What is '@{}' meaning in PowerShell

Question

I have line of scripts for review here, I noticed variable declaration with a value:

function readConfig {
    Param([string]$fileName)
    $config = @{}
    Get-Content $fileName | Where-Object {
        $_ -like '*=*'
    } | ForEach-Object {
        $key, $value = $_ -split '\s*=\s*', 2
        $config[$key] = $value
    }
    return $config
}

I wonder what @{} means in $config = @{}?

Answer 1

@{} in PowerShell defines a hashtable, a data structure for mapping unique keys to values (in other languages this data structure is called "dictionary" or "associative array").

@{} on its own defines an empty hashtable, that can then be filled with values, e.g. like this:

$h = @{}
$h['a'] = 'foo'
$h['b'] = 'bar'

Hashtables can also be defined with their content already present:

$h = @{
    'a' = 'foo'
    'b' = 'bar'
}

Note, however, that when you see similar notation in PowerShell output, e.g. like this:

abc: 23
def: @{"a"="foo";"b"="bar"}

that is usually not a hashtable, but the string representation of a custom object.