Set default Wave Out Audio Device - VB.Net / DRVM_MAPPER_PREFERRED_SET

You can set the default audio playback device in windows 2000, Me, and XP with the DRVM_MAPPER_PREFERRED_SET message, which is sent with waveOutMessage().

It doesn't work on vista - you get MMSYSERR_NOTSUPPORTED (8), returned.
You can do it in Vista - see http://vachanger.sourceforge.net/

Anyway, the code uses the Windows Media .Net library.

Imports MultiMedia ' http://windowsmedianet.sourceforge.net/
Imports System.Runtime.InteropServices
Imports System.ComponentModel

Public Class Form1

    Private DevicesComboBox As New ComboBox
    Private DefaultDeviceLabel As New Label
    Private WithEvents SetDefaultButton As New Button
    Private Const DRVM_MAPPER_PREFERRED_GET As Integer = &H2015
    Private Const DRVM_MAPPER_PREFERRED_SET As Integer = &H2016
    Private WAVE_MAPPER As New IntPtr(-1)

    ' This just brings together a device ID and a WaveOutCaps so 
    ' that we can store them in a combobox.
    Private Structure WaveOutDevice

        Private m_id As Integer
        Public Property Id() As Integer
            Get
                Return m_id
            End Get
            Set(ByVal value As Integer)
                m_id = value
            End Set
        End Property

        Private m_caps As WaveOutCaps
        Public Property WaveOutCaps() As WaveOutCaps
            Get
                Return m_caps
            End Get
            Set(ByVal value As WaveOutCaps)
                m_caps = value
            End Set
        End Property

        Sub New(ByVal id As Integer, ByVal caps As WaveOutCaps)
            m_id = id
            m_caps = caps
        End Sub

        Public Overrides Function ToString() As String
            Return WaveOutCaps.szPname
        End Function

    End Structure

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        ' I do use the IDE for this stuff normally... (in case anyone is wondering)
        Me.Controls.AddRange(New Control() {DevicesComboBox, DefaultDeviceLabel, SetDefaultButton})
        DevicesComboBox.Location = New Point(5, 5)
        DevicesComboBox.DropDownStyle = ComboBoxStyle.DropDownList
        DevicesComboBox.Width = Me.ClientSize.Width - 10
        DevicesComboBox.Anchor = AnchorStyles.Left Or AnchorStyles.Right
        DefaultDeviceLabel.Location = New Point(DevicesComboBox.Left, DevicesComboBox.Bottom + 5)
        DefaultDeviceLabel.AutoSize = True
        SetDefaultButton.Location = New Point(DefaultDeviceLabel.Left, DefaultDeviceLabel.Bottom + 5)
        SetDefaultButton.Text = "Set Default"
        SetDefaultButton.AutoSize = True
        RefreshInformation()
    End Sub

    Private Sub RefreshInformation()
        PopulateDeviceComboBox()
        DisplayDefaultWaveOutDevice()
    End Sub

    Private Sub PopulateDeviceComboBox()
        DevicesComboBox.Items.Clear()
        ' How many wave out devices are there? WaveOutGetNumDevs API call.
        Dim waveOutDeviceCount As Integer = waveOut.GetNumDevs()
        For i As Integer = 0 To waveOutDeviceCount - 1
            Dim caps As New WaveOutCaps
            ' Get a name - its in a WAVEOUTCAPS structure. 
            ' The name is truncated to 31 chars by the api call. You probably have to 
            ' dig around in the registry to get the full name.
            Dim result As Integer = waveOut.GetDevCaps(i, caps, Marshal.SizeOf(caps))
            If result <> MMSYSERR.NoError Then
                Dim err As MMSYSERR = DirectCast(result, MMSYSERR)
                Throw New Win32Exception("GetDevCaps() error, Result: " & result.ToString("x8") & ", " & err.ToString)
            End If
            DevicesComboBox.Items.Add(New WaveOutDevice(i, caps))
        Next
        DevicesComboBox.SelectedIndex = 0
    End Sub

    Private Sub DisplayDefaultWaveOutDevice()
        Dim currentDefault As Integer = GetIdOfDefaultWaveOutDevice()
        Dim device As WaveOutDevice = DirectCast(DevicesComboBox.Items(currentDefault), WaveOutDevice)
        DefaultDeviceLabel.Text = "Defualt: " & device.WaveOutCaps.szPname
    End Sub

    Private Function GetIdOfDefaultWaveOutDevice() As Integer        
        Dim id As Integer = 0
        Dim hId As IntPtr
        Dim flags As Integer = 0
        Dim hFlags As IntPtr
        Dim result As Integer
        Try
            ' It would be easier to declare a nice overload with ByRef Integers.
            hId = Marshal.AllocHGlobal(4)
            hFlags = Marshal.AllocHGlobal(4)
            ' http://msdn.microsoft.com/en-us/library/bb981557.aspx
            result = waveOut.Message(WAVE_MAPPER, DRVM_MAPPER_PREFERRED_GET, hId, hFlags)
            If result <> MMSYSERR.NoError Then
                Dim err As MMSYSERR = DirectCast(result, MMSYSERR)
                Throw New Win32Exception("waveOutMessage() error, Result: " & result.ToString("x8") & ", " & err.ToString)
            End If
            id = Marshal.ReadInt32(hId)
            flags = Marshal.ReadInt32(hFlags)
        Finally
            Marshal.FreeHGlobal(hId)
            Marshal.FreeHGlobal(hFlags)
        End Try
        ' There is only one flag, DRVM_MAPPER_PREFERRED_FLAGS_PREFERREDONLY, defined as 1
        ' "When the DRVM_MAPPER_PREFERRED_FLAGS_PREFERREDONLY flag bit is set, ... blah ...,  
        ' the waveIn and waveOut APIs use only the current preferred device and do not search 
        ' for other available devices if the preferred device is unavailable. 
        Return id
    End Function

    Private Sub SetDefaultButton_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles SetDefaultButton.Click
        If DevicesComboBox.Items.Count = 0 Then Return
        Dim selectedDevice As WaveOutDevice = DirectCast(DevicesComboBox.SelectedItem, WaveOutDevice)
        SetDefault(selectedDevice.Id)
        RefreshInformation()
    End Sub

    Private Sub SetDefault(ByVal id As Integer)
        Dim defaultId As Integer = GetIdOfDefaultWaveOutDevice()
        If defaultId = id Then Return ' no change.
        Dim result As Integer
        ' So here we say "change the Id of the device that has id id to 0", which makes it the default.
        result = waveOut.Message(WAVE_MAPPER, DRVM_MAPPER_PREFERRED_SET, New IntPtr(id), IntPtr.Zero)
        If result <> MMSYSERR.NoError Then
            Dim err As MMSYSERR = DirectCast(result, MMSYSERR)
            Throw New Win32Exception("waveOutMessage() error, Result: " & result.ToString("x8") & ", " & err.ToString)
        End If
    End Sub

End Class