es=pBlockTable->getAt()为什么返回值是eLockViolation

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代码如下： AcGePoint3d startPoint(165.0,137.0,0.0);
AcGePoint3d endPoint(224.0,216.0,0.0);
//initiate the instance of the line object
AcDbLine *pLine=new AcDbLine(startPoint,endPoint);
if(pLine==NULL)
{
return 0;
}
//open the blocktable
AcDbBlockTable *pBlockTable;
Acad::ErrorStatus
es=acdbHostApplicationServices()->workingDatabase()
->getSymbolTable(pBlockTable,AcDb::kForRead);
if(es!=Acad::eOk)
{
delete pLine;
return 0;
}
//opent the blocktable segment of the model space
//get the current blocktable record
//close the blocktable
AcDbBlockTableRecord *pBlockTableRecord;
es=pBlockTable->getAt(ACDB_MODEL_SPACE,
pBlockTableRecord,AcDb::kForWrite); 得到的返回值 es为 eLockViolation，该怎么解决啊？ 急啊，大侠救我，有些情况下就能得到eOK 的返回值。我是菜鸟！·！·！