change volume of mp3 playing via wmplib in c#

Question

Is it somehow possible to change the volume of a mp3-file that is playing via wmplib? Changing the volume of the program itself would be ok as well.

Are there any solutions to do this?

Answer 1

The idea is to send WM_APPCOMMAND message (also see this answer).

For WPF use WindowInteropHelper to get the Handle of the Window:

class MainWindow : Window
{
    ...

    private const int APPCOMMAND_VOLUME_MUTE = 0x80000;
    private const int WM_APPCOMMAND = 0x319;
    private const int APPCOMMAND_VOLUME_UP = 10 * 65536;
    private const int APPCOMMAND_VOLUME_DOWN = 9 * 65536;

    [DllImport("user32.dll")]
    public static extern IntPtr SendMessageW(IntPtr hWnd, int Msg, IntPtr wParam, IntPtr lParam);

    private void VolumeUp()
    {
        // APPCOMMAND_VOLUME_UP or APPCOMMAND_VOLUME_DOWN
        var windowInteropHelper = new WindowInteropHelper(this);
        SendMessageW(windowInteropHelper.Handle, (IntPtr)WM_APPCOMMAND, windowInteropHelper.Handle, (IntPtr)APPCOMMAND_VOLUME_UP);
    }
}

For Windows Forms use Control.Handle Property:

class MainForm : Form
{
    ...

    private void VolumeUp()
    {
        SendMessageW(Handle, (IntPtr)WM_APPCOMMAND, Handle, (IntPtr)APPCOMMAND_VOLUME_UP);
    }
}