画点要素 engine

private void axMapControl1_OnMouseDown(object sender, ESRI.ArcGIS.Controls.IMapControlEvents2_OnMouseDownEvent e)
        {

            IPoint point = new PointClass();
            point.X = e.mapX;
            point.Y = e.mapY;
            IActiveView activeView = axMapControl1.ActiveView;
            IScreenDisplay screenDisplay = activeView.ScreenDisplay;
            screenDisplay.StartDrawing(screenDisplay.hDC, (short)esriScreenCache.esriNoScreenCache);
            screenDisplay.SetSymbol(new SimpleMarkerSymbolClass());
            screenDisplay.DrawPoint(point);
            screenDisplay.FinishDrawing();

        }
这样可以画出一个点,但是刷新之后就不见了,而且当我想选择指定的符号点来作图时,应该怎样处理呢?
其中可以设置SimpleMarkerSymbolClass,完整代码,待续...

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