Hi OguzSariyildiz,
If you surround your code snippet with a try {} catch {} you should see an error message because of your where clause is not a valid where clause.
The where clause string for a QueryFilter has to be specified in sql where clause format:
{
string selectedCounty = "San Bernardino";
QueuedTask.Run(() =>
{
var featLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().Where(fl => fl.Name.Contains("Counties")).FirstOrDefault();
QueryFilter qf = new QueryFilter {
WhereClause = $@"name = '{selectedCounty}'"
};
featLayer.Select(qf);
MapView.Active.ZoomToSelected();
});
}