List<T>.Add(T) Method

public:
 virtual void Add(T item);

public void Add(T item);

abstract member Add : 'T -> unit
override this.Add : 'T -> unit

Public Sub Add (item As T)

Parameters

Implements

Examples

The following example demonstrates how to add, remove, and insert a simple business object in a List<T>.

using System;
using System.Collections.Generic;
// Simple business object. A PartId is used to identify the type of part
// but the part name can change.
public class Part : IEquatable<Part>
 {
 public string PartName { get; set; }

 public int PartId { get; set; }

 public override string ToString()
 {
 return "ID: " + PartId + " Name: " + PartName;
 }
 public override bool Equals(object obj)
 {
 if (obj == null) return false;
 Part objAsPart = obj as Part;
 if (objAsPart == null) return false;
 else return Equals(objAsPart);
 }
 public override int GetHashCode()
 {
 return PartId;
 }
 public bool Equals(Part other)
 {
 if (other == null) return false;
 return (this.PartId.Equals(other.PartId));
 }
 // Should also override == and != operators.
 }
public class Example
{
 public static void Main()
 {
 // Create a list of parts.
 List<Part> parts = new List<Part>();

 // Add parts to the list.
 parts.Add(new Part() { PartName = "crank arm", PartId = 1234 });
 parts.Add(new Part() { PartName = "chain ring", PartId = 1334 });
 parts.Add(new Part() { PartName = "regular seat", PartId = 1434 });
 parts.Add(new Part() { PartName = "banana seat", PartId = 1444 });
 parts.Add(new Part() { PartName = "cassette", PartId = 1534 });
 parts.Add(new Part() { PartName = "shift lever", PartId = 1634 });

 // Write out the parts in the list. This will call the overridden ToString method
 // in the Part class.
 Console.WriteLine();
 foreach (Part aPart in parts)
 {
 Console.WriteLine(aPart);
 }

 // Check the list for part #1734. This calls the IEquatable.Equals method
 // of the Part class, which checks the PartId for equality.
 Console.WriteLine("\nContains(\"1734\"): {0}",
 parts.Contains(new Part { PartId = 1734, PartName = "" }));

 // Insert a new item at position 2.
 Console.WriteLine("\nInsert(2, \"1834\")");
 parts.Insert(2, new Part() { PartName = "brake lever", PartId = 1834 });

 //Console.WriteLine();
 foreach (Part aPart in parts)
 {
 Console.WriteLine(aPart);
 }

 Console.WriteLine("\nParts[3]: {0}", parts[3]);

 Console.WriteLine("\nRemove(\"1534\")");

 // This will remove part 1534 even though the PartName is different,
 // because the Equals method only checks PartId for equality.
 parts.Remove(new Part() { PartId = 1534, PartName = "cogs" });

 Console.WriteLine();
 foreach (Part aPart in parts)
 {
 Console.WriteLine(aPart);
 }
 Console.WriteLine("\nRemoveAt(3)");
 // This will remove the part at index 3.
 parts.RemoveAt(3);

 Console.WriteLine();
 foreach (Part aPart in parts)
 {
 Console.WriteLine(aPart);
 }

 /*

 ID: 1234 Name: crank arm
 ID: 1334 Name: chain ring
 ID: 1434 Name: regular seat
 ID: 1444 Name: banana seat
 ID: 1534 Name: cassette
 ID: 1634 Name: shift lever

 Contains("1734"): False

 Insert(2, "1834")
 ID: 1234 Name: crank arm
 ID: 1334 Name: chain ring
 ID: 1834 Name: brake lever
 ID: 1434 Name: regular seat
 ID: 1444 Name: banana seat
 ID: 1534 Name: cassette
 ID: 1634 Name: shift lever

 Parts[3]: ID: 1434 Name: regular seat

 Remove("1534")

 ID: 1234 Name: crank arm
 ID: 1334 Name: chain ring
 ID: 1834 Name: brake lever
 ID: 1434 Name: regular seat
 ID: 1444 Name: banana seat
 ID: 1634 Name: shift lever

 RemoveAt(3)

 ID: 1234 Name: crank arm
 ID: 1334 Name: chain ring
 ID: 1834 Name: brake lever
 ID: 1444 Name: banana seat
 ID: 1634 Name: shift lever


 */
 }
}

Imports System.Collections.Generic

' Simple business object. A PartId is used to identify the type of part 
' but the part name can change. 
Public Class Part
 Implements IEquatable(Of Part)
 Public Property PartName() As String
 Get
 Return m_PartName
 End Get
 Set(value As String)
 m_PartName = Value
 End Set
 End Property
 Private m_PartName As String

 Public Property PartId() As Integer
 Get
 Return m_PartId
 End Get
 Set(value As Integer)
 m_PartId = Value
 End Set
 End Property
 Private m_PartId As Integer

 Public Overrides Function ToString() As String
 Return "ID: " & PartId & " Name: " & PartName
 End Function
 Public Overrides Function Equals(obj As Object) As Boolean
 If obj Is Nothing Then
 Return False
 End If
 Dim objAsPart As Part = TryCast(obj, Part)
 If objAsPart Is Nothing Then
 Return False
 Else
 Return Equals(objAsPart)
 End If
 End Function
 Public Overrides Function GetHashCode() As Integer
 Return PartId
 End Function
 Public Overloads Function Equals(other As Part) As Boolean _
 Implements IEquatable(Of Part).Equals
 If other Is Nothing Then
 Return False
 End If
 Return (Me.PartId.Equals(other.PartId))
 End Function
 ' Should also override == and != operators.

End Class
Public Class Example
 Public Shared Sub Main()
 ' Create a list of parts.
 Dim parts As New List(Of Part)()

 ' Add parts to the list.
 parts.Add(New Part() With { _
 .PartName = "crank arm", _
 .PartId = 1234 _
 })
 parts.Add(New Part() With { _
 .PartName = "chain ring", _
 .PartId = 1334 _
 })
 parts.Add(New Part() With { _
 .PartName = "regular seat", _
 .PartId = 1434 _
 })
 parts.Add(New Part() With { _
 .PartName = "banana seat", _
 .PartId = 1444 _
 })
 parts.Add(New Part() With { _
 .PartName = "cassette", _
 .PartId = 1534 _
 })
 parts.Add(New Part() With { _
 .PartName = "shift lever", _
 .PartId = 1634 _
 })



 ' Write out the parts in the list. This will call the overridden ToString method
 ' in the Part class.
 Console.WriteLine()
 For Each aPart As Part In parts
 Console.WriteLine(aPart)
 Next


 ' Check the list for part #1734. This calls the IEquatable.Equals method
 ' of the Part class, which checks the PartId for equality.
 Console.WriteLine(vbLf & "Contains(""1734""): {0}", parts.Contains(New Part() With { _
 .PartId = 1734, _
 .PartName = "" _
 }))

 ' Insert a new item at position 2.
 Console.WriteLine(vbLf & "Insert(2, ""1834"")")
 parts.Insert(2, New Part() With { _
 .PartName = "brake lever", _
 .PartId = 1834 _
 })


 'Console.WriteLine();
 For Each aPart As Part In parts
 Console.WriteLine(aPart)
 Next

 Console.WriteLine(vbLf & "Parts[3]: {0}", parts(3))

 Console.WriteLine(vbLf & "Remove(""1534"")")

 ' This will remove part 1534 even though the PartName is different,
 ' because the Equals method only checks PartId for equality.
 parts.Remove(New Part() With { _
 .PartId = 1534, _
 .PartName = "cogs" _
 })

 Console.WriteLine()
 For Each aPart As Part In parts
 Console.WriteLine(aPart)
 Next

 Console.WriteLine(vbLf & "RemoveAt(3)")
 ' This will remove part at index 3.
 parts.RemoveAt(3)

 Console.WriteLine()
 For Each aPart As Part In parts
 Console.WriteLine(aPart)
 Next
 End Sub
 '
 ' This example code produces the following output:
 ' ID: 1234 Name: crank arm
 ' ID: 1334 Name: chain ring
 ' ID: 1434 Name: regular seat
 ' ID: 1444 Name: banana seat
 ' ID: 1534 Name: cassette
 ' ID: 1634 Name: shift lever
 '
 ' Contains("1734"): False
 '
 ' Insert(2, "1834")
 ' ID: 1234 Name: crank arm
 ' ID: 1334 Name: chain ring
 ' ID: 1834 Name: brake lever
 ' ID: 1434 Name: regular seat
 ' ID: 1444 Name: banana seat
 ' ID: 1534 Name: cassette
 ' ID: 1634 Name: shift lever
 '
 ' Parts[3]: ID: 1434 Name: regular seat
 '
 ' Remove("1534")
 '
 ' ID: 1234 Name: crank arm
 ' ID: 1334 Name: chain ring
 ' ID: 1834 Name: brake lever
 ' ID: 1434 Name: regular seat
 ' ID: 1444 Name: banana seat
 ' ID: 1634 Name: shift lever
 ' '
 ' RemoveAt(3)
 '
 ' ID: 1234 Name: crank arm
 ' ID: 1334 Name: chain ring
 ' ID: 1834 Name: brake lever
 ' ID: 1444 Name: banana seat
 ' ID: 1634 Name: shift lever
 ' 

End Class

// Simple business object. A PartId is used to identify the type of part 
// but the part name can change. 
[<CustomEquality; NoComparison>]
type Part = { PartId : int ; mutable PartName : string } with
 override this.GetHashCode() = hash this.PartId
 override this.Equals(other) =
 match other with
 | :? Part as p -> this.PartId = p.PartId
 | _ -> false
 override this.ToString() = sprintf "ID: %i Name: %s" this.PartId this.PartName

[<EntryPoint>]
let main argv = 
 // We refer to System.Collections.Generic.List<'T> by its type 
 // abbreviation ResizeArray<'T> to avoid conflicts with the F# List module. 
 // Note: In F# code, F# linked lists are usually preferred over
 // ResizeArray<'T> when an extendable collection is required.
 let parts = ResizeArray<_>()
 parts.Add({PartName = "crank arm" ; PartId = 1234})
 parts.Add({PartName = "chain ring"; PartId = 1334 })
 parts.Add({PartName = "regular seat"; PartId = 1434 })
 parts.Add({PartName = "banana seat"; PartId = 1444 })
 parts.Add({PartName = "cassette"; PartId = 1534 })
 parts.Add({PartName = "shift lever"; PartId = 1634 })

 // Write out the parts in the ResizeArray. This will call the overridden ToString method
 // in the Part type
 printfn ""
 parts |> Seq.iter (fun p -> printfn "%O" p)

 // Check the ResizeArray for part #1734. This calls the IEquatable.Equals method 
 // of the Part type, which checks the PartId for equality. 
 printfn "\nContains(\"1734\"): %b" (parts.Contains({PartId=1734; PartName=""}))
 
 // Insert a new item at position 2.
 printfn "\nInsert(2, \"1834\")"
 parts.Insert(2, { PartName = "brake lever"; PartId = 1834 })

 // Write out all parts
 parts |> Seq.iter (fun p -> printfn "%O" p)

 printfn "\nParts[3]: %O" parts.[3]

 printfn "\nRemove(\"1534\")"
 // This will remove part 1534 even though the PartName is different, 
 // because the Equals method only checks PartId for equality.
 // Since Remove returns true or false, we need to ignore the result
 parts.Remove({PartId=1534; PartName="cogs"}) |> ignore

 // Write out all parts
 printfn ""
 parts |> Seq.iter (fun p -> printfn "%O" p)

 printfn "\nRemoveAt(3)"
 // This will remove the part at index 3.
 parts.RemoveAt(3)

 // Write out all parts
 printfn ""
 parts |> Seq.iter (fun p -> printfn "%O" p)

 0 // return an integer exit code

The following example demonstrates several properties and methods of the List<T> generic class, including the Add method. The parameterless constructor is used to create a list of strings with a capacity of 0. The Capacity property is displayed, and then the Add method is used to add several items. The items are listed, and the Capacity property is displayed again, along with the Count property, to show that the capacity has been increased as needed.

Other properties and methods are used to search for, insert, and remove elements from the list, and finally to clear the list.

List<string> dinosaurs = new List<string>();

Console.WriteLine("\nCapacity: {0}", dinosaurs.Capacity);

dinosaurs.Add("Tyrannosaurus");
dinosaurs.Add("Amargasaurus");
dinosaurs.Add("Mamenchisaurus");
dinosaurs.Add("Deinonychus");
dinosaurs.Add("Compsognathus");
Console.WriteLine();
foreach(string dinosaur in dinosaurs)
{
 Console.WriteLine(dinosaur);
}

Console.WriteLine("\nCapacity: {0}", dinosaurs.Capacity);
Console.WriteLine("Count: {0}", dinosaurs.Count);

Console.WriteLine("\nContains(\"Deinonychus\"): {0}",
 dinosaurs.Contains("Deinonychus"));

Console.WriteLine("\nInsert(2, \"Compsognathus\")");
dinosaurs.Insert(2, "Compsognathus");

Console.WriteLine();
foreach(string dinosaur in dinosaurs)
{
 Console.WriteLine(dinosaur);
}

// Shows accessing the list using the Item property.
Console.WriteLine("\ndinosaurs[3]: {0}", dinosaurs[3]);

Console.WriteLine("\nRemove(\"Compsognathus\")");
dinosaurs.Remove("Compsognathus");

Console.WriteLine();
foreach(string dinosaur in dinosaurs)
{
 Console.WriteLine(dinosaur);
}

dinosaurs.TrimExcess();
Console.WriteLine("\nTrimExcess()");
Console.WriteLine("Capacity: {0}", dinosaurs.Capacity);
Console.WriteLine("Count: {0}", dinosaurs.Count);

dinosaurs.Clear();
Console.WriteLine("\nClear()");
Console.WriteLine("Capacity: {0}", dinosaurs.Capacity);
Console.WriteLine("Count: {0}", dinosaurs.Count);

/* This code example produces the following output:

Capacity: 0

Tyrannosaurus
Amargasaurus
Mamenchisaurus
Deinonychus
Compsognathus

Capacity: 8
Count: 5

Contains("Deinonychus"): True

Insert(2, "Compsognathus")

Tyrannosaurus
Amargasaurus
Compsognathus
Mamenchisaurus
Deinonychus
Compsognathus

dinosaurs[3]: Mamenchisaurus

Remove("Compsognathus")

Tyrannosaurus
Amargasaurus
Mamenchisaurus
Deinonychus
Compsognathus

TrimExcess()
Capacity: 5
Count: 5

Clear()
Capacity: 5
Count: 0
 */

Imports System.Collections.Generic

Partial Public Class Program
 Public Shared Sub ShowPlanets()
 Dim planets As New List(Of String)

 Console.WriteLine(vbLf & "Capacity: {0}", planets.Capacity)

 planets.Add("Mercury")
 planets.Add("Venus")
 planets.Add("Earth")
 planets.Add("Mars")
 planets.Add("Jupiter")

 Console.WriteLine()
 For Each planet As String In planets
 Console.WriteLine(planet)
 Next

 Console.WriteLine(vbLf & "Capacity: {0}", planets.Capacity)
 Console.WriteLine("Count: {0}", planets.Count)

 Console.WriteLine(vbLf & "Contains(""Mars""): {0}", _
 planets.Contains("Mars"))

 Console.WriteLine(vbLf & "Insert(2, ""Saturn"")")
 planets.Insert(2, "Saturn")

 Console.WriteLine()
 For Each planet As String In planets
 Console.WriteLine(planet)
 Next
 ' Shows how to access the list using the Item property.
 Console.WriteLine(vbLf & "planets(3): {0}", planets(3))
 Console.WriteLine(vbLf & "Remove(""Jupiter"")")
 planets.Remove("Jupiter")

 Console.WriteLine()
 For Each planet As String In planets
 Console.WriteLine(planet)
 Next

 planets.TrimExcess()
 Console.WriteLine(vbLf & "TrimExcess()")
 Console.WriteLine("Capacity: {0}", planets.Capacity)
 Console.WriteLine("Count: {0}", planets.Count)

 planets.Clear()
 Console.WriteLine(vbLf & "Clear()")
 Console.WriteLine("Capacity: {0}", planets.Capacity)
 Console.WriteLine("Count: {0}", planets.Count)
 End Sub
End Class

' This code example produces the following output:
'
' Capacity: 0
'
' Mercury
' Venus
' Earth
' Mars
' Jupiter
'
' Capacity: 8
' Count: 5
'
' Contains("Mars"): True
'
' Insert(2, "Saturn")
'
' Mercury
' Venus
' Saturn
' Earth
' Mars
' Jupiter
'
' planets(3): Earth
'
' Remove("Jupiter")
'
' Mercury
' Venus
' Saturn
' Earth
' Mars
'
' TrimExcess()
' Capacity: 5
' Count: 5
'
' Clear()
' Capacity: 5
' Count: 0

[<EntryPoint>]
let main argv = 
 // We refer to System.Collections.Generic.List<'T> by its type 
 // abbreviation ResizeArray<'T> to avoid conflict with the List module. 
 // Note: In F# code, F# linked lists are usually preferred over
 // ResizeArray<'T> when an extendable collection is required.
 let dinosaurs = ResizeArray<_>()
 
 // Write out the dinosaurs in the ResizeArray.
 let printDinosaurs() =
 printfn ""
 dinosaurs |> Seq.iter (fun p -> printfn "%O" p) 
 
 
 printfn "\nCapacity: %i" dinosaurs.Capacity
 
 dinosaurs.Add("Tyrannosaurus")
 dinosaurs.Add("Amargasaurus")
 dinosaurs.Add("Mamenchisaurus")
 dinosaurs.Add("Deinonychus")
 dinosaurs.Add("Compsognathus")
 
 printDinosaurs()
 
 printfn "\nCapacity: %i" dinosaurs.Capacity
 printfn "Count: %i" dinosaurs.Count
 
 printfn "\nContains(\"Deinonychus\"): %b" (dinosaurs.Contains("Deinonychus"))
 
 printfn "\nInsert(2, \"Compsognathus\")"
 dinosaurs.Insert(2, "Compsognathus")
 
 printDinosaurs()
 
 // Shows accessing the list using the Item property.
 printfn "\ndinosaurs[3]: %s" dinosaurs.[3]
 
 printfn "\nRemove(\"Compsognathus\")"
 dinosaurs.Remove("Compsognathus") |> ignore
 
 printDinosaurs()
 
 dinosaurs.TrimExcess()
 printfn "\nTrimExcess()"
 printfn "Capacity: %i" dinosaurs.Capacity
 printfn "Count: %i" dinosaurs.Count
 
 dinosaurs.Clear()
 printfn "\nClear()"
 printfn "Capacity: %i" dinosaurs.Capacity
 printfn "Count: %i" dinosaurs.Count
 
 0 // return an integer exit code
 
 (* This code example produces the following output:
 
Capacity: 0
 
Tyrannosaurus
Amargasaurus
Mamenchisaurus
Deinonychus
Compsognathus
 
Capacity: 8
Count: 5
 
Contains("Deinonychus"): true
 
Insert(2, "Compsognathus")
 
Tyrannosaurus
Amargasaurus
Compsognathus
Mamenchisaurus
Deinonychus
Compsognathus
 
dinosaurs[3]: Mamenchisaurus
 
Remove("Compsognathus")
 
Tyrannosaurus
Amargasaurus
Mamenchisaurus
Deinonychus
Compsognathus
 
TrimExcess()
Capacity: 5
Count: 5
 
Clear()
Capacity: 5
Count: 0
 *)

Remarks

List<T> accepts null as a valid value for reference types and allows duplicate elements.

If Count already equals Capacity, the capacity of the List<T> is increased by automatically reallocating the internal array, and the existing elements are copied to the new array before the new element is added.

If Count is less than Capacity, this method is an O(1) operation. If the capacity needs to be increased to accommodate the new element, this method becomes an O(n) operation, where n is Count.

See also

• AddRange(IEnumerable<T>)
• Insert(Int32, T)
• Remove(T)
• Count