[Débutant] Problème avec WaveInAddBuffer en VB.net

Bonjour,
Je voudrais écrire un programme accordeur qui donne la fréquence d'un son joué devant un micro, en visual basic 2010 sous Windows 7 familial sur un PC 64 bits. Voici un extrait du programme :

Code:

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    Public LongWhdr As Int32
    Public Phwi As Int32
    Public Structure WaveHdr
        Public lpData As Int32                    
        Public dwBufferLength As Int32      
        Public dwBytesRecorded As Int32  
        Public dwUser As Int32
        Public dwFlags As Int32
        Public dwLoops As Int32
        Public lpNext As Int32
        Public Reserved As Int32
    End Structure
   Public Xwhdr As WaveHdr
   Public Whdr(0 To 1) As WaveHdr
 
 Public Declare Function waveInAddBuffer Lib "winmm.dll" (ByVal hWaveIn As Int32, ByRef lpWaveInHdr As WaveHdr, ByVal uSize As Int32) As Int32
   Public Declare Function waveInOpen Lib "winmm.dll" (ByRef lphWaveIn As Int32, ByVal uDeviceID As Int32,                                                ByRef lpFormat As WaveFormatex, ByVal dwCallback As Int32, ByVal dwInstance As Int32,                                                 ByVal dwFlags As Int32) As Int32
   Public Declare Function waveInPrepareHeader Lib "winmm.dll" (ByVal hWaveIn As Int32, ByRef lpWaveInHdr As WaveHdr,  ByVal uSize As Int32) As Int32
 
        LongWhdr = Len(Xwhdr)
        Res = waveInOpen(Phwi, No_Periph, Wfe, Evt, 0, CallBack_Event)
        Lig = "WaveInOpen"
        Debug.Print(Lig)
        Res = waveInPrepareHeader(Phwi, Whdr(0), LongWhdr)
        Lig = " Res prepare " & Str(Res) & " " & Res.ToString
        Debug.Print(Lig)
        Res = waveInAddBuffer(Phwi, Whdr(0), LongWhdr)
        Lig = "waveInAddBuffer 0 " & Str(Res) & " " & Res.ToString
        Debug.Print (Lig)

J'obtiens bien les deux premiers debug "mouchards", puis je suis bloqué à la dernière instruction waveInAddBuffer sur le message:

Citation:

A first chance exception of type 'System.AccessViolationException' occurred in Accordeur.exe

Que se passe t-il ? Comment avancer ? Merci à tous ceux qui ont une idée.