Controller:

        [MapActionFilter]
        public void GetCurrentExtentFeatures(Map map, GeoCollection<object> args)
        {
            RectangleShape currentExtent = new RectangleShape(double.Parse(args[0].ToString()), double.Parse(args[1].ToString()), double.Parse(args[2].ToString()), double.Parse(args[3].ToString()));
            
            LayerOverlay overlay = map.CustomOverlays["MsSql2008"] as LayerOverlay;
            MsSql2008FeatureLayer sql2008Layer = overlay.Layers[0] as MsSql2008FeatureLayer;
            if (sql2008Layer != null)
            {
                Collection<Feature> allfeaturesInBoundingBox = sql2008Layer.QueryTools.GetFeaturesInsideBoundingBox(currentExtent, ReturningColumnsType.NoColumns);
                //To Do like add those features into MarkerOverlay.
            }
        }

View:

    function GetCurrentExtentFeatures() {
        var currentBounds = Map1.getExtent();
        var currentExtent = []; 
        currentExtent.push(currentBounds.left);
        currentExtent.push(currentBounds.top);
        currentExtent.push(currentBounds.right);
        currentExtent.push(currentBounds.bottom);

        Map1.ajaxCallAction('@ViewContext.RouteData.Values["Controller"].ToString()', 'GetCurrentExtentFeatures', currentExtent, function (result) {
            //ToDo like refresh layer Map1.redrawLayer("MarkerOverlay");
        })
    }