You have to cast it explicitly to a Action
in order for the conversion to System.Delegate
to kick in.
That is:
_map.Dispatcher.BeginInvoke((Action)(() =>
{
_map.Children.Clear();
foreach (var projectedPin in pinsToAdd.Where(pin => PointIsVisibleInMap(pin.ScreenLocation, _map)))
{
_map.Children.Add(projectedPin.GetElement(ClusterTemplate));
}
}));