Quick answer: Reference the generic instantiation once in code (e.g., new List<MyType>()) so IL2CPP sees it statically, or add a link.xml preserve entry for the constructed type.

Your game runs fine in the Editor and in the Mono build. The IL2CPP iOS build throws on startup: ExecutionEngineException: Attempting to call method 'System.Collections.Generic.List`1[MyType]::.ctor' for which no ahead of time (AOT) code was generated. The list works in C# anywhere else.

How IL2CPP Generic Stripping Works

IL2CPP converts your C# IL to C++ at build time. Generics require monomorphization: each instantiation (List<int>, List<MyType>, Dictionary<string, Player>) becomes a separate concrete C++ class. The build process walks the call graph and generates code only for the instantiations it can see.

If a generic instantiation is only created via:

...IL2CPP won’t see it during static analysis. The C++ class doesn’t exist. At runtime, the AOT framework throws.

Fix 1: Add a Static Reference

using System.Collections.Generic;
using UnityEngine;

public static class AotTypeHints
{
    [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.BeforeSceneLoad)]
    static void EnsureTypes()
    {
        var _ = new List<MyType>();
        var __ = new Dictionary<string, Player>();
        var ___ = new HashSet<Item>();
        System.GC.KeepAlive(_);
        System.GC.KeepAlive(__);
        System.GC.KeepAlive(___);
    }
}

The references force IL2CPP to generate code for these instantiations. GC.KeepAlive prevents the compiler from eliding them as dead. Place in Plugins/ or any non-Editor folder.

Fix 2: link.xml Preserve

Create Assets/link.xml:

<linker>
  <assembly fullname="Assembly-CSharp">
    <type fullname="System.Collections.Generic.List`1[[MyType, Assembly-CSharp]]" preserve="all" />
    <type fullname="System.Collections.Generic.Dictionary`2[[System.String, mscorlib],[Player, Assembly-CSharp]]" preserve="all" />
  </assembly>
</linker>

The fully-qualified instantiated type names tell the linker to preserve these generic specializations. Tedious for many types; the static-reference approach scales better.

Fix 3: Lower Managed Stripping Level

For projects with many reflection-driven types and limited bandwidth to enumerate them: Player Settings → Other Settings → Managed Stripping Level → Low. Increases binary size 5–15% but eliminates stripping-related crashes broadly.

Don’t reach for this first — it’s the “turn it off” option. Use static references for known cases and Low only when the call graph genuinely can’t be enumerated.

Fix 4: Mark Methods with [Preserve]

If reflection calls a specific method on a generic instantiation, decorate it:

using UnityEngine.Scripting;

[Preserve]
public List<MyType> GetItems() => items;

Stops the linker from stripping the method even when it can’t see direct callers.

Verifying

Build IL2CPP for the target platform. Watch the device logs (Xcode console for iOS, adb logcat for Android). Reproduce the path that triggered the crash. ExecutionEngineException should not appear; the operation should succeed.

Understanding the issue

This bug class falls into a pattern that's worth understanding beyond the specific case. In Unity Engine, the underlying behavior is shaped by how the engine layers its abstractions - the public API you call, the runtime systems that respond, and the platform-specific implementations underneath. A bug at any layer can produce symptoms that look like they originate at a different layer. Triaging effectively means recognizing which layer the symptom belongs to, even when the gameplay code is what's visible.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

The triage path for this kind of bug is long. The symptom appears in gameplay, but the cause is in a different system. The reporter describes the gameplay effect; the engineer has to translate that into a hypothesis about the underlying cause. Misdirection is common.

At the engine level, the behavior comes from a deliberate design decision in Unity. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

After applying the fix, the verification step has three parts: confirm the original repro is resolved, confirm no obvious regressions in adjacent functionality, and (for shipping titles) deploy to a small player cohort first and watch the crash and report rates. Each step catches something the others miss.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

Live games surface this bug class at scale. What's a rare edge case in development becomes a daily occurrence once you have a few thousand concurrent players. The class isn't 'this player has a unique setup'; it's 'one in N thousand sessions will trigger this exact combination'.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

If this issue manifests under high load (many actors, many particles, many network connections), profile the post-fix code path with realistic counts. The original cost was a bug; the new cost is real work, and real work has a budget.

Diagnostic approach

The diagnostic tools available depend on your engine and platform. Use the engine's native profilers and debug overlays before reaching for external tools. The native tools have context that external tools lack - they know which subsystem owns the code, which assets are loaded, and what state the engine is in.

For Unity-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

Modern engine versions ship better tooling for this kind of issue than older versions. If you're on an older release, the diagnostic step may take significantly longer because the tools you'd want don't exist yet. Sometimes the right answer is upgrading rather than fighting through limited tooling.

Within Unity, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Platform-specific edge cases are worth enumerating explicitly. iOS handles backgrounding differently than Android; Windows handles focus changes differently than macOS. A fix that works on the development platform may not work on every target. Test on each shipping platform deliberately.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

When this bug class affects multiple teams (often the case for cross-system issues), early communication prevents duplicate work. The team that owns the symptom may not own the cause. A 15-minute conversation at the start of triage often saves hours of independent investigation.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“IL2CPP needs to see your generics statically. Either reference them in code or list them in link.xml — otherwise they won’t exist in shipping.”

Build an AotTypeHints class as a project convention — add to it whenever you introduce a new reflection-loaded generic.