensive calculation used to compute the default value of $FirstParameter, I would like to avoid using it when it is not necessary:
function PrintStuff {
[CmdletBinding(DefaultParameterSetName='FirstSet')]
Param(
[Parameter(ParameterSetName='FirstSet')]
[String]$FirstParameter=(ReallyExpensiveFunction),
[Parameter(ParameterSetName='SecondSet')]
[String]$SecondParameter
)
if (-not ($FirstParameter -eq $null)) {Write-Host $FirstParameter}
Write-Host "$($PSCmdlet.ParameterSetName)"
}
function ReallyExpensiveFunction {
# Very expensive calculation
"I Am First"
}
However, at the moment running it would still give me the results below:
PS C:\> PrintStuff
# I Am First
# FirstSet
PS C:\> PrintStuff -SecondParameter "don't print this"
# I Am First
# SecondSet
As per above, when SecondSet is used $FirstParameter is still being defined. Is there a way to get only SecondSet printed when the second parameter set is used?
Bear in mind, I am looking to find out if there is a solution which would allow me to keep ReallyExpensiveFunction as the default value for $FirstParameter, and avoid solutions which would involve transferring the logic to the body of the function, such as:
...
Param(
[Parameter(ParameterSetName='FirstSet')]
[String]$FirstParameter,
...
)
if ($PSCmdlet.ParameterSetName -eq 'FirstSet' -and ($FirstParameter -eq '')) {
$FirstParameter = ReallyExpensiveFunction
}
...
Sorry if the pitch is too specific, but I am curious to find out if this is possible.
Unfortunately, the answer is no. ParameterSet allows to present a simpler interface to user for complex argument sets by filtering out the non-relevant ones. However, PowerShell goes through each parameter, whether it is in the selected parameterset or not and assign the default value to the parameter, if you specify one. So, simply put in the context of your question, ParameterSet may be thought as just a filter for presentation.
I have a strange one I have searched the existing Q&A and haven't found a match.
I have written my functions using parameter validation using the basic format
function FunctioName
{
[CmdletBinding()]
Param(
[parameter(Mandatory)]
[String]$VariableName
)
When I set the parameter to Mandatory as above I get a parameter binding exception indicating a null value was passed. Running the script in debug I can see the function parameter being passed is not null and is a valid string.
When I run the script in the exact same way without the mandatory flag the string is passed into the function and it executes correctly.
Has anyone got any ideas, what could be the issue. This problem is affecting a number of functions in my application interestingly it appears that the affected functions all have only a single parameter functions with multiple parameters do not appear to be affected.
Ok thanks guys for your feedback its much appreciated. BTW i am using powershell 5 .
Further to the issue, looking into it further I found that the variable was being passed to the function as an array of strings, however an empty string value was being appended into the array which I believe was the cause for the issue. This is where it starts to get interesting, I will need to give a bit more background.
The script I am running queries active directory for user attributes meeting specific conditions, those that match I create an array of strings with each value a delimited value of the user,hostname and other attribute properties.
To ensure that I am getting the latest values I use the ASDI GetInfo method,which seems to trigger the odd behavior.
At a high level the functions are
Function GetuserAttr
{
$inscopeusers = New-Object System.Collections.ArrayList
$accountlist = (Get-ADUser -Filter { attribute1 -eq "value"} -Properties attribute1).SamAccountName
foreach ($user in $accountlist)
{
$DN = getDN($user) # basically a funtion I wrote to create ASDI object for user account.
$DN.GetInfo() # this method call appears to cause issues
$attr1 = $DN.Get("Attribute1")
$attr2 = $DN.Get("Attribute2")
$hoststring = "$($user)|$($attr1)|$($attr2)"
$inscopeusers.Add($hoststring) > null
}
return $inscopeusers
}
The string array returned in this function is fed into a number of other functions, one of which is the one that was giving the error that I originally brought up.
The thing is when I use the GetInfo method the array returned by this function contains several null values in the array, when I remove the command the array has no null strings.
Even more strange when I am operating on the array in other functions it appears that the array looses some of its properties when the GetInfo method is used. So for instance I am able to use the foreach loop to iterate through array values but I cannot access an array value by index such as $array[1].
By simply commenting out the GetInfo method call in the function the array returned seems to function normally and you can access array values by index.
I have another function that also uses GetInfo and returns a hash table, when I try to operate on the returned hashtable I cannot access values using a key value such as $hashtable['key'], but I can access them using $hashtable.key. I know this is really weird and can't really think what it could be
Has any one else experienced a similar problem.
You're missing an argument.
Function Test
{
[CmdletBinding()]
Param(
[Parameter(Mandatory=$True)]
[String]
$Variable
)
Write "$Variable"
}
I have a script that I am running both remotely and locally in different situations. In the remote situations I would like to return some additional job information. However, the script also needs to complete with a Restart, LogOff, etc.
However, if I Return $Results, no code that follows is executed. Whereas if I Write-Host the return info, I see it as additional data for the job, and the script completion code that follows can still execute.
I can make this work, but it causes two issues.
I have two bits of data I need to return, which I had planned to do in an array. But with Write-Host I need to use the kludge of $value1|$value2 and then split the string on | to get at the two bits of data.
Write-Host somewhat pollutes my console so I need some extra code to manage that.
Is there any way to return intermediate info and continue execution? Or am I going to have no choice but to go the Write-Host route? Or is there perhaps another approach that is better than the Write-Host hack?
You seem to be under the impression that you have to use the return keyword to make a function(?) return something. PowerShell works a little differently, though. PowerShell functions return the entire non-captured output on the success output stream. The return keyword is just for making a function return at that particular point.
There is no difference at all between
function Foo {
return 'something' # return with value
}
and
function Foo {
'something'
return # echo first, then return
}
and
function Foo {
'something' # just echo, return implicitly
}
That way you can have a function generate the output, then do more stuff before actually returning:
function Foo {
'foo'
'bar'
Get-ChildItem . -Recurse | Out-Null # output not returned (suppressed)
'baz' > 'C:\some.txt' # output not returned (redirected to file)
}
The caller will receive the returned values (in this case 'foo', 'bar') only after the function call returns, though. If you need immediate feedback you'll need other means.
Don't use Return. Just output $Results, and let the rest of the script continue on.