SortedList<TKey,TValue>.Item[TKey] 属性

定义

命名空间:

System.Collections.Generic

程序集:

System.Collections.dll

Source:

SortedList.cs

获取或设置与指定的键关联的值。

public:
 property TValue default[TKey] { TValue get(TKey key); void set(TKey key, TValue value); };

public TValue this[TKey key] { get; set; }

member this.Item('Key) : 'Value with get, set

Default Public Property Item(key As TKey) As TValue

参数

key

TKey

要获取或设置其值的键。

属性值

TValue

与指定的键相关联的值。 如果找不到指定的键，则 get 操作会引发一个 KeyNotFoundException，而 set 操作会创建一个使用指定键的新元素。

实现

Item[TKey]

例外

ArgumentNullException

key 为 null。

KeyNotFoundException

已检索该属性且集合中不存在 key。

示例

下面的代码示例使用 Item[] C#) 中索引器 (属性来检索值，演示 KeyNotFoundException 当请求的键不存在时会引发 ，并显示可以替换与键关联的值。

该示例还演示了 TryGetValue 当程序经常必须尝试不在排序列表中的键值时，如何使用 方法作为一种更有效的方法来检索值。

此代码示例是为 SortedList<TKey,TValue> 类提供的一个更大示例的一部分。

// 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 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 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 Item property is another name for the indexer, so you
// can omit its name when accessing elements.
printfn $"""For key = "rtf", value = {openWith["rtf"]}."""

// The indexer can be used to change the value associated
// with a key.
openWith["rtf"] <- "winword.exe"
printfn $"""For key = "rtf", value = {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 throws an exception if the requested key is
// not in the list.
try
{
    Console::WriteLine("For key = \"tif\", value = {0}.",
        openWith["tif"]);
}
catch (KeyNotFoundException^)
{
    Console::WriteLine("Key = \"tif\" is not found.");
}

// The indexer throws an exception if the requested key is
// not in the list.
try
{
    Console.WriteLine("For key = \"tif\", value = {0}.",
        openWith["tif"]);
}
catch (KeyNotFoundException)
{
    Console.WriteLine("Key = \"tif\" is not found.");
}

' The default Item property throws an exception if the requested
' key is not in the list.
Try
    Console.WriteLine("For key = ""tif"", value = {0}.", _
        openWith("tif"))
Catch 
    Console.WriteLine("Key = ""tif"" is not found.")
End Try

// The indexer throws an exception if the requested key is
// not in the list.
try
    printfn $"""For key = "tif", value = {openWith["tif"]}."""
with 
    | :? KeyNotFoundException ->
        printfn "Key = \"tif\" is not found."

// When a program often has to try keys that turn out not to
// be in the list, TryGetValue can be a more efficient
// way to retrieve values.
String^ value = "";
if (openWith->TryGetValue("tif", value))
{
    Console::WriteLine("For key = \"tif\", value = {0}.", value);
}
else
{
    Console::WriteLine("Key = \"tif\" is not found.");
}

// When a program often has to try keys that turn out not to
// be in the list, TryGetValue can be a more efficient
// way to retrieve values.
string value = "";
if (openWith.TryGetValue("tif", out value))
{
    Console.WriteLine("For key = \"tif\", value = {0}.", value);
}
else
{
    Console.WriteLine("Key = \"tif\" is not found.");
}

' When a program often has to try keys that turn out not to
' be in the list, TryGetValue can be a more efficient 
' way to retrieve values.
Dim value As String = ""
If openWith.TryGetValue("tif", value) Then
    Console.WriteLine("For key = ""tif"", value = {0}.", value)
Else
    Console.WriteLine("Key = ""tif"" is not found.")
End If

// When a program often has to try keys that turn out not to
// be in the list, TryGetValue can be a more efficient
// way to retrieve values.
match openWith.TryGetValue("tif") with
| true, value ->
    printfn "For key = \"tif\", value = {value}."
| false, _ ->
    printfn "Key = \"tif\" is not found."

注解

通过此属性，可以使用以下语法来访问集合中的特定元素：myCollection[key]。

键不能为 null，但如果列表中的值类型 是引用类型， TValue则值可以是 。

如果在检索值时找不到键， KeyNotFoundException 则会引发 。 如果在设置值时找不到键，则会添加键和值。

还可以使用 Item[] 属性通过设置 中SortedList<TKey,TValue>不存在的键的值来添加新元素;例如 。 myCollection["myNonexistentKey"] = myValue 但是，如果 指定的键已存在于 中 SortedList<TKey,TValue>，设置 属性 Item[] 将覆盖旧值。 相反， Add 方法不修改现有元素。

C# 语言使用 this 关键字来定义索引器，而不是实现 Item[] 属性。 Visual Basic 将 Item[] 实现为默认属性，该属性提供相同的索引功能。

检索此属性的值是 O (日志 n) 操作，其中 n 为 Count。 如果键已在 中SortedList<TKey,TValue>，则设置 属性是 O (日志 n) 操作。 如果该键不在列表中，则设置 属性是未排序数据的 O (n) 操作;如果新元素添加到列表末尾，则 O (日志 n) 。 如果插入导致重设大小，则操作为 O (n) 。

另请参阅

• Add(TKey, TValue)