List<T>.TrimExcess Methode

Definitie

Naamruimte:: System.Collections.Generic

Assembly's:: mscorlib.dll, System.Collections.dll

Assembly:: mscorlib.dll

Assembly:: System.Collections.dll

Assembly:: netstandard.dll

Belangrijk

Bepaalde informatie heeft betrekking op een voorlopige productversie die aanzienlijk kan worden gewijzigd voordat deze wordt uitgebracht. Microsoft biedt geen enkele expliciete of impliciete garanties met betrekking tot de informatie die hier wordt verstrekt.

Hiermee stelt u de capaciteit in op het werkelijke aantal elementen in de List<T>, als dat getal kleiner is dan een drempelwaarde.

public:
 void TrimExcess();

public void TrimExcess();

member this.TrimExcess : unit -> unit

Public Sub TrimExcess ()

Voorbeelden

In het volgende voorbeeld ziet u hoe u de capaciteit en het aantal van een List<T> object controleert dat een eenvoudig zakelijk object bevat en hoe u de TrimExcess methode gebruikt om extra capaciteit te verwijderen.

using System;
using System.Collections.Generic;
// Simple business object. A PartId is used to identify a part
// but the part name be different for the same Id.
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 base.GetHashCode();
    }
    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()
    {
        List<Part> parts = new List<Part>();

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

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

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

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

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

        parts.Clear();
        Console.WriteLine("\nClear()");
        Console.WriteLine("Capacity: {0}", parts.Capacity);
        Console.WriteLine("Count: {0}", parts.Count);
    }
    /*
     This code example produces the following output.
            Capacity: 0

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

            Capacity: 8
            Count: 5

            TrimExcess()
            Capacity: 5
            Count: 5

            Clear()
            Capacity: 5
            Count: 0
     */
}

Imports System.Collections.Generic

' Simple business object. A PartId is used to identify a 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 MyBase.GetHashCode()
    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()
        Dim parts As New List(Of Part)()

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

        ' 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 _
        })



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

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

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

        parts.Clear()
        Console.WriteLine(vbLf & "Clear()")
        Console.WriteLine("Capacity: {0}", parts.Capacity)
        Console.WriteLine("Count: {0}", parts.Count)
    End Sub
    '
    '     This code example produces the following output. 
    '            Capacity: 0
    '
    '            ID: 1234   Name: crank arm
    '            ID: 1334   Name: chain ring
    '            ID: 1434   Name: seat
    '            ID: 1534   Name: cassette
    '            ID: 1634   Name: shift lever
    '
    '            Capacity: 8
    '            Count: 6
    '
    '            TrimExcess()
    '            Capacity: 6
    '            Count: 6
    '
    '            Clear()
    '            Capacity: 6
    '            Count: 0
    '     

End Class

In het volgende voorbeeld ziet u de TrimExcess methode. Verschillende eigenschappen en methoden van de List<T> klasse worden gebruikt om items toe te voegen, in te voegen en te verwijderen uit een lijst met tekenreeksen. Vervolgens wordt de TrimExcess methode gebruikt om de capaciteit te verminderen zodat deze overeenkomt met het aantal en worden de Capacity eigenschappen Count weergegeven. Als de ongebruikte capaciteit minder dan 10 procent van de totale capaciteit was, zou de lijst niet zijn gewijzigd. Ten slotte wordt de inhoud van de lijst gewist.

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
    *)

Opmerkingen

Deze methode kan worden gebruikt om de geheugenoverhead van een verzameling te minimaliseren als er geen nieuwe elementen aan de verzameling worden toegevoegd. De kosten van het opnieuw toewijzen en kopiëren van een grote List<T> kunnen aanzienlijk zijn, dus de TrimExcess methode doet niets als de lijst meer dan 90 procent van de capaciteit heeft. Dit voorkomt dat er een grote herlocatiekosten ontstaan voor een relatief kleine winst.

Note

De huidige drempelwaarde van 90 procent kan in toekomstige releases veranderen.

Deze methode is een O(n)-bewerking, waarbij n is Count.

Als u de List<T> oorspronkelijke status opnieuw wilt instellen, roept u de Clear methode aan voordat u de TrimExcess methode aanroept. Als u een lege List<T> functie bijwerkt, wordt de capaciteit van de List<T> standaardcapaciteit ingesteld.

De capaciteit kan ook worden ingesteld met behulp van de Capacity eigenschap.

Van toepassing op

Zie ook

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