If you have IEnumerable<XElement> and want to call various LINQ extension methods such as Enumerable.Where() or ​​Enumerable.ToDictionary() on it using PowerShell then first load the necessary assemblies:

[void][System.Reflection.Assembly]::LoadWithPartialName('System.Core')
[void][System.Reflection.Assembly]::LoadWithPartialName("System.Xml.Linq")

And here’s the code for Where():

$whereCond = [System.Func``2[System.Xml.Linq.XElement, bool]] { $args[0].Attribute("Name").Value.Contains('_') -eq $false };
$elems = [System.Linq.Enumerable]::ToDictionary($elems, $whereCond)

and for ToDctionary():

$keySelector = [System.Func``2[System.Xml.Linq.XElement, string]] { $args[0].Attribute("Name").Value }
$elementSelector = [System.Func``2[System.Xml.Linq.XElement, string]] { $args[0].Attribute("Value").Value }
$dic = [System.Linq.Enumerable]::ToDictionary($elems, $keySelector, $elementSelector)