Lambda 表达式 (Visual Basic)
Lambda 表达式是没有名称的函数或子例程，可在委托有效的任何位置使用。 Lambda 表达式可以是函数或子例程，可以是单行或多行。 你可以将值从当前范围传递到 Lambda 表达式。
注意
RemoveHandler 语句会引发异常。 不能为 RemoveHandler 的委托参数传入一个 Lambda 表达式。
使用 Function 或 Sub 关键字创建 Lambda 表达式，就像创建标准函数或子例程一样。 但是，Lambda 表达式包含在语句中。
下面的示例是一个 Lambda 表达式，它递增其参数并返回值。 示例演示了函数的单行和多行 Lambda 表达式语法。
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))
下面的示例是一个 Lambda 表达式，它将值写入控制台。 示例演示了子例程的单行和多行 Lambda 表达式语法。
Dim writeline1 = Sub(x) Console.WriteLine(x)
Dim writeline2 = Sub(x)
Console.WriteLine(x)
End Sub
' Write "Hello".
writeline1("Hello")
' Write "World"
writeline2("World")
请注意，在以上示例中，将 Lambda 表达式分配给了变量名称。 每当引用该变量时，都会调用 Lambda 表达式。 还可以同时声明和调用 Lambda 表达式，如以下示例所示。
Console.WriteLine((Function(num As Integer) num + 1)(5))
Lambda 表达式可以作为函数调用的值返回（如本主题后面的上下文部分中的示例所示），或者作为实参传递给采用委托类型的形参，如下例所示。
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 表达式语法
Lambda 表达式的语法类似于标准函数或子例程的语法。 不同之处如下：
Lambda 表达式不具有名称。
Lambda 表达式不能包含修饰符，如
Overloads或Overrides。单行 Lambda 函数不使用
As子句来指定返回类型。 而是从 Lambda 表达式主体计算出的值推断返回类型。 例如，如果 Lambda 表达式的主体为cust.City = "London"，则其返回类型为Boolean。在多行 lambda 函数中，可使用
As子句指定返回类型，也可省略As子句来推断返回类型。 在多行 lambda 函数中省略As子句时，可推断返回类型为多行 lambda 函数中所有Return语句的基准类型。 基准类型是所有其他类型可以扩大到的唯一类型。 如果无法确定此唯一类型，则基准类型是数组中所有其他类型可以缩小到的唯一类型。 如果无法确定为这两种唯一类型之一，则基准类型是Object。 在这种情况下，如果Option Strict设置为On，会发生编译器错误。例如，如果提供给
Return语句的表达式包含Integer、Long和Double类型的值，则生成的数组类型是Double。Integer和Long都扩大到Double，并且仅扩大到Double。 因此，Double是基准类型。 有关详细信息，请参阅 Widening and Narrowing Conversions。单行函数的主体必须是一个返回值的表达式，而不是语句。 单行函数中没有
Return语句。 单行函数返回的值是函数主体中表达式的值。单行子例程的主体必须是单行语句。
单行函数和子例程不包括
End Function或End Sub语句。可以使用
As关键字指定 Lambda 表达式参数的数据类型，也可以推断参数的数据类型。 所有参数都必须具有指定的数据类型，或者必须推断所有数据类型。不允许使用
Optional和Paramarray参数。不允许使用泛型参数。
异步 lambda
通过使用 Async 和 Await 运算符关键字，可以轻松创建包含异步处理的 Lambda 表达式和语句。 例如，下面的 Windows 窗体示例包含一个调用和等待异步方法 ExampleMethodAsync的事件处理程序。
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
你可以在 AddHandler 语句中使用异步 lambda 添加同一事件处理程序。 若要添加此处理程序，请在 lambda 参数列表前添加一个 Async 修饰符，如下例所示。
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
若要详细了解如何创建和使用异步方法，请参阅使用 Async 和 Await 的异步编程。
上下文
Lambda 表达式与定义它的范围共享其上下文。 它的访问权限与在包含范围内编写的任何代码相同。 这包括对成员变量、函数和子项、Me 以及包含范围内的参数和局部变量的访问权限。
对包含范围内的局部变量和参数的访问权限在该范围的生存期后仍然有效。 只要引用 Lambda 表达式的委托不可用于垃圾回收，就保留对原始环境中的变量的访问权限。 在下面的示例中，变量 target 是 makeTheGame 的局部变量，即在其中定义 Lambda 表达式 playTheGame 的方法。 请注意，在 takeAGuess 中分配给 Main 的已返回的 Lambda 表达式仍然可以访问局部变量 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
下面的示例演示了嵌套 Lambda 表达式的各种访问权限。 当返回的 Lambda 表达式从 Main 作为 aDel 执行时，它会访问这些元素：
定义它的类的字段：
aField定义它的类的属性：
aProp定义它的
functionWithNestedLambda方法的参数：level1functionWithNestedLambda的局部变量：localVar嵌套它的 Lambda 表达式的参数：
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
转换为委托类型
Lambda 表达式可以隐式转换为兼容的委托类型。 若要了解有关兼容性的一般要求，请参阅宽松委托转换。 例如，下面的代码示例演示了隐式转换为 Func(Of Integer, Boolean) 或匹配委托签名的 Lambda 表达式。
' 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
下面的代码示例演示了隐式转换为 Sub(Of Double, String, Double) 或匹配委托签名的 Lambda 表达式。
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
将 Lambda 表达式分配给委托或将其作为参数传递给过程时，可指定参数名称但省略其数据类型，以便从委托中获取类型。
示例
以下示例定义了一个 Lambda 表达式。如果可为 null 的值类型参数具有赋值，则该表达式返回
True；如果其值为Nothing，则返回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))下面的示例定义了一个 Lambda 表达式，它返回数组中最后一个元素的索引。
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
