PowerShell append 对象到变量列表

Question

Fairly new to PowerShell and wanting to learn how to append objects to a variable list. Below is the error message:

Method invocation failed because [System.IO.FileInfo] does not contain a method named 'op_Addition'.
At C:\Users\Username\Desktop\Sandbox\New folder\BoxProjectFiles.ps1:117 char:4
+             $MechDWGFile += $file
+             ~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (op_Addition:String) [], RuntimeException
    + FullyQualifiedErrorId : MethodNotFound

and code:

LogWrite "`n-------------Mechanical Drawing(s)------------"
foreach ($file in $MechDWGList)
{
    # Where the file name contains one of these filters
    foreach($filter in $MechDWGFilterList)
    {
        if($file.Name -like $filter)
        {
            $MechDWGFile += $file # this is where the error is happening, so I know it is probably something simple
            LogWrite $file.FullName
        }
    }
}

PowerShell 5.1 and Windows 10 OS is being used.

Could someone help me understand what's wrong with my syntax?

Answer 1

Based on the error message, $MechDWGFile already contains a single [FileInfo] object - the type of object returned by Get-Item or Get-ChildItem .

The += operator is overloaded in PowerShell, meaning its behavior depends on the type of object you have on the left-hand side - in this case $MechDWGFile which contains a [FileInfo] object.

$file also contains such an object, but [FileInfo] + [FileInfo] doesn't make any sense, which is why you see the error.

To make the += operator work, you need to create an array with the @() array subexpression operator:

$MechDWGFile = @()

# ...

# This now works
$MechDWGFile += $file

If you've initialized $MechDWGFile with output from Get-Item or Get-ChildItem , simply nest the existing pipeline in @() :

# `Get-ChildItem |Select -First 1` will only output 1 object, 
# but the @(...) makes PowerShell treat it as a 1-item array
# which in turn allows you to use `+=` later
$MechDWGFile = @(Get-ChildItem -Recurse -File |Select -First 1)

Answer 2

One big caveat about arrays in PowerShell—they are immutable .

Although you can "add" elements to an array in PowerShell using the overloaded += operator, what it essentially does is creates a new array with the elements of the first operand and the second operand combined.

This may be more of a personal preference, but when I plan on looping over a bunch of items to populate an array, I use an [ArrayList] instead. ArrayLists are mutable by design. Why this isn't the default in PowerShell, I have no idea.

Adding a new element to an existing array:

$Array1 = Get-ChildItem -Path 'C:\'
$Array2 = @()

# Add only the directories to our second array
foreach ($Item in $Array1) {
    if ($Item.PSIsContainer) {
        $Array2 += $Item    # This creates a new array every time
    }
}

$Array2

Adding a new element to an existing [ArrayList] :

$Array1 = Get-ChildItem -Path 'C:\'
$Array2 = [System.Collections.ArrayList]@()

# Add only the directories to our ArrayList
foreach ($Item in $Array1) {
    if ($Item.PSIsContainer) {
        $null = $Array2.Add($Item)    # This does not create a new object each time.

        # The $null= prepend is because [ArrayList].Add() will output the ID of the
        #   last element added, so we assign that to null, keeping our console clean.
    }
}

$Array2

Side note: Like Mathias has stated, your variable $MechDWGFile may not have been initialized as an array, which is why the += overload operator didn't work and instead threw an error.