C# + MapX 创建一个 feature 为什么不成功?

代码:
        
                            MapXLib.Point Pnt = new MapXLib.PointClass();
MapXLib.FeatureFactory FeaFac;
MapXLib.Layer Lyr;
MapXLib.Feature Ftr;

Lyr = this.axMap1.Layers._Item(1);
FeaFac = this.axMap1.FeatureFactory;

Lyr.Editable = true;
Pnt.Set(this.axMap1.CenterX,this.axMap1.CenterY); Ftr = FeaFac.CreateSymbol(Pnt,this.axMap1.DefaultStyle);
Lyr.AddFeature(Ftr);//此句报错..............(1)
Lyr.Refresh();把(1)换成 Lyr.AddFeature(Ftr,null) 还是报错
为什么?