C# 根据索引从列表中返回一个对象
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C# Return an object from a list based on index
提问by Gvs
I need to make some changes so that I only retrieve one object at a time from my list instead of the entire list as I do now.
我需要进行一些更改，以便我一次只从列表中检索一个对象，而不是像现在一样检索整个列表。
Right now ive got a private list in class B with a property returning the entire list basically making it public anyway and I want to change this.
现在我在 B 类中得到了一个私有列表，其中一个属性返回整个列表，基本上使它成为公开的，我想改变它。
The way class a looks(the ui class operating with the list) is, I enter some data validate it and send it to class B which in turn packages it into a list of objects based on the input.
类 a 的外观（使用列表操作的 ui 类）的方式是，我输入一些数据对其进行验证并将其发送到类 B，后者又根据输入将其打包成一个对象列表。
Then class A needs to loop this list and add it to a listview for displaying it which looks like this at the moment:
然后类 A 需要循环这个列表并将其添加到一个列表视图中以显示它，目前看起来像这样：
ListViewItem lvi = new ListViewItem();
foreach ([Object] o in CLassB.getList())
{
lvi = new ListViewItem(o.property0);
lvi.SubItems.Add(o.property1);
lvi.SubItems.Add(o.property2);
lvi.SubItems.Add(o.property3);
}
}
Object is my abstract class which controls how the different types of items are added and getList() is my method in class B returning the entire list.
Object 是我的抽象类，它控制如何添加不同类型的项目，getList() 是我在 B 类中返回整个列表的方法。
The thing is these propertys are the common ones all classes share the there are some that arent, like a textbox that you enter specific text about the object and so on which is displayed like this:
问题是这些属性是所有类共享的公共属性，有一些属性没有，例如您输入有关对象的特定文本的文本框等，其显示如下：
private void lvRegistered_ItemSelectionChanged(object sender, ListViewItemSelectionChangedEventArgs e)
{
if (Listview.SelectedItems.Count > 0)
{
foreach ([Object] ob in ClassB.getList())
{
if (Listview.SelectedItems[0].SubItems[0].Text == ob.id.ToString())
{
TextBox.Clear();
TextBox.Text = ob.property4;
}
}
}
}
Now this all works great at the moment but now I have a returned list to operate on but I dont want to return the list making it public I want to return one object of the list based on an index number (yes the functionality will be exactly the same, I made a method returning the count of the private list so I can loop over it and return all). This is for practicing OOP for when I dont want to return everything.
现在这一切都很好，但现在我有一个返回的列表进行操作，但我不想返回列表使其公开我想根据索引号返回列表的一个对象（是的，功能将完全相同同样，我创建了一个返回私有列表计数的方法，这样我就可以遍历它并返回全部）。这是为了在我不想返回所有内容时练习 OOP。
How would this be done? All I can think of is making a new list, take an int as input and search my private list and find the index then add that to the other list and return that list, but I dont know if this is good practice or the best way to do it? Well I havent looked into how to "copy" one element over to next list either but might aswell check and see if theres a better way to do things?
这将如何完成？我能想到的就是创建一个新列表，将 int 作为输入并搜索我的私人列表并找到索引，然后将其添加到另一个列表并返回该列表，但我不知道这是好的做法还是最佳方法去做吧？好吧，我还没有研究如何将一个元素“复制”到下一个列表，但不妨检查一下是否有更好的做事方法？
Just get the feeling im "taking the long way around" this way
只是感觉我“走很长的路”这样
采纳答案by ken2k
Not sure to understand, but how about a simple GetByIdmethod?
不确定是否理解，但是一个简单的GetById方法怎么样？
public class Foo
{
public int Id { get; set; }
public string Name { get; set; }
}
public class Test
{
private List<Foo> list = new List<Foo>();
public void Add(Foo foo)
{
this.list.Add(foo);
}
public Foo GetById(int id)
{
return this.list.FirstOrDefault(z => z.Id == id);
}
}
....
Test test = new Test();
test.Add(new Foo { Id = 1, Name = "1" });
test.Add(new Foo { Id = 2, Name = "2" });
test.Add(new Foo { Id = 3, Name = "3" });
Foo foo2 = test.GetById(2);
回答by Ray
I'm not sure I understand completely, but it sounds like you could resolve this be creating an indexer on class B to return the item you want:
我不确定我是否完全理解，但听起来您可以通过在 B 类上创建索引器来返回您想要的项目来解决这个问题：
public object this[int index] {
get {
return list[index];
}
}
change 'object' to whatever your class type actually is.
将“对象”更改为您的类实际类型。
You can then access the items as if class B was an array:
然后，您可以像 B 类是数组一样访问这些项目：
object item = classB[5];
回答by Makah
1) You can use List.AsReadOnly() Method.
1) 您可以使用 List.AsReadOnly() 方法。
public ReadOnlyCollection<Double> MyList {
get {
return myList.AsReadOnly();
}
}
private List<Double> myList;
2) Use index method in the class.
2）在类中使用索引方法。
public Double this[int index] {
get {
return myList[index];
}
set {
myList[index] = value;
}
}
private List<Double> myList;
回答by Gvs
Did about the same as suggested by Ken2k but instead I went with this:
与 Ken2k 的建议大致相同，但我选择了这个：
public [Class] getListItem(int index)
{
return myList.ElementAt(id);
}
which in turn is used in my other class that displays the info like:
反过来，它在我的另一个类中使用，显示如下信息：
for (int i = 0; i < am.getCount(); i++)
{
ListViewItem = new ListViewItem([class reference].getList(i).[property].ToString());
ListViewItems.SubItems.Add([class reference].getList(i).[property]);
}
Thanks everyone for all your help on this.
感谢大家在这方面的所有帮助。
