Lambda 表达式 (Visual Basic)Lambda Expressions (Visual Basic)

lambda 表达式是一个函数或子例程,没有名称,可以在委托有效的地方使用。A lambda expression is a function or subroutine without a name that can be used wherever a delegate is valid. Lambda 表达式可以是函数或子例程,可以是单行表达式或多行表达式。Lambda expressions can be functions or subroutines and can be single-line or multi-line. 可以将值从当前作用域传递到 lambda 表达式。You can pass values from the current scope to a lambda expression.

备注

RemoveHandler语句是一个例外。The RemoveHandler statement is an exception. 不能为 的委托参数传递 lambda 表达式RemoveHandlerYou cannot pass a lambda expression in for the delegate parameter of RemoveHandler.

使用FunctionorSub关键字创建 lambda 表达式,就像创建标准函数或子例程一样。You create lambda expressions by using the Function or Sub keyword, just as you create a standard function or subroutine. 但是,lambda 表达式包含在语句中。However, lambda expressions are included in a statement.

下面的示例是 lambda 表达式,该表达式递增其参数并返回值。The following example is a lambda expression that increments its argument and returns the value. 该示例显示了函数的单行和多行 lambda 表达式语法。The example shows both the single-line and multi-line lambda expression syntax for a function.

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 lambda 表达式,该表达式将值写入控制台。The following example is a lambda expression that writes a value to the console. 该示例显示了子例程的单行和多行 lambda 表达式语法。The example shows both the single-line and multi-line lambda expression syntax for a subroutine.

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,在前面的示例中,lambda 表达式被分配给一个变量名称。Notice that in the previous examples the lambda expressions are assigned to a variable name. 每当引用变量时,都会调用 lambda 表达式。Whenever you refer to the variable, you invoke the lambda expression. 您还可以同时声明和调用 lambda 表达式,如以下示例所示。You can also declare and invoke a lambda expression at the same time, as shown in the following example.

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"部分中的示例中所示),也可以作为参数传递给采用委托类型的参数,如以下示例所示。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 表达式语法Lambda Expression Syntax

lambda 表达式的语法类似于标准函数或子例程的语法。The syntax of a lambda expression resembles that of a standard function or subroutine. 不同之处如下:The differences are as follows:

  • lambda 表达式没有名称。A lambda expression does not have a name.

  • Lambda 表达式不能具有修饰符,如OverloadsOverridesLambda expressions cannot have modifiers, such as Overloads or Overrides.

  • 单行 lambda 函数不使用子句As来指定返回类型。Single-line lambda functions do not use an As clause to designate the return type. 相反,类型是从 lambda 表达式的正文计算到的值推断的。Instead, the type is inferred from the value that the body of the lambda expression evaluates to. 例如,如果 lambda 表达式的正文为cust.City = "London",则其返回类型Boolean为 。For example, if the body of the lambda expression is cust.City = "London", its return type is Boolean.

  • 在多行 lambda 函数中,可以使用As子句指定返回类型,或者省略As子句,以便推断返回类型。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. 当多As行 lambda 函数省略子句时,返回类型被推断为多行 lambda 函数中所有Return语句中的主要类型。When 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. 主导类型是所有其他类型都可以扩展到的唯一类型。The dominant type is a unique type that all other types can widen to. 如果无法确定此唯一类型,则主导类型是数组中所有其他类型都可以缩小到的唯一类型。If this unique type cannot be determined, the dominant type is the unique type that all other types in the array can narrow to. 如果无法确定为这两种唯一类型之一,则基准类型是 ObjectIf neither of these unique types can be determined, the dominant type is Object. 在这种情况下,如果Option Strict设置为On,则会发生编译器错误。In this case, if Option Strict is set to On, a compiler error occurs.

    例如Return,如果提供给语句的表达式包含 类型Integer的值 ,LongDouble, 生成的数组的类型Double为 。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. IntegerLong加宽Double至和Double仅。Both Integer and Long widen to Double and only Double. 因此, Double 是基准类型。Therefore, Double is the dominant type. 有关详细信息,请参阅 Widening and Narrowing ConversionsFor more information, see Widening and Narrowing Conversions.

  • 单行函数的正文必须是返回值的表达式,而不是语句。The body of a single-line function must be an expression that returns a value, not a statement. 单行函数Return没有语句。There is no Return statement for single-line functions. 单行函数返回的值是函数正文中表达式的值。The value returned by the single-line function is the value of the expression in the body of the function.

  • 单行子例程的正文必须是单行语句。The body of a single-line subroutine must be single-line statement.

  • 单行函数和子例程不包括 或End Function``End Sub语句。Single-line functions and subroutines do not include an End Function or End Sub statement.

  • 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。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. 所有参数都必须具有指定的数据类型,或者必须推断所有参数。Either all parameters must have specified data types or all must be inferred.

  • Optional不允许Paramarray和参数。Optional and Paramarray parameters are not permitted.

  • 不允许使用通用参数。Generic parameters are not permitted.

异步 lambdaAsync Lambdas

通过使用AsyncAwait 运算符关键字,可以轻松地创建包含异步处理的 lambda 表达式和语句。You can easily create lambda expressions and statements that incorporate asynchronous processing by using the Async and Await Operator keywords. 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。For example, the following Windows Forms example contains an event handler that calls and awaits an async method, ExampleMethodAsync.

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

您可以通过在AddHandler 语句中使用异步 lambda 来添加相同的事件处理程序。You can add the same event handler by using an async lambda in an AddHandler Statement. 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。To add this handler, add an Async modifier before the lambda parameter list, as the following example shows.

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

有关如何创建和使用异步方法的详细信息,请参阅使用异步和 Await 进行异步编程For more information about how to create and use async methods, see Asynchronous Programming with Async and Await.

上下文Context

lambda 表达式与其定义上下文的范围共享上下文。A lambda expression shares its context with the scope within which it is defined. 它具有与在包含作用域中编写的任何代码相同的访问权限。It has the same access rights as any code written in the containing scope. 这包括访问包含作用域中的成员变量、函数和子以及Me参数和局部变量。This includes access to member variables, functions and subs, Me, and parameters and local variables in the containing scope.

对包含作用域中的局部变量和参数的访问可以超出该作用域的生存期。Access to local variables and parameters in the containing scope can extend beyond the lifetime of that scope. 只要引用 lambda 表达式的委托不能用于垃圾回收,则将保留对原始环境中变量的访问。As long as a delegate referring to a lambda expression is not available to garbage collection, access to the variables in the original environment is retained. 在下面的示例中,变量target是 局部的makeTheGame, 在 其中定义 lambda 表达式playTheGame的方法。In the following example, variable target is local to makeTheGame, the method in which the lambda expression playTheGame is defined. 请注意,分配给takeAGuess``Main的返回 lambda 表达式仍有权访问局部变量targetNote that the returned lambda expression, assigned to takeAGuess in Main, still has access to the local variable 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

下面的示例演示嵌套 lambda 表达式的广泛访问权限。The following example demonstrates the wide range of access rights of the nested lambda expression. 当返回的 lambda 表达式从Main``aDel执行 时,它将访问以下元素:When the returned lambda expression is executed from Main as aDel, it accesses these elements:

  • 在其中定义它的类的字段:aFieldA field of the class in which it is defined: aField

  • 定义它的类的属性:aPropA property of the class in which it is defined: aProp

  • 方法functionWithNestedLambda的参数,level1A parameter of method functionWithNestedLambda, in which it is defined: level1

  • functionWithNestedLambda局部变量:localVarA local variable of functionWithNestedLambda: localVar

  • 嵌套在 lambda 表达式中的参数: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.
        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Converting to a Delegate Type

lambda 表达式可以隐式转换为兼容的委托类型。A lambda expression can be implicitly converted to a compatible delegate type. 有关兼容性的一般要求的信息,请参阅放宽委托转换For information about the general requirements for compatibility, see Relaxed Delegate Conversion. 例如,以下代码示例显示隐式转换为Func(Of Integer, Boolean)或匹配委托签名的 lambda 表达式。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, AddressOf IsMultipleOfTen)
    CheckForMultipleOfTen(values, Function(num) num Mod 10 = 0)
End Sub

下面的代码示例显示隐式转换为Sub(Of Double, String, Double)或匹配委托签名的 lambda 表达式。The following code example shows a lambda expression that implicitly converts to Sub(Of Double, String, Double) or a matching delegate signature.

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它们作为参数传递给过程时,可以指定参数名称,但省略其数据类型,允许从委托获取类型。When you assign lambda expressions to delegates or pass them as arguments to procedures, you can specify the parameter names but omit their data types, letting the types be taken from the delegate.

示例Examples

  • 下面的示例定义 lambda 表达式,如果空True值类型参数具有赋值,并且False其值为Nothing,则返回该表达式。The following example defines a lambda expression that returns True if the nullable value type 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))
    
  • 下面的示例定义一个 lambda 表达式,该表达式返回数组中最后一个元素的索引。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
    

请参阅See also