New Post: A sample code for add symbology to layer

    IMapFeatureLayer lay = (IMapFeatureLayer)map1.Layers[0];
            lay.Symbology.ClearCategories();
            PolygonSymbolizer lightblue = new PolygonSymbolizer(Color.LightBlue);
            lightblue.OutlineSymbolizer = new LineSymbolizer(Color.Blue, 1);
            lay.Symbolizer = lightblue;