Dictionary<TKey,TValue>.IDictionary.Item[Object] Dictionary<TKey,TValue>.IDictionary.Item[Object] Dictionary<TKey,TValue>.IDictionary.Item[Object] Property

定义

获取或设置具有指定键的值。Gets or sets the value with the specified key.

property System::Object ^ System::Collections::IDictionary::Item[System::Object ^] { System::Object ^ get(System::Object ^ key); void set(System::Object ^ key, System::Object ^ value); };
object System.Collections.IDictionary.Item[object key] { get; set; }
 Property Item(key As Object) As Object Implements IDictionary.Item

参数

key
Object Object Object

要获取的值的键。The key of the value to get.

属性值

与指定键关联的值,如果 key 不在字典中或者 key 属于不能分配给 Dictionary<TKey,TValue> 的键类型 TKey 的类型,则为 nullThe value associated with the specified key, or null if key is not in the dictionary or key is of a type that is not assignable to the key type TKey of the Dictionary<TKey,TValue>.

实现

异常

正在分配值,并且 key 属于不能分配给 Dictionary<TKey,TValue> 的键类型 TKey 的类型。A value is being assigned, and key is of a type that is not assignable to the key type TKey of the Dictionary<TKey,TValue>.

-or- 正在分配值,并且 value 属于不能分配给 Dictionary<TKey,TValue> 的值类型 TValue 的类型。A value is being assigned, and value is of a type that is not assignable to the value type TValue of the Dictionary<TKey,TValue>.

示例

下面的代码示例演示如何将IDictionary.Item[Object] System.Collections.IDictionary接口Dictionary<TKey,TValue>的属性(中C#的索引器)用于和属性不同于Dictionary<TKey,TValue>.Item[TKey]属性的方式。The following code example shows how to use the IDictionary.Item[Object] property (the indexer in C#) of the System.Collections.IDictionary interface with a Dictionary<TKey,TValue>, and ways the property differs from the Dictionary<TKey,TValue>.Item[TKey] property.

该示例显示,与Dictionary<TKey,TValue>.Item[TKey]属性一样,属性可以更改与现有键关联的值,如果指定键不在字典中,则Dictionary<TKey,TValue>.IDictionary.Item[Object]可用于添加新的键/值对。The example shows that, like the Dictionary<TKey,TValue>.Item[TKey] property, the Dictionary<TKey,TValue>.IDictionary.Item[Object] property can change the value associated with an existing key and can be used to add a new key/value pair if the specified key is not in the dictionary. 该示例还显示了与Dictionary<TKey,TValue>.Item[TKey]属性不同的,如果key不在字典中,则Dictionary<TKey,TValue>.IDictionary.Item[Object]属性不会引发异常,而是返回空引用。The example also shows that unlike the Dictionary<TKey,TValue>.Item[TKey] property, the Dictionary<TKey,TValue>.IDictionary.Item[Object] property does not throw an exception if key is not in the dictionary, returning a null reference instead. 最后,该示例演示如果Dictionary<TKey,TValue>.IDictionary.Item[Object] key不是正确的数据类型,获取属性将返回空引用; 如果key不是正确的数据类型,则设置属性会引发异常。Finally, the example demonstrates that getting the Dictionary<TKey,TValue>.IDictionary.Item[Object] property returns a null reference if key is not the correct data type, and that setting the property throws an exception if key is not the correct data type.

此代码示例摘自为IDictionary.Add方法提供的更大示例,包括输出。The code example is part of a larger example, including output, provided for the IDictionary.Add method.

using System;
using System.Collections;
using System.Collections.Generic;

public class Example
{
    public static void Main()
    {
        // Create a new dictionary of strings, with string keys,
        // and access it using the IDictionary interface.
        //
        IDictionary openWith = new Dictionary<string, string>();

        // Add some elements to the dictionary. There are no 
        // duplicate keys, but some of the values are duplicates.
        // IDictionary.Add throws an exception if incorrect types
        // are supplied for key or value.
        openWith.Add("txt", "notepad.exe");
        openWith.Add("bmp", "paint.exe");
        openWith.Add("dib", "paint.exe");
        openWith.Add("rtf", "wordpad.exe");
Imports System.Collections
Imports System.Collections.Generic

Public Class Example
    
    Public Shared Sub Main() 

        ' Create a new dictionary of strings, with string keys,
        ' and access it using the IDictionary interface.
        '
        Dim openWith As IDictionary = _
            New Dictionary(Of String, String)
        
        ' Add some elements to the dictionary. There are no 
        ' duplicate keys, but some of the values are duplicates.
        ' IDictionary.Add throws an exception if incorrect types
        ' are supplied for key or value.
        openWith.Add("txt", "notepad.exe")
        openWith.Add("bmp", "paint.exe")
        openWith.Add("dib", "paint.exe")
        openWith.Add("rtf", "wordpad.exe")
// The Item property is another name for the indexer, so you 
// can omit its name when accessing elements. 
Console.WriteLine("For key = \"rtf\", value = {0}.", 
    openWith["rtf"]);

// The indexer can be used to change the value associated
// with a key.
openWith["rtf"] = "winword.exe";
Console.WriteLine("For key = \"rtf\", value = {0}.", 
    openWith["rtf"]);

// If a key does not exist, setting the indexer for that key
// adds a new key/value pair.
openWith["doc"] = "winword.exe";

// The indexer returns null if the key is of the wrong data 
// type.
Console.WriteLine("The indexer returns null" 
    + " if the key is of the wrong type:");
Console.WriteLine("For key = 2, value = {0}.", 
    openWith[2]);

// The indexer throws an exception when setting a value
// if the key is of the wrong data type.
try
{
    openWith[2] = "This does not get added.";
}
catch (ArgumentException)
{
    Console.WriteLine("A key of the wrong type was specified" 
        + " when assigning to the indexer.");
}
' The Item property is the default property, so you 
' can omit its name when accessing elements. 
Console.WriteLine("For key = ""rtf"", value = {0}.", _
    openWith("rtf"))

' The default Item property can be used to change the value
' associated with a key.
openWith("rtf") = "winword.exe"
Console.WriteLine("For key = ""rtf"", value = {0}.", _
    openWith("rtf"))

' If a key does not exist, setting the default Item property
' for that key adds a new key/value pair.
openWith("doc") = "winword.exe"

' The default Item property returns Nothing if the key
' is of the wrong data type.
Console.WriteLine("The default Item property returns Nothing" _
    & " if the key is of the wrong type:")
Console.WriteLine("For key = 2, value = {0}.", _
    openWith(2))

' The default Item property throws an exception when setting
' a value if the key is of the wrong data type.
Try
    openWith(2) = "This does not get added."
Catch 
    Console.WriteLine("A key of the wrong type was specified" _
        & " when setting the default Item property.")
End Try
// Unlike the default Item property on the Dictionary class
// itself, IDictionary.Item does not throw an exception
// if the requested key is not in the dictionary.
Console.WriteLine("For key = \"tif\", value = {0}.", 
    openWith["tif"]);
' Unlike the default Item property on the Dictionary class
' itself, IDictionary.Item does not throw an exception
' if the requested key is not in the dictionary.
Console.WriteLine("For key = ""tif"", value = {0}.", _
    openWith("tif"))
    }
}

    End Sub

End Class

注解

此属性提供了使用以下C#语法访问集合中特定值的功能: myCollection[key]myCollection(key)在 Visual Basic 中)。This property provides the ability to access a specific value in the collection by using the following C# syntax: myCollection[key] (myCollection(key) in Visual Basic).

你还可以使用Item[Object]属性来添加新元素,方法是设置字典中不存在的键的值; myCollection["myNonexistentKey"] = myValue例如。You can also use the Item[Object] property to add new elements by setting the value of a key that does not exist in the dictionary; for example, myCollection["myNonexistentKey"] = myValue. 但是,如果字典中已存在指定的键,则设置Item[Object]属性会覆盖旧值。However, if the specified key already exists in the dictionary, setting the Item[Object] property overwrites the old value. 与此Add相反,方法不会修改现有元素。In contrast, the Add method does not modify existing elements.

该C#语言使用this关键字来定义索引器,而不是实现IDictionary.Item[Object]属性。The C# language uses the this keyword to define the indexers instead of implementing the IDictionary.Item[Object] property. Visual Basic 将 IDictionary.Item[Object] 实现为默认属性,该属性提供相同的索引功能。Visual Basic implements IDictionary.Item[Object] as a default property, which provides the same indexing functionality.

获取或设置此属性的值将接近 O (1)操作。Getting or setting the value of this property approaches an O(1) operation.

适用于

另请参阅