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

定义

命名空间:

System.Collections.Generic

程序集:

System.Runtime.dll

确定是否 IDictionary<TKey,TValue> 包含带有指定键的元素。

public:
 bool ContainsKey(TKey key);

public bool ContainsKey (TKey key);

abstract member ContainsKey : 'Key -> bool

Public Function ContainsKey (key As TKey) As Boolean

参数

key

TKey

要在 IDictionary<TKey,TValue> 中定位的键。

返回

Boolean

如果 true 包含具有键的元素，则为 IDictionary<TKey,TValue>；否则为 false。

例外

ArgumentNullException

key 为 null。

示例

下面的代码示例演示如何在调用 Add 方法之前使用 ContainsKey 方法测试键是否存在。 它还演示了如何使用 TryGetValue 方法，如果程序频繁尝试不在字典中的键值，该方法可能是检索值的更有效方法。 最后，它演示如何在 Item[] C#) 中使用属性 (索引器插入项。

此代码是可以编译和执行的更大示例的一部分。 请参阅 System.Collections.Generic.IDictionary<TKey,TValue>。

// 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

注解

实现在确定对象相等性的方式上可能会有所不同;例如， List<T> 类使用 Comparer<T>.Default，而 Dictionary<TKey,TValue> 类允许用户指定 IComparer<T> 用于比较键的实现。

实现在是否允许 key 为 null方面可能会有所不同。