VB.net 中，#if 是什么语法？

#If...Then...#Else 指令

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根据条件编译选定的 Visual Basic 代码块,需要有#Const 配对，一般要先用#Const 定义条件编译器常量

'以下是例子

Module Module1

#Const i = 60

Sub Main()

#If i 30 Then

Console.WriteLine("???") '如果用#Const定义了i，该句语句才会执行，假如用的是private i as integer=60定义，该语句不会被执行

#End If

End Sub

End Module

vb.net update语法

其他的没法看，不过下面这句是肯定有问题的，应该连编译都过不了：

Dim querystring As String = "update 班级信息表 set 班级名称="TextBox1.Text",班级编号="TextBox2.Text""

改称这样试试：

Dim querystring As String

querystring = "update 班级信息表 set 班级名称=" chr(39) TextBox1.Text chr(39) ", 班级编号=" chr(39) TextBox2.Text chr(39)

vb.net中如何用事件和委托，会C#中的事件和委托，但不知VB.net中的语法，望给个简单的例子熟悉语法。

一委托：此示例演示如何将方法与委托关联然后通过委托调用该方法。

创建委托和匹配过程

创建一个名为 MySubDelegate 的委托。

Delegate Sub MySubDelegate(ByVal x As Integer)

声明一个类，该类包含与该委托具有相同签名的方法。

Class class1

Sub Sub1(ByVal x As Integer)

MsgBox("The value of x is: " CStr(x))

End Sub

End Class

定义一个方法，该方法创建该委托的实例并通过调用内置的 Invoke 方法调用与该委托关联的方法。

Protected Sub DelegateTest()

Dim c1 As New class1

' Create an instance of the delegate.

Dim msd As MySubDelegate = AddressOf c1.Sub1

' Call the method.

msd.Invoke(10)

End Sub

二、事件

下面的示例程序阐释如何在一个类中引发一个事件，然后在另一个类中处理该事件。AlarmClock 类定义公共事件 Alarm，并提供引发该事件的方法。AlarmEventArgs 类派生自 EventArgs，并定义 Alarm 事件特定的数据。WakeMeUp 类定义处理 Alarm 事件的 AlarmRang 方法。AlarmDriver 类一起使用类，将使用 WakeMeUp 的 AlarmRang 方法设置为处理 AlarmClock 的 Alarm 事件。

该示例程序使用事件和委托和引发事件中详细说明的概念。

示例

' EventSample.vb.

'

Option Explicit

Option Strict

Imports System

Imports System.ComponentModel

Imports Microsoft.VisualBasic

Namespace EventSample

' Class that contains the data for

' the alarm event. Derives from System.EventArgs.

'

Public Class AlarmEventArgs

Inherits EventArgs

Private _snoozePressed As Boolean

Private nrings As Integer

'Constructor.

'

Public Sub New(snoozePressed As Boolean, nrings As Integer)

Me._snoozePressed = snoozePressed

Me.nrings = nrings

End Sub

' The NumRings property returns the number of rings

' that the alarm clock has sounded when the alarm event

' is generated.

'

Public ReadOnly Property NumRings() As Integer

Get

Return nrings

End Get

End Property

' The SnoozePressed property indicates whether the snooze

' button is pressed on the alarm when the alarm event is generated.

'

Public ReadOnly Property SnoozePressed() As Boolean

Get

Return _snoozePressed

End Get

End Property

' The AlarmText property that contains the wake-up message.

'

Public ReadOnly Property AlarmText() As String

Get

If _snoozePressed Then

Return "Wake Up!!! Snooze time is over."

Else

Return "Wake Up!"

End If

End Get

End Property

End Class

' Delegate declaration.

'

Public Delegate Sub AlarmEventHandler(sender As Object, _

e As AlarmEventArgs)

' The Alarm class that raises the alarm event.

'

Public Class AlarmClock

Private _snoozePressed As Boolean = False

Private nrings As Integer = 0

Private stopFlag As Boolean = False

' The Stop property indicates whether the

' alarm should be turned off.

'

Public Property [Stop]() As Boolean

Get

Return stopFlag

End Get

Set

stopFlag = value

End Set

End Property

' The SnoozePressed property indicates whether the snooze

' button is pressed on the alarm when the alarm event is generated.

'

Public Property SnoozePressed() As Boolean

Get

Return _snoozePressed

End Get

Set

_snoozePressed = value

End Set

End Property

' The event member that is of type AlarmEventHandler.

'

Public Event Alarm As AlarmEventHandler

' The protected OnAlarm method raises the event by invoking

' the delegates. The sender is always this, the current instance

' of the class.

'

Protected Overridable Sub OnAlarm(e As AlarmEventArgs)

RaiseEvent Alarm(Me, e)

End Sub

' This alarm clock does not have

' a user interface.

' To simulate the alarm mechanism it has a loop

' that raises the alarm event at every iteration

' with a time delay of 300 milliseconds,

' if snooze is not pressed. If snooze is pressed,

' the time delay is 1000 milliseconds.

'

Public Sub Start()

Do

nrings += 1

If stopFlag Then

Exit Do

Else

If _snoozePressed Then

System.Threading.Thread.Sleep(1000)

If (True) Then

Dim e As New AlarmEventArgs(_snoozePressed, nrings)

OnAlarm(e)

End If

Else

System.Threading.Thread.Sleep(300)

Dim e As New AlarmEventArgs(_snoozePressed, nrings)

OnAlarm(e)

End If

End If

Loop

End Sub

End Class

' The WakeMeUp class has a method AlarmRang that handles the

' alarm event.

'

Public Class WakeMeUp

Public Sub AlarmRang(sender As Object, e As AlarmEventArgs)

Console.WriteLine((e.AlarmText + ControlChars.Cr))

If Not e.SnoozePressed Then

If e.NumRings Mod 10 = 0 Then

Console.WriteLine(" Let alarm ring? Enter Y")

Console.WriteLine(" Press Snooze? Enter N")

Console.WriteLine(" Stop Alarm? Enter Q")

Dim input As String = Console.ReadLine()

If input.Equals("Y") Or input.Equals("y") Then

Return

Else

If input.Equals("N") Or input.Equals("n") Then

CType(sender, AlarmClock).SnoozePressed = True

Return

Else

CType(sender, AlarmClock).Stop = True

Return

End If

End If

End If

Else

Console.WriteLine(" Let alarm ring? Enter Y")

Console.WriteLine(" Stop Alarm? Enter Q")

Dim input As String = Console.ReadLine()

If input.Equals("Y") Or input.Equals("y") Then

Return

Else

CType(sender, AlarmClock).Stop = True

Return

End If

End If

End Sub

End Class

' The driver class that hooks up the event handling method of

' WakeMeUp to the alarm event of an Alarm object using a delegate.

' In a forms-based application, the driver class is the

' form.

'

Public Class AlarmDriver

Public Shared Sub Main()

' Instantiates the event receiver.

Dim w As New WakeMeUp()

' Instantiates the event source.

Dim clock As New AlarmClock()

' Wires the AlarmRang method to the Alarm event.

AddHandler clock.Alarm, AddressOf w.AlarmRang

clock.Start()

End Sub

End Class

End Namespace

如何使用VB.NET中可选参数调用方法

VB.NET可选参数的默认值必须是一个常数表达式。

过程定义中跟在可选参数后的每个参数也都必须是可选的。

下面的语法显示带VB.NET可选参数的过程声明：

Sub sub name（ByVal parameter 1 As data type 1,

Optional ByVal parameter 2 As data type 2 = default value）

调用带VB.NET可选参数的过程

过程在运行时无法检测到给定的参数是否已被省略，或者调用代码是否已显式提供默认值。如果需要弄清楚这一点，可以设置一个不可能的值作为默认值。下面的过程定义了可选参数 office,并测试其默认值 QJZ 以查看它在调用中是否已被省略：

Visual Basic

Sub notify（ByVal company As String, Optional ByVal office As String = "QJZ"）

If office = "QJZ" Then

Debug.WriteLine（"office not supplied -- using Headquarters"）

office = "Headquarters" End If

' Insert code to notify headquarters or specified office.

End Sub

如果可选参数是像 String 这样的引用类型，只要它不是该变量所预期的值，就可以使用 Nothing 作为默认值。

VB.NET可选参数和重载

定义带可选参数的过程的另一种方法是使用重载。如果有一个可选参数，可以定义过程的两个重载版本，一个接受此参数，另一个则不带参数。此方法随可选参数数目的增加而变得更复杂。然而，这样做的优点是可以完全确定调用程序是否提供了每个VB.NET可选参数。

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