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.

RemoveHandler语句是一个异常。The RemoveHandler statement is an exception. 不能为的委托参数传递中的 lambda 表达式 RemoveHandlerYou 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 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，可以使用子句指定返回类型 As ，或省略 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. 如果 As 为多行 lambda 函数省略子句，则会将返回类型推断为来自 Return 多行 lambda 函数中所有语句的基准类型。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，如果将 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，如果提供给语句的表达式 Return 包含类型为、和的值， Integer Long Double 则生成的数组的类型为 DoubleFor example, if the expressions supplied to the Return statement contain values of type Integer, Long, and Double, the resulting array is of type Double. Integer和都 Long 仅扩大到 DoubleDoubleBoth 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 Return 适用于单行函数的语句。There 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.

• 您可以使用关键字指定 lambda 表达式参数的数据类型 As ，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不允许使用和参数。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

Public Class Form1

Async Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Await ExampleMethodAsync()
TextBox1.Text = vbCrLf & "Control returned to button1_Click."
End Sub

' The following line simulates a task-returning asynchronous process.
End Function

End Class


Public Class Form1

Async Sub(sender1, e1)
Await ExampleMethodAsync()
TextBox1.Text = vbCrLf & "Control returned to Button1_ Click."
End Sub
End Sub

' The following line simulates a task-returning asynchronous process.
End Function

End Class


上下文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、函数和 sub、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.

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


• 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

• 定义它的方法的参数 functionWithNestedLambdalevel1A parameter of method functionWithNestedLambda, in which it is defined: level1

• 的局部变量 functionWithNestedLambdalocalVarA 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.
lambdaScopeDemoInstance.functionWithNestedLambda(2)

' Now the returned lambda expression is called, with 4 as the
' value of parameter level3.

' Change a few values to verify that the lambda expression has
lambdaScopeDemoInstance.aField = 20
lambdaScopeDemoInstance.Prop = 30
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


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, 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


示例Examples

• 下面的示例定义一个 lambda 表达式，该表达式 True 在可以为 null 的值类型参数具有赋值时返回， False 如果其值为，则返回 NothingThe 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