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

定義

判斷 Dictionary<TKey,TValue> 是否包含特定索引鍵。Determines whether the Dictionary<TKey,TValue> contains the specified key.

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 TKey TKey TKey

要在 Dictionary<TKey,TValue> 中尋找的索引鍵。The key to locate in the Dictionary<TKey,TValue>.

傳回

如果 true 包含具有指定索引鍵的項目,則為 Dictionary<TKey,TValue>,否則為 falsetrue if the Dictionary<TKey,TValue> contains an element with the specified key; otherwise, false.

實作

例外狀況

範例

下列程式碼範例示範如何在ContainsKey Add呼叫方法之前,使用方法來測試索引鍵是否存在。The following code example shows how to use the ContainsKey method to test whether a key exists prior to calling the Add method. 它也會示範如何使用TryGetValue方法來抓取值,這是當程式經常嘗試不在字典中的索引鍵時,用來取得值的有效方式。It also shows how to use the TryGetValue method to retrieve values, which is an efficient way to retrieve values when a program frequently tries keys that are not in the dictionary. 最後,它會使用Item[TKey]屬性(中C#的索引子),顯示最有效率的方法來測試金鑰是否存在。Finally, it shows the least efficient way to test whether keys exist, by using the Item[TKey] property (the indexer in C#).

這個程式碼範例是針對Dictionary<TKey,TValue>類別提供之較大範例的一部分(openWith這是此範例中使用的字典名稱)。This code example is part of a larger example provided for the Dictionary<TKey,TValue> class (openWith is the name of the Dictionary used in this example).

// 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

這個方法會使用 O (1)運算。This method approaches an O(1) operation.

適用於

另請參閱