Powershell set combobox index from function - winforms

Question

I want to modify the selection of a Windows form combobox from a function.
In the code below, the combobox is created in a function New-ComboBox and returned.
In a different function Set-ComboBoxSelection, I want to access this combobox and modify its selection. The function is able to read the selection from the combobox, but not to modify it.
Minimal example code:
Add-Type -assembly System.Windows.Forms
$main_form = New-Object System.Windows.Forms.Form
$main_form.Text ='Test combobox'
$main_form.Width = 300
$main_form.Height = 50
$main_form.AutoSize = $true
function New-ComboBox($cbstrings, $defaultIndex, $x, $y, $w){
$combobox = New-Object System.Windows.Forms.ComboBox
$combobox.Width = $w
$combobox.Location = New-Object System.Drawing.Point($x, $y)
$combobox.DropDownStyle = "DropDownList"
$main_form.Controls.Add($combobox)
Foreach ($strings in $cbstrings)
{ $combobox.Items.Add($strings); }
$combobox.SelectedIndex = $defaultIndex
return $combobox
}
function Set-ComboBoxSelection(){
Write-Host $combobox.SelectedIndex
Write-Host $combobox.SelectedItem
$combobox.SelectedIndex = 2
$combobox.SelectedItem = $combobox.Items[2]
}
$combobox = New-ComboBox "v1","v2","v3" 1 10 10 200
Set-ComboBoxSelection
$main_form.ShowDialog()
This will return:
The property 'SelectedIndex' cannot be found on this object.
Verify that the property exists and can be set.
The property 'SelectedItem' cannot be found on this object.
Verify that the property exists and can be set.
The values SelectedIndex and SelectedItem will however correctly return 2 and v3 from Set-ComboBoxSelection.
Why do I have "read" access but not "write" access on this combobox?
For a TextBox, created within a function and returned, I'm able to set the Text value from within a different function.

Answer 1

The issue in your code comes from Items.Add which adds a few integer to the output so while you expect having a ComboBox as output, but the output is an array of objects.
To solve the problem you can suppress unwanted outputs:
$combobox.Items.Add($strings) | Out-Null
More information
In PowerShell, in addition to what you return explicitly using return statement, or using Write-Out, if you write a literal, a variable or a function call which returns an output or use write those values also will be added to the function output.
For example, F2 in the following code will have two return values true and 1. To be more precise, the return value will be and array of objects object[], instead of int:
Function F1()
{
return $true
}
Function F2()
{
F1
return 1
}
To prevent the problem, you can:
| Out-Null: Use | out-null after functions to suppress their output, in above example F1 | Out-Null
[void]: Cast the out-put to void, in above example [void](F1)
> $null: Write output to null, in above example F1 > $null
Capture the output in a variable, in above example $null = F1
Enough of the concepts, the When creating PowerShell function, you need to be careful about what is returned from the function. You need to prevent unwanted return values from functions, for example return value of Items.Add is also returned from function and as a result, instead of just returning a single ComboBox, you are returning an array of objects.