Why does my IL2CPP native plugin callback crash on UI access?

Native callbacks from C/C++ plugins fire on whatever thread the plugin uses, not the Unity main thread. Touching UnityEngine APIs from non-main threads crashes. Marshal the work back via a captured SynchronizationContext.

What's MonoPInvokeCallback for?

Required attribute on static methods passed as function pointers to native code. Without it, IL2CPP doesn't generate the native trampoline; the callback may fail silently or crash.

How do I capture the main thread context?

In an Awake method on the main thread, store SynchronizationContext.Current. Use it later from any thread via context.Post(action, null) to enqueue work onto Unity's main loop.

Fix: Unity IL2CPP Native Callback Wrong Thread

Quick answer: Decorate the static callback with [MonoPInvokeCallback(typeof(NativeDelegate))]. From inside, capture the main thread SynchronizationContext at startup; _main.Post(_ => {...}, null) to do UnityEngine work safely.

Native plugin fires a download-complete callback. Your Unity code tries to update a Text component. Crash — the callback ran on a worker thread.

The Symptom

IL2CPP build crashes inside an UnityEngine API call from a P/Invoke callback. Editor (Mono) often forgives; IL2CPP doesn’t.

The Fix

using AOT;
using System.Threading;

public class NativeBridge : MonoBehaviour
{
    private static SynchronizationContext _main;

    void Awake() { _main = SynchronizationContext.Current; }

    [MonoPInvokeCallback(typeof(NativeCallback))]
    private static void OnDownloaded(IntPtr data, int len)
    {
        var bytes = new byte[len];
        Marshal.Copy(data, bytes, 0, len);

        _main.Post(_ =>
        {
            // Safe: this runs on the main thread next loop
            UpdateUI(bytes);
        }, null);
    }
}

MonoPInvokeCallback ensures IL2CPP keeps the trampoline. Post marshals to the main thread.

Static-Only

P/Invoke callback handlers must be static (or a delegate referring to a static). Use a static dispatch with a stable instance reference passed by IntPtr if you need the instance.

Verifying

IL2CPP build. Trigger native callback. UI updates without crash. Without Post: crash inside UnityEngine.UI on touch.

“MonoPInvokeCallback. Capture context. Post to main. Native plays nice.”

Related Issues

For IL2CPP strip reflection, see strip reflection. For async unobserved, see async exceptions.

Static callback. Marshal back. Main thread runs.