List<T>.TrimExcess Metod
Definition
Viktigt
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Anger kapaciteten till det faktiska antalet element i List<T>, om det talet är mindre än ett tröskelvärde.
public:
void TrimExcess();public void TrimExcess();member this.TrimExcess : unit -> unitPublic Sub TrimExcess ()Exempel
I följande exempel visas hur du kontrollerar kapaciteten och antalet av ett List<T> som innehåller ett enkelt affärsobjekt och visar hur du använder TrimExcess metoden för att ta bort extra kapacitet.
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
I följande exempel visas TrimExcess metoden. Flera egenskaper och metoder för List<T> klassen används för att lägga till, infoga och ta bort objekt från en lista med strängar. TrimExcess Sedan används metoden för att minska kapaciteten för att matcha antalet, och Capacity egenskaperna och Count visas. Om den outnyttjade kapaciteten hade varit mindre än 10 procent av den totala kapaciteten skulle listan inte ha ändrats. Slutligen rensas innehållet i listan.
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
*)
Kommentarer
Den här metoden kan användas för att minimera minneskostnaderna för en samling om inga nya element läggs till i samlingen. Kostnaden för att omplacera och kopiera en stor List<T> kan dock vara betydande, så TrimExcess metoden gör ingenting om listan är på mer än 90 procent av kapaciteten. Detta undviker att medföra en stor omfördelningskostnad för en relativt liten vinst.
Note
Det aktuella tröskelvärdet på 90 procent kan ändras i framtida versioner.
Den här metoden är en O(n)-åtgärd, där n är Count.
Om du vill återställa en List<T> till dess ursprungliga tillstånd anropar du Clear metoden innan du TrimExcess anropar metoden. Om du trimmar ett tomt List<T> värde anges kapaciteten för List<T> till standardkapaciteten.
Kapaciteten kan också anges med hjälp av Capacity egenskapen .