C# Signature:

  [DllImport("winmm.dll", SetLastError = true, CharSet = CharSet.Auto)]
  public static extern uint waveOutPrepareHeader(IntPtr hWaveOut, IntPtr pwh, int uSize);
  // Need to use IntPtr as WAVEHEADER struct must be in a fixed memory location. See Tips & Tricks below

[DllImport("winmm.dll", SetLastError = true, CharSet = CharSet.Auto)]

public static extern uint waveOutPrepareHeader(IntPtr hWaveOut, ref WAVEHDR lpWaveOutHdr, int uSize);

VB Signature:

Declare Function waveOutPrepareHeader Lib "winmm.dll" (TODO) As TODO

User-Defined Types:

None.

Alternative Managed API:

Do you know one? Please contribute it!

Notes:

None.

Tips & Tricks:

The same WAVEHDR structure used with waveOutPrepareHeader() is also used with

the waveOutWrite() and waveOutUnPrepareHeader() functions. The latter is called

after the audio playback has been stopped with a call to waveOutReset(); The

audio driver will asynchronously set the WHDR_DONE bit in WAVEHDR.dwflags when

it has released the audio data block.

This means that the WAVEHDR struct passed to the waveOutWrite() function must be

allocated in unmanaged memory so it will survive after the call to waveOutWrite().

What I do is preallocate a block of unmanaged memory in my class's Open() method:

    waveHdrPtr = Marshal.AllocHGlobal(Marshal.SizeOf(waveHdr));

Then in the output method I use this:

    waveHdr.lpData = wavedata;  // audio buffer
    waveHdr.dwBufferLength = wf.nSamplesPerSec * wf.nBlockAlign;  // it's size
    waveHdr.dwFlags = 0;    // clear before waveOutPrepareHeader()

    Marshal.StructureToPtr(waveHdr, waveHdrPtr, true);    // copy to unmanaged memory

    if ((MMRESULT = waveOutPrepareHeader(waveDevice, waveHdrPtr, Marshal.SizeOf(waveHdr))) != MMSYSERR_NOERROR)
    {
       waveOutGetErrorText(MMRESULT, errmsg, MAXERRORLENGTH);

       MessageBox.Show(
       "waveOutPrepareHeader(): " + errmsg.ToString(),
        this.Text,
        MessageBoxButtons.OK,
        MessageBoxIcon.Error);

        return;
    }

    WAVEHDR wh = (WAVEHDR)Marshal.PtrToStructure(waveHdrPtr, typeof(WAVEHDR));  // copy struct back from unmanaged memory

    waveHdr = wh;  // reset managed struct

    waveHdr.dwFlags |= (WHDR_BEGINLOOP | WHDR_ENDLOOP);  // add the looping flag bits
    waveHdr.dwLoops = MINUS_ONE; ;

    Marshal.StructureToPtr(waveHdr, waveHdrPtr, true);   // and update the unmanaged struct.

Now we are ready to write:

    if ((MMRESULT = waveOutWrite(waveDevice, waveHdrPtr, Marshal.SizeOf(waveHdr))) != MMSYSERR_NOERROR)
        [snipped]

When we are done listening to this playback:

    while ((waveHdr.dwFlags & WHDR_DONE) == 0)  // wait for it
    {
       if ((MMRESULT = waveOutReset(waveDevice)) != MMSYSERR_NOERROR)
       {
        waveOutGetErrorText(MMRESULT, errmsg, MAXERRORLENGTH);

        MessageBox.Show(
        "waveOutReset(): " + errmsg.ToString(),
        this.Text,
        MessageBoxButtons.OK,
        MessageBoxIcon.Error);
       }

       WAVEHDR wh = (WAVEHDR)Marshal.PtrToStructure(waveHdrPtr, typeof(WAVEHDR)); // copy unmanaged struct

       waveHdr = wh;  // update managed struct
    }

When the WHDR_DONE bit sets we can call the waveOutUnPrepareHeader() function to release

the resources previously set up:

    if ((MMRESULT = waveOutUnprepareHeader(waveDevice, waveHdrPtr, Marshal.SizeOf(waveHdr))) != MMSYSERR_NOERROR)
          [snipped]

I release the unmanaged memory allocated in the Open() method in the class's Close method:

        Marshal.FreeHGlobal(waveHdrPtr);

Sample Code:

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