Snippet_FilterSelect

This works just as Filter Selection but in Reverse. It allows you to first select the Category of the Selection by 
picking up an Object of that Category and follow up with a standard rectangular selection. After finishing the selection, only 
objects from that Category will be selected.

Author: dnenov

1-csharp-code/Snippets_FilterSelect

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+/*
+Make a selection of objects in view filtering by category 
+This works just as Filter Selection but in Reverse. It allows you to first select the Category of the Selection by 
+picking up an Object of that Category and follow up with a standard rectangular selection. After finishing the selection, only 
+objects from that Category will be selected.
+TESTED REVIT API: 2018
+The snippet can be used as is in a Revit Application Macro for test purposes
+Author: Deyan Nenov | github.com/ArchilizerLtd
+This file is shared on www.revitapidocs.com
+For more information visit http://github.com/gtalarico/revitapidocs
+License: http://github.com/gtalarico/revitapidocs/blob/master/LICENSE.md
+*/
+
+public void FilterSelect()
+{
+			UIDocument uidoc = ActiveUIDocument;
+			Document doc = uidoc.Document;
+						
+			Reference refer = uidoc.Selection.PickObject(ObjectType.Element, "Set the filter"); //Pick an object by which Category you will filter
+			
+			IList<Element> elements = uidoc.Selection.PickElementsByRectangle();
+						
+			uidoc.Selection.SetElementIds(elements
+				.Where(x => x.Category.Id.IntegerValue.Equals(doc.GetElement(refer).Category.Id.IntegerValue))
+				.Select(x => x.Id)
+				.ToList());
+}