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

Lambda 表达式是没有名称的函数或子例程, 只要委托有效, 就可以使用该函数。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, 可以是单行或多行。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语句是一个异常。The RemoveHandler statement is an exception. 不能为的委托参数RemoveHandler传递中的 lambda 表达式。You cannot pass a lambda expression in for the delegate parameter of RemoveHandler.

您可以使用FunctionSub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.

下面的示例是一个 lambda 表达式, 它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))

下面的示例是一个将值写入控制台的 lambda 表达式。The following example is a lambda expression that writes a value to the console. 该示例显示了子程序的单行和多行 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 表达式被分配给变量名。Notice that in the previous examples the lambda expressions are assigned to a variable name. 只要引用变量, 就会调用 lambda 表达式。Whenever you refer to the variable, you invoke the lambda expression. 还可以同时声明和调用 lambda 表达式, 如下例所示。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的示例中所示), 或者作为参数传递给采用委托类型的参数, 如下面的示例中所示。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 表达式的语法与标准函数或子例程的语法相似。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As子句来指定返回类型。Single-line lambda functions do not use an As clause to designate the return type. 相反, 该类型是从 lambda 表达式体的计算结果为的值推断出来的。Instead, the type is inferred from the value that the body of the lambda expression evaluates to. 例如, 如果 lambda 表达式的主体为cust.City = "London", 则其返回类型为。 BooleanFor 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As省略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. 如果为多行 lambda 函数省略Return 子句,则会将返回类型推断为来自多行lambda函数中所有语句的基准类型。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. 主导类型是所有其他类型可以扩大到的唯一类型。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. 如果这两种唯一类型都无法确定,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Option Strict , 如果将On设置为, 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为、 LongDouble的值, 则生成的数组的类型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. IntegerLongDouble扩大到和。 DoubleBoth 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,请参阅扩大和缩小转换For more information, see Widening and Narrowing Conversions.

  • 单行函数的主体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. 没有适用于单行函数的语句。ReturnThere is no Return statement for single-line functions. 单行函数返回的值是函数体中表达式的值。The value returned by the single-line function is the value of the expression in the body of the function.

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

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

  • 您可以使用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, 或者都必须被推断。Either all parameters must have specified data types or all must be inferred.

  • OptionalParamarray允许使用和参数。Optional and Paramarray parameters are not permitted.

  • 不允许使用泛型参数。Generic parameters are not permitted.

异步 lambdaAsync Lambdas

通过使用AsyncAwait 运算符关键字, 你可以轻松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  

可以通过在AddHandler 语句中使用 async lambda 来添加同一事件处理程序。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 async 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.

快捷Context

Lambda 表达式将其上下文与定义它的范围共享。A lambda expression shares its context with the scope within which it is defined. 它与在包含范围内编写的任何代码具有相同的访问权限。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、 Me函数和 sub、以及参数和局部变量。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范围的生存期。Access to local variables and parameters in the containing scope can extend beyond the lifetime of that scope. 只要引用 lambda 表达式的委托不可用于垃圾回收, 就会保留对原始环境中的变量的访问。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, target变量是的makeTheGame局部变量, 其中定义了 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takeAGuess给中Main的返回的 lambda 表达式仍有权访问本地变量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

下面的示例演示了嵌套 lambda 表达式的各种访问权限。The following example demonstrates the wide range of access rights of the nested lambda expression. 当从Main aDel执行返回的 lambda 表达式时, 它将访问这些元素:When the returned lambda expression is executed from Main as aDel, it accesses these elements:

  • 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

  • 定义它的方法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

  • 它在其中进行嵌套的 lambda 表达式的参数: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, 请参阅宽松委托转换For information about the general requirements for compatibility, see Relaxed Delegate Conversion. 例如, 下面的代码示例演示一个隐式转换为Func(Of Integer, Boolean)或匹配的委托签名的 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

下面的代码示例演示一个隐式转换为Sub(Of Double, String, Double)或匹配的委托签名的 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

当你将 lambda 表达式分配给委托或将其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, 从而使类型从委托中获取。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

  • 下面的示例定义一个 lambda 表达式, 该True表达式在可以为 null 的参数具有分配的False值时返回, Nothing如果其值为, 则返回。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))
    
  • 下面的示例定义了一个 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