用动软.net代码生成器生成的 model 一些不解。private DateTime? _resultdate;

用动软.net代码生成器生成的 model 层

private int _id;
private string _goodsname;
private DateTime? _dealdate;
private string _whoadd;
private DateTime? _adddate;
private int? _statenow;

public int ID
{
set{ _id=value;}
get{return _id;}
}
public string goodsname
{
set{ _goodsname=value;}
get{return _goodsname;}
}
public DateTime? finddate
{
set{ _finddate=value;}
get{return _finddate;}
}
public int? stateNow
{
set{ _statenow=value;}
get{return _statenow;}
}


private DateTime? _adddate;


是什么意思？为什么加个问号 ? 啊