list.add似乎要添加对原始对象的引用? [英] list.add seems to be adding a reference to the original object?

问题描述

我创建了两个自定义类(NTDropDown和NTBaseFreight)，这些类用于存储从数据库检索到的数据.我初始化NTBaseFreight的列表和NTDropDown的2个列表.

I have created a couple custom classes (NTDropDown and NTBaseFreight) which I use to store data that I retrieve from a DB. I initialize a List of NTBaseFreight and 2 lists for NTDropDown.

我可以成功地使用List.Add将货运添加到货运清单中，但是在调试代码时，我的2个下拉列表仅包含1个NTDropDown，其值始终与NTDropDown相同(我是假设这是一个引用问题，但是我在做什么错呢?

I can successfully use List.Add to add freights to the freights list, but as I debug the code, my 2 dropdown lists contain only 1 NTDropDown, which always has the same values as NTDropDown (I'm assuming this is a referencing problem, but what am I doing wrong)?

举个例子，在第二行，如果载波和carrier_label是"001", "MyTruckingCompany"，并且我在frt_carriers的if语句上放了一个空格，则frt_carriers和frt_modes在它们的列表中将只包含1个项目. ，值为"001", "MyTruckingCompany" ... NTDropDown中的相同值.

To give an example, on the second row, if the carrier and carrier_label were "001", "MyTruckingCompany" and I put a break on the if statement for frt_carriers, both frt_carriers and frt_modes would contain only 1 item in their list, with the values "001", "MyTruckingCompany"...the same values in NTDropDown.

代码:

List<NTDropDown> frt_carriers = new List<NTDropDown>();
List<NTDropDown> frt_modes = new List<NTDropDown>();
List<NTBaseFreight> freights = new List<NTBaseFreight>();
NTDropDown tempDropDown = new NTDropDown();
NTBaseFreight tempFreight = new NTBaseFreight();

//....Code to grab data from the DB...removed

while (myReader.Read())
{
    tempFreight = readBaseFreight((IDataRecord)myReader);

    //check if the carrier and mode are in the dropdown list (add them if not)
    tempDropDown.value = tempFreight.carrier;
    tempDropDown.label = tempFreight.carrier_label;
    if (!frt_carriers.Contains(tempDropDown)) frt_carriers.Add(tempDropDown);

    tempDropDown.value = tempFreight.mode;
    tempDropDown.label = tempFreight.mode_label;
    if (!frt_modes.Contains(tempDropDown)) frt_modes.Add(tempDropDown);

    //Add the freight to the list
    freights.Add(tempFreight);
}

推荐答案

是的，引用类型列表实际上只是引用列表.

Yes, a list of reference types is actually just a list of references.

您必须为要存储在列表中的每个对象创建一个新实例.

You have to create a new instance for each object that you want to store in the list.

此外，Contains方法比较引用，因此包含相同数据的两个对象不被视为相等.在列表中对象的属性中寻找一个值.

Also, the Contains method compares references, so two objects containing the same data are not considered to be equal. Look for a value in the properties of the objects in the list.

if (!frt_carriers.Any(c => c.label == tempFreight.carrier_label)) {
  NTDropDown tempDropDown = new NTDropDown {
    value = tempFreight.carrier,
    label = tempFreight.carrier_label
  };
  frt_carriers.Add(tempDropDown);
}

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