取消牵引注记时，引发nullReferenceException

IAnnotationFeature pAnn = pFeature as IAnnotationFeature;

ISymbolCollectionElement pCollecionElement = pAnn

.Annotation as ISymbolCollectionElement;

ITextBackground pGround = pCollecionElement.Background as ITextBackground;



//pGround 不为null,代表这个注记使用了牵引线样式

//我猜想取消牵引注记就是 将 pCollencionElement.Background 赋值为null

//但是一直失败。

if (pGround != null)

{

//方法一:直接赋值为null,在这里会出现nullReferenceException 的错误，而pCollectionElement,和background 并不为null

pCollecionElement.Background = null;

//方法二:直接释放background也不行。



//ILineCallout plc = pGround as ILineCallout;

//Marshal.ReleaseComObject(plc);我尝试释放对象，来取消牵引注记样式，但是不成功

pAnn.Annotation = pCollecionElement as IElement;

}