使用IUniqueValueRenderer在同一层中根据字段渲染出不同颜色

public static void UniqueValueRenderer(ILayer layer)
  {
   IFeatureLayer pFLayer = layer  as IFeatureLayer;
   IGeoFeatureLayer geoLayer = layer as IGeoFeatureLayer;
   IFeatureClass fcls = pFLayer.FeatureClass;
   IQueryFilter pQueryFilter = new  QueryFilterClass();
   IFeatureCursor fCursor = fcls.Search(pQueryFilter,false);
   IRandomColorRamp rx = new RandomColorRampClass();
   rx.MinSaturation = 15;
   rx.MaxSaturation = 30;
   rx.MinValue = 85;
   rx.MaxValue = 100;
   rx.StartHue = 0;
   rx.EndHue = 360;
   rx.Size =100;
   bool ok;;
   rx.CreateRamp(out ok);
   IEnumColors RColors = rx.Colors;
   RColors.Reset();
   IUniqueValueRenderer pRender = new UniqueValueRendererClass();
   pRender.FieldCount=1;
   pRender.set_Field(0,"乡名");
   IFeature pFeat = fCursor.NextFeature();
   int index = pFeat.Fields.FindField("乡名");
   while(pFeat!=null)
   {
    ISimpleFillSymbol symd = new SimpleFillSymbolClass();
    symd.Style=esriSimpleFillStyle.esriSFSSolid;
    symd.Outline.Width=1;
    symd.Color = RColors.Next();
    string valuestr = pFeat.get_Value(index).ToString();
    pRender.AddValue(valuestr,valuestr,symd as ISymbol);
    pFeat = fCursor.NextFeature();
   }
   geoLayer.Renderer = pRender as IFeatureRenderer;
  }