var target = pattern.Target;

var name = pattern.Name;

var fpElem = FillPatternElement.GetFillPatternElementByName(doc, target, name);

using (var tran = new Transaction(doc, "FillPattern"))

{
       tran.Start();

       var selectionIds = app.ActiveUIDocument.Selection.GetElementIds();

       if (selectionIds != null && selectionIds.Any())

       {

            foreach (var id in selectionIds)

            {

                    FilledRegion elem = doc.GetElement(id) as FilledRegion;

                    if (elem != null && !elem.IsMasking)

                    {

                          OverrideGraphicSettings overrideGraphicSettings = new OverrideGraphicSettings();

                          overrideGraphicSettings = doc.ActiveView.GetElementOverrides(elem.Id);

                          overrideGraphicSettings.SetProjectionFillPatternId(fpElem.Id);

                          doc.ActiveView.SetElementOverrides(elem.Id, overrideGraphicSettings);

                    }

             }

        }

        tran.Commit();
}