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

定義

取得或設定與指定之索引鍵相關聯的值。Gets or sets the value associated with the specified key.

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

參數

key
TKey TKey TKey TKey

要取得或設定之值的索引鍵。The key of the value to get or set.

屬性值

TValue TValue TValue TValue

與指定之索引鍵關聯的值。The value associated with the specified key. 如果找不到指定的索引鍵,則取得作業會擲回 KeyNotFoundException,且設定作業會使用指定的索引鍵建立新項目。If the specified key is not found, a get operation throws a KeyNotFoundException, and a set operation creates a new element with the specified key.

實作

例外狀況

會擷取該屬性,而且 key 不存在於集合中。The property is retrieved and key does not exist in the collection.

範例

下列程式碼範例會使用Item[TKey]屬性 (C# 中為索引子) 來擷取值,示範KeyNotFoundException要求的索引鍵不存在,而且可以取代顯示索引鍵關聯的值時擲回。The following code example uses the Item[TKey] property (the indexer in C#) to retrieve values, demonstrating that a KeyNotFoundException is thrown when a requested key is not present, and showing that the value associated with a key can be replaced.

此範例也示範如何使用TryGetValue方法作為更有效率的方式,來擷取值,如果程式通常必須嘗試不在字典中的索引鍵值。The example also shows how to use the TryGetValue method as a more efficient way to retrieve values if a program often must try key values that are not in the dictionary.

此程式碼範例是針對提供之較大範例的一部分Dictionary<TKey,TValue>類別。This code example is part of a larger example provided for the Dictionary<TKey,TValue> class. openWith 是此範例中使用此字典的名稱。openWith is the name of the Dictionary used in this example.

// Create a new dictionary of strings, with string keys.
//
Dictionary<String^, String^>^ openWith =
    gcnew Dictionary<String^, String^>();

// Add some elements to the dictionary. There are no
// duplicate keys, but some of the values are duplicates.
openWith->Add("txt", "notepad.exe");
openWith->Add("bmp", "paint.exe");
openWith->Add("dib", "paint.exe");
openWith->Add("rtf", "wordpad.exe");

// The Add method throws an exception if the new key is
// already in the dictionary.
try
{
    openWith->Add("txt", "winword.exe");
}
catch (ArgumentException^)
{
    Console::WriteLine("An element with Key = \"txt\" already exists.");
}
// Create a new dictionary of strings, with string keys.
//
Dictionary<string, string> openWith = 
    new Dictionary<string, string>();

// Add some elements to the dictionary. There are no 
// duplicate keys, but some of the values are duplicates.
openWith.Add("txt", "notepad.exe");
openWith.Add("bmp", "paint.exe");
openWith.Add("dib", "paint.exe");
openWith.Add("rtf", "wordpad.exe");

// The Add method throws an exception if the new key is 
// already in the dictionary.
try
{
    openWith.Add("txt", "winword.exe");
}
catch (ArgumentException)
{
    Console.WriteLine("An element with Key = \"txt\" already exists.");
}
' Create a new dictionary of strings, with string keys.
'
Dim openWith As New Dictionary(Of String, String)

' Add some elements to the dictionary. There are no 
' duplicate keys, but some of the values are duplicates.
openWith.Add("txt", "notepad.exe")
openWith.Add("bmp", "paint.exe")
openWith.Add("dib", "paint.exe")
openWith.Add("rtf", "wordpad.exe")

' The Add method throws an exception if the new key is 
' already in the dictionary.
Try
    openWith.Add("txt", "winword.exe")
Catch 
    Console.WriteLine("An element with Key = ""txt"" already exists.")
End Try
// 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 indexer throws an exception if the requested key is
// not in the dictionary.
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 dictionary.
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 dictionary.
Try
    Console.WriteLine("For key = ""tif"", value = {0}.", _
        openWith("tif"))
Catch 
    Console.WriteLine("Key = ""tif"" is not found.")
End Try
// When a program often has to try keys that turn out not to
// be in the dictionary, 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 dictionary, 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 dictionary, 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

備註

這個屬性讓您能夠使用下列 C# 語法來存取集合中的特定項目: myCollection[key] (myCollection(key) Visual Basic 中)。This property provides the ability to access a specific element in the collection by using the following C# syntax: myCollection[key] (myCollection(key) in Visual Basic).

您也可以使用Item[TKey]屬性來設定索引鍵的值加入新項目不存在於Dictionary<TKey,TValue>You can also use the Item[TKey] property to add new elements by setting the value of a key that does not exist in the Dictionary<TKey,TValue>. 當您設定屬性值,如果索引鍵存在於Dictionary<TKey,TValue>,該索引鍵相關聯的值會取代所指派的值。When you set the property value, if the key is in the Dictionary<TKey,TValue>, the value associated with that key is replaced by the assigned value. 如果索引鍵不在Dictionary<TKey,TValue>,索引鍵和值加入至字典。If the key is not in the Dictionary<TKey,TValue>, the key and value are added to the dictionary. 相反地,Add方法不會修改現有的項目。In contrast, the Add method does not modify existing elements.

金鑰不能null,值可以是,如果型別值但TValue是參考型別。A key cannot be null, but a value can be, if the value type TValue is a reference type.

C#語言會使用 this 關鍵字來定義索引,而不必實作Item[TKey]屬性。The C# language uses the this keyword to define the indexers instead of implementing the Item[TKey] property. Visual Basic 會將 Item[TKey] 實作為預設屬性,這樣會提供相同的索引功能。Visual Basic implements Item[TKey] as a default property, which provides the same indexing functionality.

取得或設定這個屬性的值接近 o (1) 作業。Getting or setting the value of this property approaches an O(1) operation.

適用於

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