Question

使用VB.net中的现有代码。正常工作，直到将其他功能添加到代码中以执行其他任务为止。例如，当下面的代码在检测到插入的USB拇指驱动器时使按钮可见时，以下代码在“立即”窗口中引发异常。我猜这是因为按钮尚未完全创建而发生？无论如何，寻找有关如何解决此问题的方向。我要做的就是在插入USB驱动器时使该按钮可见，而在卸下USB驱动器时使该按钮不可见。预先感谢！

代码：

Imports System.Management ' Also had to add System.Management as a reference
Imports System
Imports System.ComponentModel

Public Class Form1

    Private WithEvents m_MediaConnectWatcher As ManagementEventWatcher
    Public USBDriveName As String
    Public USBDriveLetter As String

    Public Sub StartDetection()
        ' __InstanceOperationEvent will trap both Creation and Deletion of class instances
        Dim query2 As New WqlEventQuery("SELECT * FROM __InstanceOperationEvent WITHIN 1 " _
  & "WHERE TargetInstance ISA 'Win32_DiskDrive'")

        m_MediaConnectWatcher = New ManagementEventWatcher With {
            .Query = query2
        }
        m_MediaConnectWatcher.Start()
    End Sub


    Private Sub Arrived(ByVal sender As Object, ByVal e As System.Management.EventArrivedEventArgs) Handles m_MediaConnectWatcher.EventArrived

        Dim mbo, obj As ManagementBaseObject

        ' the first thing we have to do is figure out if this is a creation or deletion event
        mbo = CType(e.NewEvent, ManagementBaseObject)
        ' next we need a copy of the instance that was either created or deleted
        obj = CType(mbo("TargetInstance"), ManagementBaseObject)

        Select Case mbo.ClassPath.ClassName
            Case "__InstanceCreationEvent"
                If obj("InterfaceType") = "USB" Then
                    MsgBox("Technician maintenance key has been plugged in")
                    Button1.Visible = True
                Else
                    MsgBox(obj("InterfaceType"))
                End If
            Case "__InstanceDeletionEvent"
                If obj("InterfaceType") = "USB" Then
                    MsgBox("Technician maintenance key has been unplugged.")
                    Button1.Visible = False
                    If obj("Caption") = USBDriveName Then
                        USBDriveLetter = ""
                        USBDriveName = ""
                    End If
                Else
                    MsgBox(obj("InterfaceType"))
                End If
            Case Else
                MsgBox("nope: " & obj("Caption"))
        End Select
    End Sub

    Private Function GetDriveLetterFromDisk(ByVal Name As String) As String
        Dim oq_part, oq_disk As ObjectQuery
        Dim mos_part, mos_disk As ManagementObjectSearcher
        Dim obj_part, obj_disk As ManagementObject
        Dim ans As String = ""

        ' WMI queries use the "\" as an escape charcter
        Name = Replace(Name, "\", "\\")

        ' First we map the Win32_DiskDrive instance with the association called
        ' Win32_DiskDriveToDiskPartition. Then we map the Win23_DiskPartion
        ' instance with the assocation called Win32_LogicalDiskToPartition

        oq_part = New ObjectQuery("ASSOCIATORS OF {Win32_DiskDrive.DeviceID=""" & Name & """} WHERE AssocClass = Win32_DiskDriveToDiskPartition")
        mos_part = New ManagementObjectSearcher(oq_part)
        For Each obj_part In mos_part.Get()

            oq_disk = New ObjectQuery("ASSOCIATORS OF {Win32_DiskPartition.DeviceID=""" & obj_part("DeviceID") & """} WHERE AssocClass = Win32_LogicalDiskToPartition")
            mos_disk = New ManagementObjectSearcher(oq_disk)
            For Each obj_disk In mos_disk.Get()
                ans &= obj_disk("Name") & ","
            Next
        Next

        Return ans.Trim(","c)
    End Function

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        StartDetection()
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        m_MediaConnectWatcher.Stop()
        Application.Exit()
    End Sub

    Private Sub Form1_Closing(sender As Object, e As CancelEventArgs) Handles Me.Closing
        m_MediaConnectWatcher.Stop()
        Application.Exit()
    End Sub
End Class

Answer 1

当创建控件的线程（在您的情况下为Button1）以外的线程尝试调用该控件（ Button1.Visible = True或Button1.Visible = False ）时，调试器将引发一个InvalidOperationException消息，控件控件名称是从不是在其上创建线程的线程访问的。这可能是您看到的消息。从不同线程访问控件的一种方法是使用 Invoke（）：

Private Sub ShowButton()
    'InvokeRequired compares the thread ID of the  
    'calling thread to the thread ID of the creating thread.  
    'If these threads are different, it returns true.
    'If the calling thread is different from the thread that  
    'created the Button1 control, this method calls itself asynchronously using the  
    'Invoke method
    If Me.InvokeRequired Then
        Me.Invoke(Sub() ShowButton())
        Return
    End If

    Button1.Visible = True
End Sub

Private Sub HideButton()
    If Me.InvokeRequired Then
        Me.Invoke(Sub() HideButton())
        Return
    End If

    Button1.Visible = False
End Sub

并在您的代码中替换

Button1.Visible = True

使用

ShowButton()

和

Button1.Visible = False

使用

HideButton()

在VB.NET中检测到USB设备时如何使WinForm控件可见= True

1 个答案: