How to add item to the beginning of List?

Question

I want to add a "Select One" option to a drop down list bound to a 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:

// 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;

score 854 · Accepted Answer · answered Dec 24 '08 at 00:37

854

Use the Insert method:

ti.Insert(0, initialItem);

answered Dec 24 '08 at 00:37

Matt Hamilton

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@BrianF, Yes you are right. Doc: [`This method is an O(n) operation, where n is Count.`](https://msdn.microsoft.com/ru-ru/library/sey5k5z4%28v=vs.110%29.aspx?f=255&MSPPError=-2147217396) – 23W Feb 15 '17 at 12:51
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@23W You should probably link to the English page if you're going to link to MSDN. – mbomb007 May 09 '17 at 16:00
Would it be possible to insert at the end of the list? – Gary Henshall Dec 11 '17 at 14:36
3

@GaryHenshall yes, use the `Add` method, it inserts at the end. – Martin Asenov Jan 02 '18 at 08:19
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Since .NET 4.7.1, you can use `Append()` and `Prepend()`. [Check this answer](https://stackoverflow.com/a/59643605/1248177) – aloisdg Jan 08 '20 at 10:13
Doesn't work for me -- cannot implicitly convert type 'void' to 'System.Collections.Generic.List – MC9000 Mar 07 '21 at 00:53
1

Wouldnt this replace an existing value that is already in the list? – Zapnologica Jul 30 '21 at 07:00
No this does not replace the first item – Pieter Jan 12 '22 at 15:39

aloisdg · Answer 2 · 2020-03-11T13:59:45.873

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Since .NET 4.7.1, you can use the side-effect free Prepend() and Append(). The output is going to be an 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 ));

edited Mar 11 '20 at 13:59

answered Jan 08 '20 at 10:11

aloisdg

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This solution is best when working with enumerable – Tấn Nguyên Apr 18 '21 at 16:12
All List implements IEnumerable, but yes both prepend and append will return an IEnumerable. This may or may be not what you are looking for. – aloisdg Apr 19 '21 at 12:18

x0n · Answer 3 · 2008-12-24T00:42:50.280

25

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.

<asp:DropDownList ID="ddl" runat="server" AppendDataBoundItems="true">
    <asp:ListItem Value="0" Text="Please choose..."></asp:ListItem>
</asp:DropDownList>

http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.listcontrol.appenddatabounditems.aspx

-Oisin

edited Dec 24 '08 at 00:42

answered Dec 24 '08 at 00:37

x0n

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That's pretty cool. The OP didn't specify ASP.NET but that's a nice trick to know. – Matt Hamilton Dec 24 '08 at 00:54

Sina Lotfi · Answer 4 · 2018-06-18T13:46:45.373

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Use Insert method of List<T>:

List.Insert Method (Int32, T): Inserts an element into the List at the specified index.

var names = new List<string> { "John", "Anna", "Monica" };
names.Insert(0, "Micheal"); // Insert to the first element

edited Jun 18 '18 at 13:46

answered Feb 03 '18 at 06:25

Sina Lotfi

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score 4 · Answer 5 · answered Nov 06 '18 at 21:11

Use 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.

How to add item to the beginning of List?

5 Answers5

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