Lambda 運算式
更新：2007 年 11 月
「Lambda 運算式」(Lambda Expression) 是沒有名稱的函式，會計算並傳回單一值。委派型別 (Delegate Type) 有效時，即可使用 Lambda 運算式。
.gif "注意事項") 注意事項： |
|---|
RemoveHandler 陳述式 (Statement) 是例外狀況 (Exception)。您無法針對 RemoveHandler 的委派參數傳遞 Lambda 運算式。 |
下列範例是會遞增其引數並傳回值的 Lambda 運算式。
Function (num As Integer) num + 1
因為 Lambda 運算式是運算式，所以只能做為陳述式的一部分。
例如，特別是在要多次使用函式的情況下，您可能會將函式指派至變數名稱。
Dim add1 = Function(num As Integer) num + 1
若要呼叫函式，請傳送參數的值。
' The following line prints 6.
Console.WriteLine(add1(5))
或者，您可以同時宣告並執行函式。
Console.WriteLine((Function(num As Integer) num + 1)(5))
Lambda 運算式可以傳回做為函式呼叫 (Function Call) 的值 (請見本主題稍後「內容」章節中的範例)，或傳遞做為委派參數的引數。在下列範例中，布林 (Boolean) Lambda 運算式會做為引數傳遞至方法 testResult。這個方法會將布林測試套用至整數引數 value，而且當套用至 value 時，如果 Lambda 運算式傳回 True，則顯示「成功」(Success)，如果 Lambda 運算式傳回 False，則顯示「失敗」(Failure)。
Module Module2
Sub Main()
' The following line will print Success, because 4 is even.
testResult(4, Function(num) num Mod 2 = 0)
' The following line will print Failure, because 5 is not > 10.
testResult(5, Function(num) num > 10)
End Sub
' Sub testResult takes two arguments, an integer value and a
' Boolean function.
' If the function returns True for the integer argument, Success
' is displayed.
' If the function returns False for the integer argument, Failure
' is displayed.
Sub testResult(ByVal value As Integer, ByVal fun As Func(Of Integer, Boolean))
If fun(value) Then
Console.WriteLine("Success")
Else
Console.WriteLine("Failure")
End If
End Sub
End Module
查詢中的 Lambda 運算式
在 Language-Integrated Query (LINQ) 中，Lambda 運算式是許多標準查詢運算子的基礎。編譯器 (Compiler) 會建立 Lambda 運算式以擷取基本查詢方法 (例如 Where、Select、Order By、Take While 等等) 中定義的計算。
例如，以下列查詢為例：
Dim londonCusts = From cust In db.Customers
Where cust.City = "London"
Select cust
這個範例會編譯成下列程式碼：
Dim londonCusts = db.Customers _
.Where(Function(cust) cust.City = "London") _
.Select(Function(cust) cust)
如需查詢方法的詳細資訊，請參閱查詢 (Visual Basic)。
Lambda 運算式語法
Lambda 運算式的語法類似標準函式。其差異如下：
Lambda 運算式沒有名稱。
Lambda 運算式不能有修飾詞 (Modifier)，例如 Overloads 或 Overrides。
Lambda 運算式不會使用 As 子句指定函式的傳回型別。而是從 Lambda 運算式評估之主體的值來推斷型別。例如，如果 Lambda 運算式的主體是 Where cust.City = "London"，其傳回型別為 Boolean。
函式的主體必須是運算式，而不是陳述式。主體可以由對函式程序的呼叫組成，但不可由對子程序的呼叫組成。
沒有 Return 陳述式。函式傳回的值就是函式主體中運算式的值。
沒有 End Function 陳述式。
所有參數都必須具有指定的資料型別，不然所有參數就都必須經過推斷。
不允許使用 Optional 和 Paramarray 參數。
不允許使用泛型參數。
因為這些限制以及 Lambda 運算式的使用方式，所產生的結果通常都是簡短不複雜的。
內容
任一 Lambda 運算式定義於某個方法中時，該運算式會將本身的內容與該方法共用。它的存取權限與包含方法中撰寫的任何程式碼相同。這包括對包含方法中成員變數、函式和子函式、Me 及參數和本機變數的存取權。
對於包含方法中的本機變數和參數的存取權期，可以延伸超過該方法的存留期 (Lifetime)。只要委派 (Delegate) 參考無法進行記憶體回收的 Lambda 運算式，就會保留對於原始環境中變數的存取權。在下列範例中，變數 target 是 makeTheGame 的本機變數，而後者是 Lambda 運算式 playTheGame 在其中定義的方法。請注意，在 Main 中指派給 takeAGuess 的傳回 Lambda 運算式，仍然可以存取本機變數 target。
Module Module1
Sub Main()
' Variable takeAGuess is a Boolean function. It stores the target
' number that is set in makeTheGame.
Dim takeAGuess As gameDelegate = makeTheGame()
' Set up the loop to play the game.
Dim guess As Integer
Dim gameOver = False
While Not gameOver
guess = CInt(InputBox("Enter a number between 1 and 10 (0 to quit)", "Guessing Game", "0"))
' A guess of 0 means you want to give up.
If guess = 0 Then
gameOver = True
Else
' Tests your guess and announces whether you are correct. Method takeAGuess
' is called multiple times with different guesses. The target value is not
' accessible from Main and is not passed in.
gameOver = takeAGuess(guess)
Console.WriteLine("Guess of " & guess & " is " & gameOver)
End If
End While
End Sub
Delegate Function gameDelegate(ByVal aGuess As Integer) As Boolean
Public Function makeTheGame() As gameDelegate
' Generate the target number, between 1 and 10. Notice that
' target is a local variable. After you return from makeTheGame,
' it is not directly accessible.
Randomize()
Dim target As Integer = CInt(Int(10 * Rnd() + 1))
' Print the answer if you want to be sure the game is not cheating
' by changing the target at each guess.
Console.WriteLine("(Peeking at the answer) The target is " & target)
' The game is returned as a lambda expression. The lambda expression
' carries with it the environment in which it was created. This
' environment includes the target number. Note that only the current
' guess is a parameter to the returned lambda expression, not the target.
' Does the guess equal the target?
Dim playTheGame = Function(guess As Integer) guess = target
Return playTheGame
End Function
End Module
下列範例示範巢狀 Lambda 運算式的廣泛存取權限。當傳回的 Lambda 運算式是從 Main 執行為 aDel 時，它會存取下列項目：
在其中進行定義的類別 (Class) 欄位：aField
在其中進行定義的類別屬性：aProp
在其中進行定義之方法 functionWithNestedLambda 的參數：level1
functionWithNestedLambda 的本機變數：localVar
形成巢狀之 Lambda 運算式的參數：level2
Module Module3
Sub Main()
' Create an instance of the class, with 1 as the value of
' the property.
Dim lambdaScopeDemoInstance = New LambdaScopeDemoClass _
With {.Prop = 1}
' Variable aDel will be bound to the nested lambda expression
' returned by the call to functionWithNestedLambda.
' The value 2 is sent in for parameter level1.
Dim aDel As aDelegate = _
lambdaScopeDemoInstance.functionWithNestedLambda(2)
' Now the returned lambda expression is called, with 4 as the
' value of parameter level3.
Console.WriteLine("First value returned by aDel: " & aDel(4))
' Change a few values to verify that the lambda expression has
' access to the variables, not just their original values.
lambdaScopeDemoInstance.aField = 20
lambdaScopeDemoInstance.Prop = 30
Console.WriteLine("Second value returned by aDel: " & aDel(40))
End Sub
Delegate Function aDelegate(ByVal delParameter As Integer) _
As Integer
Public Class LambdaScopeDemoClass
Public aField As Integer = 6
Dim aProp As Integer
Property Prop() As Integer
Get
Return aProp
End Get
Set(ByVal value As Integer)
aProp = value
End Set
End Property
Public Function functionWithNestedLambda _
(ByVal level1 As Integer) As aDelegate
Dim localVar As Integer = 5
' When the nested lambda expression is executed the first
' time, as aDel from Main, the variables have these values:
' level1 = 2
' level2 = 3, after aLambda is called in the Return statement
' level3 = 4, after aDel is called in Main
' locarVar = 5
' aField = 6
' aProp = 1
' The second time it is executed, two values have changed:
' aField = 20
' aProp = 30
' level3 = 40
Dim aLambda = Function(level2 As Integer) _
Function(level3 As Integer) _
level1 + level2 + level3 + localVar _
+ aField + aProp
' The function returns the nested lambda, with 3 as the
' value of parameter level2.
Return aLambda(3)
End Function
End Class
End Module
轉換為委派型別
Lambda 運算式可以隱含地轉換為相容的委派型別。如需一般相容性需求的詳細資訊，請參閱寬鬆委派轉換。
此外，當您將 Lambda 運算式指派為委派時，可以指定參數名稱但略過其資料型別，以便從委派採用型別。在下列範例中，Lambda 運算式會指派至型別為 ExampleDel 且名稱為 del 的變數，此委派採用兩個參數，一個為整數、一個為字串。請注意，並未指定 Lambda 運算式中參數的資料型別。但是，del 需要整數引數和字串引數，如 ExampleDel 的定義中所指定。
' Definition of function delegate ExampleDel.
Delegate Function ExampleDel(ByVal arg1 As Integer, _
ByVal arg2 As String) As Integer
' Declaration of del as an instance of ExampleDel, with no data
' type specified for the parameters, m and s.
Dim del As ExampleDel = Function(m, s) m
' Valid call to del, sending in an integer and a string.
Console.WriteLine(del(7, "up"))
' Neither of these calls is valid. Function del requires an integer
' argument and a string argument.
' Not valid.
' Console.WriteLine(del(7, 3))
' Console.WriteLine(del("abc"))
範例
下列範例定義 Lambda 運算式，在可為 Null 的引數具有指派值時傳回 True，而其值為 Nothing 時傳回 False。
Dim notNothing = Function(num? As Integer) _ num IsNot Nothing Dim arg As Integer = 14 Console.WriteLine("Does the argument have an assigned value?") Console.WriteLine(notNothing(arg))下列範例定義 Lambda 運算式，會傳回陣列中最後一個元素的索引。
Dim numbers() As Integer = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9} Dim lastIndex = Function(intArray() As Integer) _ intArray.Length - 1 For i = 0 To lastIndex(numbers) numbers(i) = numbers(i) + 1 Next
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工作
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