DBNull.Value 与null

来源：http://blog.csdn.net/beautifulsarah/article/details/54691670

DBNull.Value,, 适用于向数据库的表中插入空值。而 null,是指在程序中表示空引用。或者对象为空。就是没有实例化。

row[column]的值为DBNull.Value的话，至少说明它是从数据库中取到值了，对应了数据库中的空值；但如果row[column]的值为null的话，说明没有从数据库中取到值。DBNull.Value是一个有着内存地址的具体类型的值，映射了数据库中的null值；而NULL则是空指向，不存在对应的内存地址。任何对象赋值为null后，内存就释放了。

转换方法：

heper类中：

/// <summary>
/// 将null转换为DBNull.Value
/// </summary>
/// <param name="value"></param>
/// <returns></returns>
public object ToDbValue(object value)
{
    if (value == null)
    {
        return DBNull.Value;
    }
    else
    {
        return value;
    }
}
/// <summary>
/// 将DBNull.Value转换为null
/// </summary>
/// <param name="value"></param>
/// <returns></returns>
public object FromDbValue(object value)
{
    if (value == DBNull.Value)
    {
        return null;
    }
    else
    {
        return value;
    }
}　　

应用：

public List<DateJg> GetList(int ncpid, DateTime start, DateTime end)
{
    var list = new List<DateJg>();
    string sql = @"select *  from v_yjdatealljg where yjxzqid=9 and ncpid =@ncpid and cjrq>=@start and cjrq<=@end  order by cjrq,yjxzqid,ncpid";
    var sqlparms = new SqlParameter[] {
        new SqlParameter("@ncpid",ncpid),
        new SqlParameter("@start",start.Date),
        new SqlParameter("@end",end.Date)
    };
    var table = helper.ExecuteDataTable(sql, sqlparms);
 
    using (System.Data.IDataReader dataReader = table.CreateDataReader())
    {
        while (dataReader.Read())
        {
            list.Add(GetInfo(dataReader));
        }
    }
    return list;
}
private DateJg GetInfo(IDataReader reader)
{
    var info = new DateJg();
    info.lsjg = (decimal?)helper.FromDbValue(reader["lsjg"]);
    info.ncpid = (int)helper.FromDbValue(reader["ncpid"]);
    info.ncpmc = (string)helper.FromDbValue(reader["ncpmc"]);
    info.pfjg = (decimal?)helper.FromDbValue(reader["pfjg"]);
    info.rq = (DateTime)helper.FromDbValue(reader["cjrq"]);
    info.ttjg = (decimal?)helper.FromDbValue(reader["ttjg"]);
    info.yjxzqid = (int)helper.FromDbValue(reader["yjxzqid"]);
    info.yjxzqname = (string)helper.FromDbValue(reader["yjxzqname"]);
    return info;
}