Fix: Unity ECS Aspects Not Registering In Baker

Here is how to fix Unity ECS aspects that fail to register and never appear in your queries. You define an IAspect struct that bundles several components, write a system that iterates with SystemAPI.Query<MyAspect>, and the foreach body never runs. No errors. No warnings. Aspects rely on Roslyn source generators, and the generators have strict requirements that fail silently when violated.

The Symptom

Your aspect declaration compiles. Your system compiles. SystemAPI.Query<TAspect> returns zero results even though entities with the relevant components clearly exist in the world. The Entities Hierarchy window shows the components on those entities, but the query body never executes.

What Causes This

Struct is not partial. Source generators emit additional methods into the aspect type. Without partial, the generated code cannot be merged with your declaration and the aspect type ends up incomplete or not generated at all.

Fields are concrete components, not refs. Aspects do not hold component values; they hold refs. A field of type Translation is invalid; you need RefRW<LocalTransform> or RefRO<LocalTransform>.

Aspect references missing components. If your aspect requires RefRO<Health> but no entity has Health attached, the query matches zero entities. The aspect itself is registered fine, but no entity satisfies it.

Compile errors elsewhere block source generation. Unity’s source generators do not run if the assembly already has compile errors. A red squiggle in another file can prevent your aspect from being generated, even if the aspect itself is valid.

The Fix

Step 1: Mark the struct as partial.

using Unity.Entities;
using Unity.Transforms;

public readonly partial struct PlayerMoveAspect : IAspect
{
    public readonly Entity Self;
    public readonly RefRW<LocalTransform> Transform;
    public readonly RefRO<MoveSpeed> Speed;
    public readonly RefRW<Health> Health;
}

Note partial, readonly, and that every field is a ref type or Entity. Do not include plain LocalTransform fields.

Step 2: Use the aspect in a system.

using Unity.Entities;

public partial struct PlayerMoveSystem : ISystem
{
    public void OnUpdate(ref SystemState state)
    {
        float dt = SystemAPI.Time.DeltaTime;

        foreach (PlayerMoveAspect aspect in SystemAPI.Query<PlayerMoveAspect>())
        {
            aspect.Transform.ValueRW.Position +=
                new Unity.Mathematics.float3(1, 0, 0) * aspect.Speed.ValueRO.Value * dt;
        }
    }
}

Step 3: Make optional components actually optional. If some entities have Health and others do not, use EnabledRefRO or split into a separate aspect.

public readonly partial struct DamageableAspect : IAspect
{
    public readonly RefRW<LocalTransform> Transform;
    public readonly EnabledRefRW<Health> Health;  // Optional / toggle-enabled
}

Step 4: Force a regeneration if changes do not apply. After fixing compile errors elsewhere, switch focus away from Unity and back. The Entities source generator runs again. If it does not, right-click the assembly definition and choose Reimport.

Step 5: Confirm components exist on entities. Open the Entities Hierarchy and select an entity that should match. Verify it has every component the aspect requires. A single missing component means zero matches.

Verifying the Aspect Compiles

Aspects generate hidden helper code. After saving, open Edit → Preferences → Entities → Show Entities In Hierarchy to confirm baking ran. Use the Entities Journaling tool to inspect what queries match what entities. If the aspect type does not appear in autocomplete in SystemAPI.Query<...>, the source generator did not produce it.

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

This bug class disproportionately affects late-stage development. The work to surface it is interactive testing in realistic conditions, which only really happens after the gameplay is in place and assets are populated. Catching it early requires deliberate testing of conditions that look unimportant.

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

Verifying this fix in isolation is straightforward: reproduce the bug, apply the change, confirm the bug no longer reproduces. The harder verification is regression - did this fix introduce a new bug elsewhere? Run your standard regression suite, plus any tests that exercise the same code path with different inputs.

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

Performance implications matter when this bug class scales with player count or asset count. A bug that fires once per session is annoying; a bug that fires once per frame compounds. After fixing, profile the affected code path under realistic load. The fix that's correct for one entity may be too slow for ten thousand.

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

Third-party plugins often provide better diagnostics for their own behavior than the engine does. If the affected code is in a plugin, check the plugin's documentation for debug modes, verbose logging, or inspector tools - these can save hours of investigation when they exist.

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

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

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.

“Partial struct, ref fields only, optional components are EnabledRef. Three rules and Roslyn does the rest.”

Related Issues

For empty queries with the right components present, see Entity Query Returning Empty. For tag component issues, see Entity Query Missing Tag.

When the foreach body never runs, the aspect did not generate. Check partial. Check refs.