# Lambda 運算式 (Visual Basic)Lambda Expressions (Visual Basic)

Lambda 運算式是一個不含名稱的函式或副程式, 可以在委派有效的任何地方使用。A lambda expression is a function or subroutine without a name that can be used wherever a delegate is valid. Lambda 運算式可以是函數或副程式, 而且可以是單行或多行。Lambda expressions can be functions or subroutines and can be single-line or multi-line. 您可以將值從目前範圍傳遞至 lambda 運算式。You can pass values from the current scope to a lambda expression.

RemoveHandler語句是例外狀況。The RemoveHandler statement is an exception. 您無法在中傳遞的 lambda 運算式做為的委派RemoveHandler參數。You cannot pass a lambda expression in for the delegate parameter of RemoveHandler.

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))


Dim writeline1 = Sub(x) Console.WriteLine(x)
Dim writeline2 = Sub(x)
Console.WriteLine(x)
End Sub

' Write "Hello".
writeline1("Hello")

' Write "World"
writeline2("World")


Console.WriteLine((Function(num As Integer) num + 1)(5))


Lambda 運算式可以當做函式呼叫的值傳回 (如本主題稍後的內容章節中的範例所示), 或當做引數傳遞給採用委派類型的參數, 如下列範例所示。A lambda expression can be returned as the value of a function call (as is shown in the example in the Context section later in this topic), or passed in as an argument to a parameter that takes a delegate type, as shown in the following example.

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 Expression Syntax

Lambda 運算式的語法類似于標準函式或副程式。The syntax of a lambda expression resembles that of a standard function or subroutine. 差異如下:The differences are as follows:

• Lambda 運算式沒有名稱。A lambda expression does not have a name.

• Lambda 運算式不能有修飾詞, Overloads例如Overrides或。Lambda expressions cannot have modifiers, such as Overloads or Overrides.

• 單行 lambda 函數不會使用As子句來指定傳回類型。Single-line lambda functions do not use an As clause to designate the return type. 相反地, 型別是從 lambda 運算式主體評估為的值推斷而來。Instead, the type is inferred from the value that the body of the lambda expression evaluates to. 例如, 如果 lambda 運算式的主體是cust.City = "London", 其傳回型別會是。 BooleanFor example, if the body of the lambda expression is cust.City = "London", its return type is Boolean.

• 在多行 lambda 函式中, 您可以使用As子句來指定傳回類型, 或As省略子句, 以推斷傳回類型。In multi-line lambda functions, you can either specify a return type by using an As clause, or omit the As clause so that the return type is inferred. 當多行 lambda 函數省略Return 子句時,會將傳回型別推斷為多行lambda函式中所有語句的主要型別。AsWhen the As clause is omitted for a multi-line lambda function, the return type is inferred to be the dominant type from all the Return statements in the multi-line lambda function. 主要類型是所有其他類型都可以擴展的唯一類型。The dominant type is a unique type that all other types can widen to. 如果無法判斷此唯一類型, 則主要類型是陣列中所有其他類型都可以縮小為的唯一類型。If this unique type cannot be determined, the dominant type is the unique type that all other types in the array can narrow to. 如果這些類型皆無法決定，則主類型為 ObjectIf neither of these unique types can be determined, the dominant type is Object. 在此情況下, Option Strict如果設定為On, 則會發生編譯器錯誤。In this case, if Option Strict is set to On, a compiler error occurs.

例如Return , 如果提供給語句的運算式包含型Integer別、 LongDouble的值, 則產生的陣列會是型Double別。For example, if the expressions supplied to the Return statement contain values of type Integer, Long, and Double, the resulting array is of type Double. 和只會擴大Double為和。 Double Long IntegerBoth Integer and Long widen to Double and only Double. 因此， Double 是主類型。Therefore, Double is the dominant type. 如需詳細資訊，請參閱 Widening and Narrowing ConversionsFor more information, see Widening and Narrowing Conversions.

• 單行函數的主體必須是傳回值的運算式, 而不是語句。The body of a single-line function must be an expression that returns a value, not a statement. 沒有單行函式的語句。ReturnThere is no Return statement for single-line functions. 單行函式所傳回的值是函數主體中的運算式值。The value returned by the single-line function is the value of the expression in the body of the function.

• 單行副程式的主體必須是單行語句。The body of a single-line subroutine must be single-line statement.

• 單行函數和副程式不包含End FunctionEnd Sub語句。Single-line functions and subroutines do not include an End Function or End Sub statement.

• 您可以使用As關鍵字來指定 lambda 運算式參數的資料類型, 或可以推斷參數的資料類型。You can specify the data type of a lambda expression parameter by using the As keyword, or the data type of the parameter can be inferred. 所有參數都必須具有指定的資料類型, 否則必須推斷全部。Either all parameters must have specified data types or all must be inferred.

• OptionalParamarray允許使用和參數。Optional and Paramarray parameters are not permitted.

• 不允許泛型參數。Generic parameters are not permitted.

## 非同步 LambdaAsync Lambdas

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.
End Function

End Class


Public Class Form1

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
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.
End Function

End Class


## 內容Context

Lambda 運算式會與其定義所在的範圍共用其內容。A lambda expression shares its context with the scope within which it is defined. 其存取權限與在包含範圍中撰寫的任何程式碼相同。It has the same access rights as any code written in the containing scope. 這包括存取成員變數、函式和Me子函式, 以及包含範圍內的參數和區域變數。This includes access to member variables, functions and subs, Me, and parameters and local variables in the containing scope.

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


• 其定義所在類別的欄位:aFieldA field of the class in which it is defined: aField

• 其定義所在之類別的屬性:aPropA property of the class in which it is defined: aProp

• 方法functionWithNestedLambda的參數, 其定義如下:level1A parameter of method functionWithNestedLambda, in which it is defined: level1

• functionWithNestedLambda本機變數:localVarA local variable of functionWithNestedLambda: localVar

• Lambda 運算式的參數, 其會在其中加以嵌套:level2A parameter of the lambda expression in which it is nested: 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.
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

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


## 轉換成委派類型Converting to a Delegate Type

Lambda 運算式可以隱含地轉換成相容的委派類型。A lambda expression can be implicitly converted to a compatible delegate type. 如需相容性一般需求的相關資訊, 請參閱寬鬆委派轉換For information about the general requirements for compatibility, see Relaxed Delegate Conversion. 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。For example, the following code example shows a lambda expression that implicitly converts to Func(Of Integer, Boolean) or a matching delegate signature.

' 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, Function(num) num Mod 10 = 0)
End Sub


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")

' 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
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


## 範例Examples

• 下列範例會定義 lambda 運算式, True如果可為 null 的引數具有指派的值, 且False其值為Nothing, 則會傳回。The following example defines a lambda expression that returns True if the nullable argument has an assigned value, and False if its value is Nothing.

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。The following example defines a lambda expression that returns the index of the last element in an array.

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