Hi All,
I am upgrading my app (working well with VB6) to VB.NET. It sends MIDI
messages using the API multimedia winmm.dll .
These functions are declared:
Public Declare Function timeSetEvent Lib "winmm.dll" (ByVal uDelay As
Integer, _
ByVal uResolution As Integer, ByVal lpFunction As MidiPlayDelegate, _
ByVal dwUser As Integer, ByVal uFlags As Integer) As Integer
Delegate Function MidiPlayDelegate(ByVal lTimerID As Integer, ByVal dwMsg As
Integer, _
ByVal dwUser As Integer, ByVal lParam1 As Integer, ByVal lParam2 As
Integer) As Integer
and a function MidiPlay is available:
Public Function MidiPlay(ByVal lTimerID As Integer, ByVal dwMsg As Integer,
_
ByVal dwUser As Integer, ByVal lParam1 As Integer, ByVal lParam2 As
Integer) As Integer
...
lRet = midiOutShortMsg(hMidiOut, mididata)
...
End Function
----------------------------------------------------------------------------
-----
A MIDI port is opened by a call to midiOutOpen, then a timer is started by
the statement
lTimerID = timeSetEvent(1, 1, AddressOf MidiPlay, 0, 1)
which runs in its own thread.
On each time event the call back function MidiPlay is called, which in turn
sends the MIDI data through
lRet = midiOutShortMsg(hMidiOut, mididata)
When the program is run a System.NullReferenceException occurs in an Unknown
Module (the
MidiPlayDelegate, I guess), because
"Object reference not set to an instance of an object"
Which object reference should be set?
Thank you for your help,
ReinierVDW 2 2908
Your problem is probably on this line:
lTimerID = timeSetEvent(1, 1, AddressOf MidiPlay, 0, 1)
The delegate created by the AddressOf MidiPlay statement does not have any
references to it so it will be garbage collected. If you need a delegate
that will stick around for future callbacks you must keep a live reference
to it. For example:
in your class create a class level delegate variable
Private m_TimerCallback as MDIPlayDelegate
then in the code that sets up the timer callbacks:
m_TimerCallback = New MDIPlayDelegate(AddressOf MDIPlay)
lTimerID = timeSetEvent(1, 1, m_TimerCallback, 0, 1)
now the delegate will stick around as long as your class does.
"R. van der Welle" <r.************@onzin.freeler.nl> wrote in message
news:eA**************@tk2msftngp13.phx.gbl... Hi All,
I am upgrading my app (working well with VB6) to VB.NET. It sends MIDI messages using the API multimedia winmm.dll . These functions are declared:
Public Declare Function timeSetEvent Lib "winmm.dll" (ByVal uDelay As Integer, _ ByVal uResolution As Integer, ByVal lpFunction As MidiPlayDelegate, _ ByVal dwUser As Integer, ByVal uFlags As Integer) As Integer
Delegate Function MidiPlayDelegate(ByVal lTimerID As Integer, ByVal dwMsg
As Integer, _ ByVal dwUser As Integer, ByVal lParam1 As Integer, ByVal lParam2 As Integer) As Integer
and a function MidiPlay is available: Public Function MidiPlay(ByVal lTimerID As Integer, ByVal dwMsg As
Integer, _ ByVal dwUser As Integer, ByVal lParam1 As Integer, ByVal lParam2 As Integer) As Integer ... lRet = midiOutShortMsg(hMidiOut, mididata) ... End Function --------------------------------------------------------------------------
-- -----
A MIDI port is opened by a call to midiOutOpen, then a timer is started by the statement lTimerID = timeSetEvent(1, 1, AddressOf MidiPlay, 0, 1) which runs in its own thread. On each time event the call back function MidiPlay is called, which in
turn sends the MIDI data through lRet = midiOutShortMsg(hMidiOut, mididata)
When the program is run a System.NullReferenceException occurs in an
Unknown Module (the MidiPlayDelegate, I guess), because "Object reference not set to an instance of an object"
Which object reference should be set?
Thank you for your help,
ReinierVDW
* "R. van der Welle" <r.************@onzin.freeler.nl> scripsit: A MIDI port is opened by a call to midiOutOpen, then a timer is started by the statement lTimerID = timeSetEvent(1, 1, AddressOf MidiPlay, 0, 1) which runs in its own thread. On each time event the call back function MidiPlay is called, which in turn sends the MIDI data through lRet = midiOutShortMsg(hMidiOut, mididata)
When the program is run a System.NullReferenceException occurs in an Unknown Module (the MidiPlayDelegate, I guess), because "Object reference not set to an instance of an object"
Which object reference should be set?
Don't forget to save a reference to the delegate!
Simple sample written by Armin Zingler:
\\\
dim o as CallBack
o = new CallBack(addressof CallBackSub)
'...
private sub CallBackSub
end sub
///
--
Herfried K. Wagner [MVP]
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