C# 如何将项目添加到 List<T> 的开头？
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How to add item to the beginning of List<T>?
提问by Ash Machine
I want to add a "Select One" option to a drop down list bound to a List<T>.
我想将“选择一个”选项添加到绑定到List<T>.
Once I query for the List<T>, how do I add my initial Item, not part of the data source, as the FIRST element in that List<T>? I have:
一旦我查询了List<T>，我该如何添加我的初始Item元素，而不是数据源的一部分，作为其中的第一个元素List<T>？我有：
// populate ti from data
List<MyTypeItem> ti = MyTypeItem.GetTypeItems();
//create initial entry
MyTypeItem initialItem = new MyTypeItem();
initialItem.TypeItem = "Select One";
initialItem.TypeItemID = 0;
ti.Add(initialItem) <!-- want this at the TOP!
// then
DropDownList1.DataSource = ti;
回答by x0n
Update: a better idea, set the "AppendDataBoundItems" property to true, then declare the "Choose item" declaratively. The databinding operation will add to the statically declared item.
更新：一个更好的主意，将“AppendDataBoundItems”属性设置为true，然后以声明方式声明“Choose item”。数据绑定操作将添加到静态声明的项目。
<asp:DropDownList ID="ddl" runat="server" AppendDataBoundItems="true">
<asp:ListItem Value="0" Text="Please choose..."></asp:ListItem>
</asp:DropDownList>
-Oisin
-Oisin
回答by Sina Lotfi
Use Insertmethod of List<T>:
使用插入方法List<T>：
List.Insert Method (Int32,?T):
Insertsan element into the List at thespecified index.
List.Insert Method(Int32,?T)：将
Inserts一个元素插入到List处specified index。
var names = new List<string> { "John", "Anna", "Monica" };
names.Insert(0, "Micheal"); // Insert to the first element
回答by sonny
Use List<T>.Insert
用 List<T>.Insert
While not relevant to your specific example, if performance is important also consider using LinkedList<T>because inserting an item to the start of a List<T>requires all items to be moved over. See When should I use a List vs a LinkedList.
虽然与您的具体示例无关，但如果性能很重要，也可以考虑使用，LinkedList<T>因为将项目插入到 a 的开头List<T>需要将所有项目移过。请参阅何时应该使用 List 与 LinkedList。
回答by aloisdg moving to codidact.com
Since .NET 4.7.1, you can use the side-effect free Prepend()and Append(). The output is going to be an IEnumerable.
从 .NET 4.7.1 开始，您可以免费使用副作用Prepend()和Append(). 输出将是一个 IEnumerable。
// Creating an array of numbers
var ti = new List<int> { 1, 2, 3 };
// Prepend and Append any value of the same type
var results = ti.Prepend(0).Append(4);
// output is 0, 1, 2, 3, 4
Console.WriteLine(string.Join(", ", results ));
