更改图层符号的问题

Question

我的用符号管理器选择一个符号ISymbol，该怎样更换地图中相应图层的符号为我选择的符号呢？
下面是我的代码，请高手帮忙看看为什么不可以：
                    ILayer editLayer=listLayers[i];
                    IFeatureLayer featureLayer = editLayer as IFeatureLayer;
                    ILegendInfo legendInfo = featureLayer as ILegendInfo;
                    ILegendGroup pLegendGroup = null;
                    ILegendClass pLegendClass = null;
                    for (int j = 0; j <legendInfo.LegendGroupCount;j++)
                    {
                        pLegendGroup = legendInfo.get_LegendGroup(j);
                        for (int ij = 0; ij < pLegendGroup.ClassCount; ij++)
                        {
                            pLegendClass = pLegendGroup.get_Class(ij);
                            pLegendClass.Symbol = frmSymbolManager.YYSymbol;
                        }
                    }
谢谢！