Lambda 運算式 (Visual Basic)
Lambda 運算式是沒有名稱的函式或副程式，可在委派有效的地方使用。 Lambda 運算式可以是函式或副程式，而且可以是單行或多行。 您可以將值從目前範圍傳遞至 Lambda 運算式。
注意
RemoveHandler 陳述式是例外狀況。 您無法針對 RemoveHandler 的委派參數傳遞 Lambda 運算式。
您可以使用 Function 或 Sub 關鍵字來建立 Lambda 運算式，就像建立標準函式或副程式一樣。 不過，Lambda 運算式會包含在陳述式中。
下列範例是 Lambda 運算式，會遞增其引數並傳回值。 此範例顯示函式的單行和多行 Lambda 運算式語法。
Dim increment1 = Function(x) x + 1
Dim increment2 = Function(x)
Return x + 2
End Function
' Write the value 2.
Console.WriteLine(increment1(1))
' Write the value 4.
Console.WriteLine(increment2(2))
下列範例是 Lambda 運算式，會將值寫入主控台。 此範例顯示副程式的單行和多行 Lambda 運算式語法。
Dim writeline1 = Sub(x) Console.WriteLine(x)
Dim writeline2 = Sub(x)
Console.WriteLine(x)
End Sub
' Write "Hello".
writeline1("Hello")
' Write "World"
writeline2("World")
請注意，在先前的範例中，Lambda 運算式會指派給變數名稱。 每當您參考變數時，就會叫用 Lambda 運算式。 您也可以同時宣告及叫用 Lambda 運算式，如下列範例所示。
Console.WriteLine((Function(num As Integer) num + 1)(5))
您可以將 Lambda 運算式作為函式呼叫的值傳回 (如本主題稍後的內容區段中的範例所示)，或以引數形式傳入採用委派型別的參數，如下列範例所示。
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
' delegate function that takes an integer as input and returns
' a boolean.
' 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
Lambda 運算式語法
Lambda 運算式的語法類似於標準函式或副程式的語法。 差異如下：
Lambda 運算式沒有名稱。
Lambda 運算式不能有修飾元，例如
Overloads或Overrides。單行 Lambda 函式不會使用
As子句來指定傳回型別。 相反地，系統會從 Lambda 運算式主體評估的值推斷型別。 例如，如果 Lambda 運算式的主體為cust.City = "London"，則其傳回型別為Boolean。在多行 Lambda 函式中，您可以使用
As子句指定傳回型別，或省略As子句，以便推斷傳回型別。 當多行 Lambda 函式省略As子句時，傳回型別會推斷為多行 Lambda 函式中所有Return陳述式的主控型別。 主控型別是所有其他型別都可以擴展至的唯一型別。 如果無法決定此唯一型別，則主控型別將成為陣列中其他型別皆可精簡而成的唯一型別。 如果這些類型皆無法決定，則主類型為Object。 在此情況下，如果Option Strict設定為On，就會發生編譯器錯誤。例如，如果提供給
Return陳述式的運算式含有型別Integer、Long和Double的值，則結果陣列的型別是Double。Integer和Long擴展為Double，且只有Double。 因此，Double是主類型。 如需詳細資訊，請參閱 Widening and Narrowing Conversions。單行函式的主體必須是傳回值的運算式，而不是陳述式。 單行函式沒有
Return陳述式。 單行函式所傳回的值，是函式主體中運算式的值。單行副程式主體必須是單行陳述式。
單行函式和副程式不包含
End Function或End Sub陳述式。您可以使用
As關鍵字來指定 Lambda 運算式參數的資料類型，也可以推斷參數的資料類型。 所有參數都必須具有指定的資料類型，或必須推斷所有資料類型。不允許
Optional和Paramarray參數。不允許泛型參數。
非同步 Lambda
您可以使用 Async 和 Await 運算子關鍵字，輕鬆建立結合非同步處理的 Lambda 運算式和陳述式。 例如，下列 Windows Form 範例包含呼叫並等候非同步方法 ExampleMethodAsync的事件處理常式。
Public Class Form1
Async Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
' ExampleMethodAsync returns a Task.
Await ExampleMethodAsync()
TextBox1.Text = vbCrLf & "Control returned to button1_Click."
End Sub
Async Function ExampleMethodAsync() As Task
' The following line simulates a task-returning asynchronous process.
Await Task.Delay(1000)
End Function
End Class
您可以在 AddHandler 陳述式中使用非同步 Lambda 來新增相同的事件處理常式。 若要加入這個處理常式，請將 Async 修飾詞加入至 Lambda 參數清單前面，如下列範例所示。
Public Class Form1
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
AddHandler Button1.Click,
Async Sub(sender1, e1)
' ExampleMethodAsync returns a Task.
Await ExampleMethodAsync()
TextBox1.Text = vbCrLf & "Control returned to Button1_ Click."
End Sub
End Sub
Async Function ExampleMethodAsync() As Task
' The following line simulates a task-returning asynchronous process.
Await Task.Delay(1000)
End Function
End Class
如需如何建立和使用非同步方法的詳細資訊，請參閱使用 Async 和 Await 進行非同步程式設計。
上下文
Lambda 運算式會與其定義所在的範圍共用其內容。 它具有與在包含範圍中撰寫的任何程式碼相同的存取權限。 這包括存取包含範圍中的成員變數、函式和子函式、Me 以及參數和區域變數。
存取包含範圍中的區域變數和參數，可以延伸到該範圍的存留期之外。 只要參考 Lambda 運算式的委派無法用於記憶體回收，就會保留原始環境中變數的存取權。 在下列範例中，變數 target 是本機 makeTheGame，這是定義 Lambda 運算式 playTheGame 的方法。 請注意，在 Main 中指派給 takeAGuess 的傳回 Lambda 運算式，仍可存取區域變數 target。
Module Module6
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 運算式的各種存取權限。 當傳回的 Lambda 運算式以 aDel 的形式從 Main 執行時，它會存取下列元素：
定義類別的欄位：
aField定義類別的屬性：
aProp方法
functionWithNestedLambda的參數，其定義於：level1functionWithNestedLambda的區域變數：localVarLambda 運算式的參數，其巢狀結構於：
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
' localVar = 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
轉換成委派型別
Lambda 運算式可以隱含地轉換成相容的委派型別。 如需相容性的一般需求相關資訊，請參閱寬鬆委派轉換。 例如，下列程式碼範例顯示隱含轉換成 Func(Of Integer, Boolean) 或相符委派簽章的 Lambda 運算式。
' Explicitly specify a delegate type.
Delegate Function MultipleOfTen(ByVal num As Integer) As Boolean
' This function matches the delegate type.
Function IsMultipleOfTen(ByVal num As Integer) As Boolean
Return num Mod 10 = 0
End Function
' This method takes an input parameter of the delegate type.
' The checkDelegate parameter could also be of
' type Func(Of Integer, Boolean).
Sub CheckForMultipleOfTen(ByVal values As Integer(),
ByRef checkDelegate As MultipleOfTen)
For Each value In values
If checkDelegate(value) Then
Console.WriteLine(value & " is a multiple of ten.")
Else
Console.WriteLine(value & " is not a multiple of ten.")
End If
Next
End Sub
' This method shows both an explicitly defined delegate and a
' lambda expression passed to the same input parameter.
Sub CheckValues()
Dim values = {5, 10, 11, 20, 40, 30, 100, 3}
CheckForMultipleOfTen(values, AddressOf IsMultipleOfTen)
CheckForMultipleOfTen(values, Function(num) num Mod 10 = 0)
End Sub
下列程式碼範例顯示隱含轉換成 Sub(Of Double, String, Double) 或相符委派簽章的 Lambda 運算式。
Module Module1
Delegate Sub StoreCalculation(ByVal value As Double,
ByVal calcType As String,
ByVal result As Double)
Sub Main()
' Create a DataTable to store the data.
Dim valuesTable = New DataTable("Calculations")
valuesTable.Columns.Add("Value", GetType(Double))
valuesTable.Columns.Add("Calculation", GetType(String))
valuesTable.Columns.Add("Result", GetType(Double))
' Define a lambda subroutine to write to the DataTable.
Dim writeToValuesTable = Sub(value As Double, calcType As String, result As Double)
Dim row = valuesTable.NewRow()
row(0) = value
row(1) = calcType
row(2) = result
valuesTable.Rows.Add(row)
End Sub
' Define the source values.
Dim s = {1, 2, 3, 4, 5, 6, 7, 8, 9}
' Perform the calculations.
Array.ForEach(s, Sub(c) CalculateSquare(c, writeToValuesTable))
Array.ForEach(s, Sub(c) CalculateSquareRoot(c, writeToValuesTable))
' Display the data.
Console.WriteLine("Value" & vbTab & "Calculation" & vbTab & "Result")
For Each row As DataRow In valuesTable.Rows
Console.WriteLine(row(0).ToString() & vbTab &
row(1).ToString() & vbTab &
row(2).ToString())
Next
End Sub
Sub CalculateSquare(ByVal number As Double, ByVal writeTo As StoreCalculation)
writeTo(number, "Square ", number ^ 2)
End Sub
Sub CalculateSquareRoot(ByVal number As Double, ByVal writeTo As StoreCalculation)
writeTo(number, "Square Root", Math.Sqrt(number))
End Sub
End Module
當您將 Lambda 運算式指派給委派，或將它們作為引數傳遞至程序時，您可以指定參數名稱，但省略其資料類型，讓類型取自委派。
範例
下列範例會定義 Lambda 運算式，如果可為 Null 的實值型別引數具有指派的值，則傳回
True，如果其值為Nothing，則傳回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))下列範例會定義 Lambda 運算式，以傳回陣列中最後一個元素的索引。
Dim numbers() = {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) += 1 Next
