1 if (ds.Tables[0].Rows[0]["NullField"] == DBNull.Value)
2 if (ds.Tables[0].Rows[i]["NullField"] is DBNull)
3.isdbnull(ds.tb.row(xx).item(xx)) return Bool
4.Convert.IsDBNull()
三种常用的字符串判空串方法：
2 if (ds.Tables[0].Rows[i]["NullField"] is DBNull)
3.isdbnull(ds.tb.row(xx).item(xx)) return Bool
4.Convert.IsDBNull()
三种常用的字符串判空串方法：
- Length法：bool isEmpty = (str.Length == 0);
- Empty法：bool isEmpty = (str == String.Empty);
- General法：bool isEmpty = (str == "");
