List<T>.AsReadOnly 方法

定义

命名空间:

System.Collections.Generic

程序集:

netstandard.dll, System.Collections.dll

Source:

List.cs

返回当前集合的只读 ReadOnlyCollection<T> 包装器。

public:
 System::Collections::ObjectModel::ReadOnlyCollection<T> ^ AsReadOnly();

public System.Collections.ObjectModel.ReadOnlyCollection<T> AsReadOnly();

member this.AsReadOnly : unit -> System.Collections.ObjectModel.ReadOnlyCollection<'T>

Public Function AsReadOnly () As ReadOnlyCollection(Of T)

返回

ReadOnlyCollection<T>

一个对象，充当当前 List<T>周围的只读包装器。

示例

以下示例演示了该方法 AsReadOnly 。 创建了一个 List<T> 容量为 4 的字符串，因为列表的最终大小已知正好为 4。 该列表填充了四个字符串，该方法 AsReadOnly 用于获取包装原始列表的只读 IList<T> 泛型接口实现。

原始列表的元素使用属性（C# 中的索引器）设置为“Coelophysis” Item[] ，并且再次显示只读列表的内容，以证明它只是原始列表的包装器。

using System;
using System.Collections.Generic;

public partial class Program
{
    public static void Main()
    {
        List<string> animals = new List<string>(4);

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

        animals.Add("Cat");
        animals.Add("Dog");
        animals.Add("Squirrel");
        animals.Add("Wolf");

        Console.WriteLine();
        foreach (string animal in animals)
        {
            Console.WriteLine(animal);
        }

        Console.WriteLine("\nIList<string> roAnimals = animals.AsReadOnly()");
        IList<string> roAnimals = animals.AsReadOnly();

        Console.WriteLine("\nElements in the read-only IList:");
        foreach (string animal in roAnimals)
        {
            Console.WriteLine(animal);
        }

        Console.WriteLine("\nanimals[2] = \"Lion\"");
        animals[2] = "Lion";

        Console.WriteLine("\nElements in the read-only IList:");
        foreach (string animal in roAnimals)
        {
            Console.WriteLine(animal);
        }
    }
}

/*
    This code example produces the following output:

    Capacity: 4

    Cat
    Dog
    Squirrel
    Wolf

    IList<string> roAnimals = animals.AsReadOnly()

    Elements in the read-only IList:
    Cat
    Dog
    Squirrel
    Wolf

    animals[2] = "Lion"

    Elements in the read-only IList:
    Cat
    Dog
    Lion
    Wolf
*/

Imports System.Collections.Generic

Partial Public Class Program
    Public Shared Sub Main()

        Dim animals As New List(Of String)(4)

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

        animals.Add("Cat")
        animals.Add("Dog")
        animals.Add("Squirrel")
        animals.Add("Wolf")

        Console.WriteLine()
        For Each animal As String In animals
            Console.WriteLine(animal)
        Next

        Console.WriteLine(vbLf & _
            "Dim roAnimals As IList(Of String) = animals.AsReadOnly")
        Dim roAnimals As IList(Of String) = animals.AsReadOnly

        Console.WriteLine(vbLf & "Elements in the read-only IList:")
        For Each animal As String In roAnimals
            Console.WriteLine(animal)
        Next

        Console.WriteLine(vbLf & "animals(2) = ""Lion""")
        animals(2) = "Lion"

        Console.WriteLine(vbLf & "Elements in the read-only IList:")
        For Each animal As String In roAnimals
            Console.WriteLine(animal)
        Next

    End Sub
End Class

' This code example produces the following output:
'
' Capacity: 4
'
' Cat
' Dog
' Squirrel
' Wolf
'
' Dim roAnimals As IList(Of String) = animals.AsReadOnly
'
' Elements in the read-only IList:
' Cat
' Dog
' Squirrel
' Wolf
'
' animals(2) = "Lion"
'
' Elements in the read-only IList:
' Cat
' Dog
' Lion
' Wolf

注解

若要防止对 List<T> 对象进行任何修改，请仅通过此包装公开它。 对象 ReadOnlyCollection<T> 不公开修改集合的方法。 但是，如果对基础 List<T> 对象进行了更改，只读集合将反映这些更改。

此方法是 O(1) 操作。