C# 将对象添加到列表时出现空引用异常
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原文地址: http://stackoverflow.com/questions/17096803/
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Null Reference Exception When adding object to list
提问by zerodoc
I keep getting an NullReferenceException when I try to add an object to a list inside an object, even when all properties of the object contains data. Classes--
当我尝试将一个对象添加到一个对象内的列表时，我不断收到 NullReferenceException，即使该对象的所有属性都包含数据。班级——
public class OrderInfo
{
public virtual string OrderNum { get; set; }
public virtual string TrackingNum { get; set; }
public virtual DateTime Shipdate { get; set; }
public virtual string Cost { get; set; }
public virtual string ShipMethod { get; set; }
public virtual string ShipService { get; set; }
public virtual string Country { get; set; }
public virtual decimal Weight { get; set; }
public virtual List<OrderItemInfo> OrderiTems { get; set; }
public void AddShipmentItem(OrderItemInfo oi)
{
this.OrderiTems.Add(oi); // NULL Reference HERE
}
}
public class OrderItemInfo
{
public virtual string OrderItemCode { get; set; }
public virtual decimal? Quantity { get; set; }
public virtual decimal? Cost { get; set; }
public virtual decimal? Weight { get; set; }
public virtual string Store { get; set; }
}
Then I have code that catches if any nullable data is there.
然后我有代码来捕捉是否有任何可为空的数据。
private static OrderInfo GetOrderInfo(DataRow dr)
{
SqlConnection ShipworksConnectionString =
SqlCommand ShipworksCmd = new SqlCommand("SELECT OrderItem.Code,
InternationalShipmentCostAnalysisApp.OrderInfo ip = new InternationalShipmentCostAnalysisApp.OrderInfo
{
OrderNum = (dr[0] is DBNull) ? String.Empty : dr[0].ToString(),
TrackingNum = (dr[1] is DBNull) ? String.Empty : dr[1].ToString(),
Shipdate = (dr[2] is DBNull) ? DateTime.MinValue : Convert.ToDateTime(dr[2]),
Cost = (dr[3] is DBNull) ? String.Empty : dr[3].ToString(),
ShipMethod = (dr[4] is DBNull) ? String.Empty : dr[4].ToString(),
ShipService = (dr[5] is DBNull) ? String.Empty : dr[5].ToString(),
Country = (dr[6] is DBNull) ? String.Empty : dr[6].ToString(),
Weight = (dr[7] is DBNull) ? 0 : Convert.ToDecimal(dr[7])
};
ShipworksConnectionString.Open();
SqlDataReader rdr = ShipworksCmd.ExecuteReader();
DataTable dt = new DataTable();
dt.Load(rdr);
rdr.Close();
ShipworksConnectionString.Close();
foreach (DataRow item in dt.Rows)
{
if (item != null)
{
InternationalShipmentCostAnalysisApp.OrderItemInfo i = new InternationalShipmentCostAnalysisApp.OrderItemInfo
{
OrderItemCode = (item[0] is DBNull) ? String.Empty : item[0].ToString(),
Quantity = (item[1] is DBNull) ? 0 : Convert.ToDecimal(item[1]),
Cost = (item[2] is DBNull) ? 0 : Convert.ToDecimal(item[2]),
Weight = (item[3] is DBNull) ? 0 : Convert.ToDecimal(item[3]),
Store = (item[4] is DBNull) ? String.Empty : item[4].ToString()
};
ip.AddShipmentItem(i);
}
}
return ip;
}
采纳答案by Haney
It seems you never SET your property... You have it defined as a List<OrderItemInfo>typebut you never initialize it to an instanceof that type. Try initializing it in a constructor:
似乎您从未设置过您的属性...您已将其定义为一种List<OrderItemInfo>类型，但从未将其初始化为该类型的实例。尝试在构造函数中初始化它：
public class OrderInfo
{
public OrderInfo
{
OrderiTems = new List<OrderItemInfo>();
}
}
回答by Fabian Bigler
Is DbNull is not the same as Is NULL!
Is DbNull 与 Is NULL 不同！
You need to check for 'IS NULL', instead. That is why you are getting Null Reference Exceptions.
相反，您需要检查“IS NULL”。这就是您收到空引用异常的原因。
EDIT:as ebyrob got it right, the following line throws the exception:
编辑：由于 ebyrob 做对了，以下行引发异常：
this.OrderiTems.Add(oi); // NULL Reference HERE
回答by Fabian Bigler
OrderiTemsis null.
OrderiTems是null。
public virtual List<OrderItemInfo> OrderiTems { get; set; }
Try implementing set/get or storing an empty list into OrderiTemsin the constructor.
尝试OrderiTems在构造函数中实现 set/get 或存储一个空列表。
回答by dna
You need to instanciate the list before starting using it. A good place to do that is in the class constructor. Assuming you use the default constructor :
在开始使用它之前，您需要实例化该列表。这样做的一个好地方是在类构造函数中。假设您使用默认构造函数：
public OrderInfo()
{
this.OrderiTems = new List<OrderItemInfo>();
}
