Adds an object to the end of the List<T>.
public:
virtual void Add(T item);
public void Add (T item);
abstract thành viên Add : ‘T -> unit
override this.Add : ‘T -> unit
Public Sub Add (item As T)
Parameters
itemT
The object to be added to the end of the List<T>. The value can be null for reference types.
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.
using namespace System;
using namespace System::Collections::Generic;
void main()
List<String^>^ dinosaurs = gcnew 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();
for each(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();
for each(String^ dinosaur in dinosaurs )
Console::WriteLine(dinosaur);
Console::WriteLine(“ndinosaurs[3]: 0”, dinosaurs[3]);
Console::WriteLine(“nRemove(“Compsognathus”)”);
dinosaurs->Remove(“Compsognathus”);
Console::WriteLine();
for each(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
*/
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
Public Class Example
Public Shared Sub Main()
Dim dinosaurs As New List(Of String)
Console.WriteLine(vbLf & “Capacity: 0”, dinosaurs.Capacity)
dinosaurs.Add(“Tyrannosaurus”)
dinosaurs.Add(“Amargasaurus”)
dinosaurs.Add(“Mamenchisaurus”)
dinosaurs.Add(“Deinonychus”)
dinosaurs.Add(“Compsognathus”)
Console.WriteLine()
For Each dinosaur As String In dinosaurs
Console.WriteLine(dinosaur)
Next
Console.WriteLine(vbLf & “Capacity: 0”, dinosaurs.Capacity)
Console.WriteLine(“Count: 0”, dinosaurs.Count)
Console.WriteLine(vbLf & “Contains(“”Deinonychus””): 0″, _
dinosaurs.Contains(“Deinonychus”))
Console.WriteLine(vbLf & “Insert(2, “”Compsognathus””)”)
dinosaurs.Insert(2, “Compsognathus”)
Console.WriteLine()
For Each dinosaur As String In dinosaurs
Console.WriteLine(dinosaur)
Next
‘ Shows how to access the list using the Item property.
Console.WriteLine(vbLf & “dinosaurs(3): 0”, dinosaurs(3))
Console.WriteLine(vbLf & “Remove(“”Compsognathus””)”)
dinosaurs.Remove(“Compsognathus”)
Console.WriteLine()
For Each dinosaur As String In dinosaurs
Console.WriteLine(dinosaur)
Next
dinosaurs.TrimExcess()
Console.WriteLine(vbLf & “TrimExcess()”)
Console.WriteLine(“Capacity: 0”, dinosaurs.Capacity)
Console.WriteLine(“Count: 0”, dinosaurs.Count)
dinosaurs.Clear()
Console.WriteLine(vbLf & “Clear()”)
Console.WriteLine(“Capacity: 0”, dinosaurs.Capacity)
Console.WriteLine(“Count: 0”, dinosaurs.Count)
End Sub
End Class
‘ 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
[<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.
Applies to
See also
- AddRange(IEnumerable<T>)
- Insert(Int32, T)
- Remove(T)
- Count
