List<T>.Add(T) Method

Reference

Definition

Namespace:: System.Collections.Generic

Assembly:: System.Collections.dll

Assembly:: mscorlib.dll

Assembly:: netstandard.dll

Source:: List.cs

Source:: List.cs

Source:: List.cs

Important

Some information relates to prerelease product that may be substantially modified before it’s released. Microsoft makes no warranties, express or implied, with respect to the information provided here.

Adds an object to the end of the List<T>.

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

item: T

The object to be added to the end of the List<T>. The value can be null for reference types.

Implements

Add(T)

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

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