为什么这段c#代码不是类型安全的，为什么这段代码是类型安全的

这来自midi-dot-net http://code.google.com/p/midi-dot-net/库:

static class Win32API
{ ...
    #region Non-Typesafe Bindings
     // The bindings in this section are not typesafe, so we make them private
     // and provide typesafe variants    
     [DllImport("winmm.dll", SetLastError = true)]
     private static extern MMRESULT midiOutOpen(out HMIDIOUT lphmo, 
        UIntPtr uDeviceID, MidiOutProc dwCallback, UIntPtr dwCallbackInstance, 
        MidiOpenFlags dwFlags);
     ...
     /// <summary>
     /// Opens a MIDI output device.
     /// </summary>
     /// NOTE: This is adapted from the original Win32 function in order
     ///       to make it typesafe.
     ///
     /// Win32 docs: http://msdn.microsoft.com/en-us/library/ms711632(VS.85).aspx
     public static MMRESULT midiOutOpen(out HMIDIOUT lphmo,
        UIntPtr uDeviceID, MidiOutProc dwCallback, UIntPtr dwCallbackInstance)
     {
        return midiOutOpen(out lphmo, uDeviceID, dwCallback, dwCallbackInstance,
                  dwCallback == null ? MidiOpenFlags.CALLBACK_NULL :
                     MidiOpenFlags.CALLBACK_FUNCTION);
     }

最后一个函数如何使win32调用类型安全?