Visual studio Vb.net | Usb detection code partially working-CodePudding

I had to re-write the post because stackoverflow detects the post as already solved in another thread and i had to delete it. But the thread that referenced the forum was for C# and in anycase even reading it I can't find out a solution for my problem.

So, hello again guys :D

I did find somewhere online a code that allows me to detect usb devices and relative files, it works but not very well because each time i plug or unplug the usb device i get:

"System.NullReferenceException", i will not write the rest of the error to avoid that stackoverflow closes the thread, anyway you should have understood my issue.

Here the code:

Imports System.IO
Public Class Form1
#Region "USB EVENT"

    Private WM_DEVICECHANGE As Integer = &H219

    Public Enum WM_DEVICECHANGE_WPPARAMS As Integer
        DBT_CONFIGCHANGECANCELED = &H19
        DBT_CONFIGCHANGED = &H18
        DBT_CUSTOMEVENT = &H8006
        DBT_DEVICEARRIVAL = &H8000
        DBT_DEVICEQUERYREMOVE = &H8001
        DBT_DEVICEQUERYREMOVEFAILED = &H8002
        DBT_DEVICEREMOVECOMPLETE = &H8004
        DBT_DEVICEREMOVEPENDING = &H8003
        DBT_DEVICETYPESPECIFIC = &H8005
        DBT_DEVNODES_CHANGED = &H7
        DBT_QUERYCHANGECONFIG = &H17
        DBT_USERDEFINED = &HFFFF
    End Enum

    Private Structure DEV_BROADCAST_VOLUME
        Public dbcv_size As Int32
        Public dbcv_devicetype As Int32
        Public dbcv_reserved As Int32
        Public dbcv_unitmask As Int32
        Public dbcv_flags As Int16
    End Structure

    Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)

        Try
            If m.Msg = WM_DEVICECHANGE Then

                Dim Volume As DEV_BROADCAST_VOLUME

                Volume = DirectCast(Runtime.InteropServices.Marshal.PtrToStructure(m.LParam, GetType(DEV_BROADCAST_VOLUME)), DEV_BROADCAST_VOLUME)

                If Not GetDriveLetterFromMask(Volume.dbcv_unitmask).ToString.Trim = String.Empty Then

                    Dim DriveLetter As String = (GetDriveLetterFromMask(Volume.dbcv_unitmask) & ":\")

                    Select Case m.WParam

                        Case WM_DEVICECHANGE_WPPARAMS.DBT_DEVICEARRIVAL

                            LblUSB.Text = DriveLetter

                            GetFiles()
                            MsgBox("Usb connected")
                            '   Code When add USB

                        Case WM_DEVICECHANGE_WPPARAMS.DBT_DEVICEREMOVECOMPLETE

                            LblUSB.Text = ""

                            CheckedListBox1.Items.Clear()
                            MsgBox("Usb disconnected")
                            '   Code When Remove USB

                    End Select

                End If

            End If

        Catch ex As Exception

        End Try

        MyBase.WndProc(m)

    End Sub

    Private Function GetDriveLetterFromMask(ByRef Unit As Int32) As Char
        For i As Integer = 0 To 25
            If Unit = (2 ^ i) Then
                Return Chr(Asc("A")   i)
            End If
        Next
        Return ""
    End Function

#End Region

    Sub GetFiles()

        Try

            Dim Path = LblUSB.Text

            Dim LstFiles = My.Computer.FileSystem.GetFiles(Path, FileIO.SearchOption.SearchTopLevelOnly)

            For Each File In LstFiles
            Dim F As New IO.FileInfo(File)
            CheckedListBox1.Items.Add(F.Name)
        Next

        Catch ex As Exception

        End Try

    End Sub
End Class

The System.NullReferenceException i get is at this point:

 Volume = DirectCast(Runtime.InteropServices.Marshal.PtrToStructure(m.LParam, GetType(DEV_BROADCAST_VOLUME)), DEV_BROADCAST_VOLUME)

I did read that this error happens when the value is null and there should be a value associated, i tried different approaches but none did work and i hope that someone of you could give me a solution and an explaination to better understand that.

Thank you very much guys for the help!

CodePudding user response：

When one runs the code in VS 2019, beneath the System.NullReferenceException it states: System.Runtime.InteropServices.Marshal.PtrToStructure(...) returned Nothing which occurs because m.LParam is 0. This can be observed by placing the following code just before the DirectCast statement: Debug.WriteLine($"m.LParam: '{m.LParam.ToString()}'").

There isn't a need to attempt to get a drive letter until it's been detected that the USB device has been inserted. If one refactors the code as shown below, the issue doesn't occur:

Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)

    If m.Msg = WM_DEVICECHANGE Then
        Debug.WriteLine($"m.LParam: '{m.LParam.ToString()}'")

        Select Case CInt(m.WParam)
            Case WM_DEVICECHANGE_WPPARAMS.DBT_DEVICEARRIVAL
                Debug.WriteLine($"    DBT_DEVICEARRIVAL")

                Dim Volume As DEV_BROADCAST_VOLUME

                Volume = DirectCast(Runtime.InteropServices.Marshal.PtrToStructure(m.LParam, GetType(DEV_BROADCAST_VOLUME)), DEV_BROADCAST_VOLUME)

                If Not GetDriveLetterFromMask(Volume.dbcv_unitmask).ToString.Trim = String.Empty Then

                    Dim DriveLetter As String = (GetDriveLetterFromMask(Volume.dbcv_unitmask) & ":\")

                    LblUSB.Text = DriveLetter

                    GetFiles()
                    MsgBox("Usb connected")
                    '   Code When add USB
                End If

            Case WM_DEVICECHANGE_WPPARAMS.DBT_DEVICEREMOVECOMPLETE

                LblUSB.Text = ""

                CheckedListBox1.Items.Clear()
                MsgBox("Usb disconnected")
                '   Code When Remove USB

        End Select
    End If

    MyBase.WndProc(m)
End Sub

A side note: It's recommended to enable Option Strict for your project.

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