List<T>.AsReadOnly 方法

定义

返回当前集合的只读 ReadOnlyCollection<T> 包装器。Returns a read-only ReadOnlyCollection<T> wrapper for the current collection.

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,作为围绕当前 List<T> 的只读包装器。An object that acts as a read-only wrapper around the current List<T>.

示例

下面的示例演示 AsReadOnly 方法。The following example demonstrates the AsReadOnly method. List<T>创建了容量为4的字符串的,因为已知列表的最终大小为4。A List<T> of strings with a capacity of 4 is created, because the ultimate size of the list is known to be exactly 4. 列表中填充了四个字符串, AsReadOnly 方法用于获取 IList<T> 包装原始列表的只读泛型接口实现。The list is populated with four strings, and the AsReadOnly method is used to get a read-only IList<T> generic interface implementation that wraps the original list.

使用 c # ) 中索引器 (的属性将原始列表的元素设置为 "Coelophysis" Item[] ,并再次显示只读列表的内容,以表明它只是原始列表的包装。An element of the original list is set to "Coelophysis" using the Item[] property (the indexer in C#), and the contents of the read-only list are displayed again to demonstrate that it is just a wrapper for the original list.

using namespace System;
using namespace System::Collections::Generic;

void main()
{
    List<String^>^ dinosaurs = gcnew List<String^>(4);

    Console::WriteLine("\nCapacity: {0}", dinosaurs->Capacity);

    dinosaurs->Add("Tyrannosaurus");
    dinosaurs->Add("Amargasaurus");
    dinosaurs->Add("Mamenchisaurus");
    dinosaurs->Add("Deinonychus");

    Console::WriteLine();
    for each(String^ dinosaur in dinosaurs)
    {
        Console::WriteLine(dinosaur);
    }

    Console::WriteLine("\nIList<String^>^ roDinosaurs = dinosaurs->AsReadOnly()");
    IList<String^>^ roDinosaurs = dinosaurs->AsReadOnly();

    Console::WriteLine("\nElements in the read-only IList:");
    for each(String^ dinosaur in roDinosaurs)
    {
        Console::WriteLine(dinosaur);
    }

    Console::WriteLine("\ndinosaurs[2] = \"Coelophysis\"");
    dinosaurs[2] = "Coelophysis";

    Console::WriteLine("\nElements in the read-only IList:");
    for each(String^ dinosaur in roDinosaurs)
    {
        Console::WriteLine(dinosaur);
    }
}

/* This code example produces the following output:

Capacity: 4

Tyrannosaurus
Amargasaurus
Mamenchisaurus
Deinonychus

IList<String^>^ roDinosaurs = dinosaurs->AsReadOnly()

Elements in the read-only IList:
Tyrannosaurus
Amargasaurus
Mamenchisaurus
Deinonychus

dinosaurs[2] = "Coelophysis"

Elements in the read-only IList:
Tyrannosaurus
Amargasaurus
Coelophysis
Deinonychus
 */
using System;
using System.Collections.Generic;

public class Example
{
    public static void Main()
    {
        List<string> dinosaurs = new List<string>(4);

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

        dinosaurs.Add("Tyrannosaurus");
        dinosaurs.Add("Amargasaurus");
        dinosaurs.Add("Mamenchisaurus");
        dinosaurs.Add("Deinonychus");

        Console.WriteLine();
        foreach(string s in dinosaurs)
        {
            Console.WriteLine(s);
        }

        Console.WriteLine("\nIList<string> roDinosaurs = dinosaurs.AsReadOnly()");
        IList<string> roDinosaurs = dinosaurs.AsReadOnly();

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

        Console.WriteLine("\ndinosaurs[2] = \"Coelophysis\"");
        dinosaurs[2] = "Coelophysis";

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

/* This code example produces the following output:

Capacity: 4

Tyrannosaurus
Amargasaurus
Mamenchisaurus
Deinonychus

IList<string> roDinosaurs = dinosaurs.AsReadOnly()

Elements in the read-only IList:
Tyrannosaurus
Amargasaurus
Mamenchisaurus
Deinonychus

dinosaurs[2] = "Coelophysis"

Elements in the read-only IList:
Tyrannosaurus
Amargasaurus
Coelophysis
Deinonychus
 */
Imports System.Collections.Generic

Public Class Example

    Public Shared Sub Main()

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

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

        dinosaurs.Add("Tyrannosaurus")
        dinosaurs.Add("Amargasaurus")
        dinosaurs.Add("Mamenchisaurus")
        dinosaurs.Add("Deinonychus")

        Console.WriteLine()
        For Each dinosaur As String In dinosaurs
            Console.WriteLine(dinosaur)
        Next

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

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

        Console.WriteLine(vbLf & "dinosaurs(2) = ""Coelophysis""")
        dinosaurs(2) = "Coelophysis"

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

    End Sub
End Class

' This code example produces the following output:
'
'Capacity: 4
'
'Tyrannosaurus
'Amargasaurus
'Mamenchisaurus
'Deinonychus
'
'Dim roDinosaurs As IList(Of String) = dinosaurs.AsReadOnly
'
'Elements in the read-only IList:
'Tyrannosaurus
'Amargasaurus
'Mamenchisaurus
'Deinonychus
'
'dinosaurs(2) = "Coelophysis"
'
'Elements in the read-only IList:
'Tyrannosaurus
'Amargasaurus
'Coelophysis
'Deinonychus

注解

若要防止对该对象进行任何修改 List<T> ,只需要通过此包装器进行公开。To prevent any modifications to the List<T> object, expose it only through this wrapper. ReadOnlyCollection<T>对象不公开修改集合的方法。A ReadOnlyCollection<T> object does not expose methods that modify the collection. 但是,如果对基础对象进行了更改 List<T> ,只读集合将反映这些更改。However, if changes are made to the underlying List<T> object, the read-only collection reflects those changes.

此方法是 O (1) 操作。This method is an O(1) operation.

适用于