https://learn.microsoft.com/ja-jp/dotnet/api/system.xml.linq.extensions.descendantnodesandself?view=netframework-4.8.1&viewFallbackFrom=net-10.0
Extensions.DescendantNodesAndSelf(IEnumerableXElement) Method (System.Xml.Linq) | Microsoft Learn
Returns a collection of nodes that contains every element in the source collection, and the descendant nodes of every element in the source collection.
extensionsmethodsystemxmllinq