SortedList<TKey,TValue>.Item[TKey] 属性

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

属性值

与指定键关联的值。 如果未找到指定的键，则 get 操作将引发一个 KeyNotFoundException 设置操作，并使用指定的键创建一个新元素。

实现

例外

示例

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

该示例还演示如何将 TryGetValue 该方法用作检索值的更高效方法，如果程序经常必须尝试不在排序列表中的键值。

该代码示例是 SortedList<TKey,TValue> 类中的一个较大示例的一部分。

// 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 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 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", 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

此属性提供使用以下语法访问集合中的特定元素的功能： myCollection[key]

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

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

还可以通过使用属性 Item[] 来添加新元素，方法是设置不存在的 SortedList<TKey,TValue>键的值;例如 myCollection["myNonexistentKey"] = myValue。 但是，如果指定的键已存在于该属性中 SortedList<TKey,TValue>，则设置该 Item[] 属性将覆盖旧值。 相反，该方法 Add 不会修改现有元素。

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 实现 Item[] 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（logn） 操作，其中 n。Count 如果键已位于 O（log）中SortedList<TKey,TValue>，则设置该属性为 O（logn） 操作。 如果键不在列表中，则为未排序的数据设置属性为 O（n）操作;如果新元素在列表末尾添加，则为 O（log n）。 如果插入导致调整大小，则操作为 O（n）。

另请参阅

• Add(TKey, TValue)