Dictionary<TKey,TValue>.ContainsKey(TKey) 方法

定義

命名空間:

System.Collections.Generic

組件:

System.Collections.dll

來源:

Dictionary.cs

判斷 Dictionary<TKey,TValue> 是否包含特定索引鍵。

public:
 virtual bool ContainsKey(TKey key);

public bool ContainsKey (TKey key);

abstract member ContainsKey : 'Key -> bool
override this.ContainsKey : 'Key -> bool

Public Function ContainsKey (key As TKey) As Boolean

參數

key

TKey

要在 Dictionary<TKey,TValue> 中尋找的索引鍵。

傳回

Boolean

如果 true 包含具有指定索引鍵的項目，則為 Dictionary<TKey,TValue>，否則為 false。

實作

ContainsKey(TKey) ContainsKey(TKey)

例外狀況

ArgumentNullException

key 為 null。

範例

下列程式代碼範例示範如何使用 ContainsKey 方法，在呼叫 Add 方法之前測試索引鍵是否存在。 它也會示範如何使用 TryGetValue 方法來擷取值，這是當程式經常嘗試不在字典中的索引鍵時，擷取值的有效方式。 最後，它會使用 Item[] 屬性 (C#) 中的索引器，顯示測試索引鍵是否存在的最有效率方式。

此程式代碼範例是針對 Dictionary<TKey,TValue> 類別提供的較大範例的一部分， openWith (是本範例) 中使用的 Dictionary 名稱。

// ContainsKey can be used to test keys before inserting
// them.
if (!openWith->ContainsKey("ht"))
{
    openWith->Add("ht", "hypertrm.exe");
    Console::WriteLine("Value added for key = \"ht\": {0}",
        openWith["ht"]);
}

// ContainsKey can be used to test keys before inserting
// them.
if (!openWith.ContainsKey("ht"))
{
    openWith.Add("ht", "hypertrm.exe");
    Console.WriteLine("Value added for key = \"ht\": {0}",
        openWith["ht"]);
}

' ContainsKey can be used to test keys before inserting 
' them.
If Not openWith.ContainsKey("ht") Then
    openWith.Add("ht", "hypertrm.exe")
    Console.WriteLine("Value added for key = ""ht"": {0}", _
        openWith("ht"))
End If

// 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

// 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

備註

此方法會接近 O (1) 作業。

另請參閱

• ContainsValue(TValue)