You are trying to mix Auto implemented property with normal property. Moreover you need a backing field, otherwise you will run in to Stackoverflow exception. Your property declaration should be like:
private List<ObjectA> _ObjectAList; //private backing field
public List<ObjectA> ObjectAList
{
get
{
return _ObjectAList ?? new List<ObjectA>();
}
set
{
_ObjectAList = value;
}
}