Why is this C# code not type-safe and why does this other bit make it type-safe?

This is from the midi-dot-net http://code.google.com/p/midi-dot-net/ library:

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,
开发者_开发技巧        UIntPtr uDeviceID, MidiOutProc dwCallback, UIntPtr dwCallbackInstance)
     {
        return midiOutOpen(out lphmo, uDeviceID, dwCallback, dwCallbackInstance,
                  dwCallback == null ? MidiOpenFlags.CALLBACK_NULL :
                     MidiOpenFlags.CALLBACK_FUNCTION);
     }

How does this last function make the win32 call type-safe?

Adapting my comment as an answer...

I don't know what makes the revised version type safe, but it is a safer (less error-prone) call.

It is possible to call dll function midiOutOpen parameter with a null dwCallbackInstance with MidiOpenFlags = MidiOpenFlags.CALLBACK_FUNCTION. If the dll function does not check for null then it will cause some disturbance.

With the adopted function the midiOutOpen parameter is derived so no danger.

I don't know what SetLastError = true is but I would think the wrapper might have checked the LastError and acted appropriately (throw exception?)