Hi,

My app uses the API structure MIDIHDR to receive MIDI long
(system-exclusive) messages and I have got a declaration problem.

The SDK declaration (in C) on MIDIHDR reads:
 typedef struct {
    LPSTR lpData
    DWORD dwBufferLength
    DWORD dwBytesRecorded
    DWORD dwUser
    DWORD dwFlags
    and so on
 } MIDIHDR;
where lpData is a pointer to a string sSysEx, which is filled by an API call
back routine (using that pointer).

The structure, an instance mhdr of the header, the string and its handle are
declared on main module level:
  <StructLayout(LayoutKind.Sequential)> _
  Public Structure MIDIHDR
     Public lpData As Integer
     Public dwBufferLength As Integer
     and so on ...
  End Structure
  Public mhdr As MIDIHDR
  Public sSysEx As String = New String("<data is entered here>")
To prevent garbage collection on the string it is pinned:
  Public GCHSysEx as GCHandle = GCHandle.Alloc(sSysEx, GCHandleType.Pinned)

Then the header is filled:
  mhdr.lpData = GCHSysEx.AddrOfPinnedObject.ToInt32
  mhdr.dwBufferLength = lenMsg
  mhdr.dwFlags = 0
  lengthHeader = Len(mhdr)

The declaration of lpData as Integer in the struct works, but is obviously
incorrect and will not run on 64-bits machines.
It should be:
  Dim lpData As IntPtr
and also
  mhdr.lpData = GCHSysEx.AddrOfPinnedObject
However, this yields the runtime error:
 System.ArgumentException: File I/O of a structure with field 'lpData' of
type 'IntPtr' is not valid
That error is given on the above statement:   lengthHeader = Len(mhdr)
because IntPtr is part of a struct and the VB Len function cannot determine
its length.

My question is how the pointer should be 'managed'?

Another question about MIDIHDR. It seems that this header is garbage
collected, because sometimes an AccessViolationException is thrown when its
handle is used. How should such a structure be pinned?

Thank you for your help.
Reinier