[.NET] TinySoundFont ChannelNoteOn/ChannelNoteOff throw NullReferenceException during synthesis (1.8.4, 1.9.0-alpha.1891)

[Gungr]

Summary

In the C# port (AlphaTab NuGet 1.8.4, also present in 1.9.0-alpha.1891), TinySoundFont.ChannelNoteOn / ChannelNoteOff throw System.NullReferenceException when a MIDI file is actually synthesized — both via the audio exporter (AlphaSynth.ExportAudio → IAlphaSynthAudioExporter.Render) and via the realtime AlphaSynth.Play() path. The TypeScript/JS engine is unaffected.

Repro (minimal, .NET 8, Windows)

using AlphaTab;
using AlphaTab.Core.EcmaScript;
using AlphaTab.Midi;
using AlphaTab.Synth;

var sf2 = System.IO.File.ReadAllBytes("timgm6mb.sf2"); // any valid SoundFont
var synth = new AlphaSynth(new NAudioSynthOutput(), 50);
var ready = new System.Threading.ManualResetEventSlim();
synth.Ready.On(() => ready.Set());
synth.LoadSoundFont(new Uint8Array(sf2), false);
ready.Wait(TimeSpan.FromSeconds(20));

var file = new MidiFile { Format = MidiFileFormat.MultiTrack, Division = 960 };
var track = new MidiTrack();
track.AddEvent(new ProgramChangeEvent(0, 0, 0, 24));
track.AddEvent(new TempoChangeEvent(0, 500000));
track.AddEvent(new NoteOnEvent(0, 0, 0, 60, 100));
track.AddEvent(new NoteOffEvent(0, 960, 0, 60, 64));
file.Tracks.Add(track);

var options = new AudioExportOptions { SampleRate = 44100, MasterVolume = 1.0 };
var exporter = (AlphaSynthAudioExporter)synth.ExportAudio(options, file, null, null);
var chunk = exporter.Render(250); // -> NullReferenceException

Observed:

System.NullReferenceException: Object reference not set to an instance of an object.
   at AlphaTab.Synth.Synthesis.TinySoundFont.ChannelNoteOn(Double channel, Double key, Double vel)

Narrowing (decompiled IL, AlphaTab.dll 1.8.4)

1. TinySoundFont's constructor initializes _midiEventQueue, _mutedChannels, _soloChannels, _transpositionPitches, _liveTranspositionPitches, _metronomeChannel, and _voices = new List<Voice>() — but not _channels (matches the TS, where _channels is lazily created).
2. LoadSoundFont(Uint8Array) calls Reset() first, and Reset() sets _channels = null (TS equivalent keeps _channels and only stops voices — please verify the translation).
3. _channelInit(channel) recreates _channels lazily and its grow-loop matches the TS (i <= channel, inclusive), ending in return _channels.ChannelList[channel];.
4. Calling the font directly (via reflection) after LoadSoundFont:
   • ChannelSetPresetIndex(0, 24) → OK
   • ChannelNoteOn(0, 60, 100) → NRE (note: direct preset-level NoteOn(24, 60, 100) succeeds, which suggests the crash is in code inlined into ChannelNoteOn — e.g. inside NoteOn or the channel lookup)
   • ChannelNoteOff(0, 60) → NRE (this method iterates _voices; in the observed instance _channels held 17 fully-constructed Channel entries, so the null deref is elsewhere)
   • ChannelGetPresetIndex(0) → OK

Suggested first look: the state of _channels/_voices across the LoadSoundFont → Reset() → LoadPresets() sequence, and the NoteOn body that channelNoteOn inlines (e.g. preset lookup / voices.Add), since the TS originals of all these are null-safe and work in the JS engine.

Impact

Any C# consumer trying to synthesize audio (realtime playback or audio export) of a program-change + note-on MIDI file crashes. The audio features of the C# port are currently unusable with a loaded SoundFont.

Environment

• AlphaTab 1.8.4 (also reproduced on 1.9.0-alpha.1891), AlphaTab.Windows 1.8.4, .NET 8, Windows 10/11
• SoundFont: TimGM6mb (public-domain-ish GM set, 136 presets) — issue is not font-specific (reproduced with other GM fonts as well)

[github-actions]

Thanks for opening this. Before we dig in, a couple of things need addressing in the description:

• Use the bug report template. The bug report template doesn't appear to have been used. Please open the issue via the "Bug Report" template — the form collects the specific info we need to reproduce the problem.

If the description isn't updated within 7 days, this issue will be closed automatically and labeled state-rules-not-followed. Reopen any time — an edit to the description is all it takes.

If an AI helped draft this, AGENTS.md covers the same expectations from the AI side.