ClusterRenderer.cs

    public class ClusterRenderer : DefaultClusterRenderer
    {
        public Dictionary<string, ClusterItem> m_dicAllMarkerOnMap;
        
        public ClusterRenderer(Activity context, GoogleMap map, ClusterManager clusterManager, Dictionary<string, ClusterItem> dicAllMarkerOnMap) 
            : base(context, map, clusterManager)
        {
            m_dicAllMarkerOnMap = dicAllMarkerOnMap;
            //...
        }

        //...
                
        //After a cluster item have been rendered to a marker
        protected override void OnClusterItemRendered(Object item, Marker marker)
        {
            base.OnClusterItemRendered(item, marker);

            ClusterItem clusterItem = (ClusterItem)item;

            if (!m_dicAllMarkerOnMap.ContainsKey(marker.Id))
                m_dicAllMarkerOnMap.Add(marker.Id, clusterItem);
        }
    }