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

定义

获取与指定键关联的值。Gets the value associated with the specified key.

public:
 bool TryGetValue(TKey key, [Runtime::InteropServices::Out] TValue % value);
public bool TryGetValue (TKey key, out TValue value);
abstract member TryGetValue : 'Key * 'Value -> bool
Public Function TryGetValue (key As TKey, ByRef value As TValue) As Boolean

参数

key
TKey

要获取其值的键。The key whose value to get.

value
TValue

当此方法返回时,如果找到指定键,则返回与该键相关联的值;否则,将返回 value 参数的类型的默认值。When this method returns, the value associated with the specified key, if the key is found; otherwise, the default value for the type of the value parameter. 此参数未经初始化即被传递。This parameter is passed uninitialized.

返回

Boolean

如果实现 IDictionary<TKey,TValue> 的对象包含具有指定键的元素,则为 true;否则,为 falsetrue if the object that implements IDictionary<TKey,TValue> contains an element with the specified key; otherwise, false.

例外

keynullkey is null.

示例

该示例演示如何使用 TryGetValue 方法来检索值。The example shows how to use the TryGetValue method to retrieve values. 如果程序经常尝试不在字典中的键值,则 TryGetValue 方法比使用 Item[] 属性 (c # ) 中的索引器更有效,这会在尝试检索不存在的键时引发异常。If a program frequently tries key values that are not in a dictionary, the TryGetValue method can be more efficient than using the Item[] property (the indexer in C#), which throws exceptions when attempting to retrieve nonexistent keys.

此代码是可以编译和执行的更大示例的一部分。This code is part of a larger example that can be compiled and executed. 请参阅 System.Collections.Generic.IDictionary<TKey,TValue>See System.Collections.Generic.IDictionary<TKey,TValue>.

// 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

此方法结合了 ContainsKey 方法和属性的功能 Item[]This method combines the functionality of the ContainsKey method and the Item[] property.

如果未找到键,则 value 参数将为类型 TValue 获取适当的默认值;例如,为整数类型获取零 (0),为布尔值类型获取 false,为引用类型获取 nullIf the key is not found, then the value parameter gets the appropriate default value for the type TValue; for example, zero (0) for integer types, false for Boolean types, and null for reference types.

适用于

另请参阅