Unity csharp architecture and ecs

Unity C# Architecture & Data-Oriented ECS

This skill guide provides production-grade architectural patterns, memory management protocols, frame budget optimization techniques, and data-oriented design patterns for Unity applications using C#, Unity DOTS (Entities, Burst, C# Job System), and ScriptableObject-driven systems.

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1. Architectural Patterns

1.1 Data-Oriented Design (DOTS / ECS) vs Object-Oriented Component Design

High-performance Unity engines split responsibilities based on memory access patterns:

• Object-Oriented (MonoBehaviour): Used for high-level lifecycle control, UI binding, scene bootstrapping, and editor tooling.
• Data-Oriented (ECS / Entities): Used for mass data processing (particles, projectiles, crowd simulation, terrain processing, spatial queries) where cache layout determines performance.

+-------------------------------------------------------------------+
|                        MonoBehaviour Layer                        |
|   (Bootstrapping, UI Binding, Input Mapping, Authoring Components)  |
+-------------------------------------------------------------------+
                                  |
                   Baker System / Conversion World
                                  v
+-------------------------------------------------------------------+
|                        Unity DOTS ECS World                       |
|   Entity (ID) + IComponentData (Blittable) + ISystem (Burst-compiled) |
+-------------------------------------------------------------------+

1.2 ScriptableObject-Driven Architecture (Event Channels & Runtime Sets)

Decouple game systems without singletons using ScriptableObject event channels and runtime sets:

• Event Channel: ScriptableObjects acting as broadcast conduits. Subscribed listeners execute without direct reference to publishers.
• Runtime Set: ScriptableObjects holding dynamic lists of active game instances (e.g., active enemies, spatial triggers) avoiding Object.FindObjectsOfType.

1.3 Service Locator vs Dependency Injection

For MonoBehaviour-based systems:

• Use VContainer or Zenject for explicit compile-time dependency injection.
• Avoid generic untyped Service Locators that introduce hidden temporal dependencies during scene loads.

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2. Memory Management & Cache Alignment

2.1 Native Container Lifecycle & Safety Handles

All Native collections (NativeArray, NativeParallelHashMap, NativeList, NativeQueue) require explicit allocation lifetime management:

Allocator	Duration	Use Case
Allocator.Temp	Single frame (1 frame max)	Short-lived job inputs/outputs within a single method.
Allocator.TempJob	4 frames max	Jobs spanning multiple frames; must be disposed upon completion.
Allocator.Persistent	Indefinite	Long-lived game state, spatial partition grids, resource pools.

Rule: Always pair Allocator.Persistent allocations with Dispose() in OnDestroy() or OnStopRunning().

2.2 Cache Line Efficiency & Struct Packing

Cache lines on modern modern CPUs are 64 bytes. Layout ECS structs sequentially ([StructLayout(LayoutKind.Sequential)]) and align data types from largest to smallest to eliminate padding bytes:

// BAD: 24 bytes due to padding gaps
public struct UnalignedComponent : IComponentData
{
    public bool IsActive;    // 1 byte (+ 3 bytes padding)
    public double Priority;  // 8 bytes
    public byte Category;    // 1 byte (+ 3 bytes padding)
    public int Health;       // 4 bytes
}

// GOOD: 16 bytes tightly aligned
[StructLayout(LayoutKind.Sequential)]
public struct AlignedComponent : IComponentData
{
    public double Priority;  // 8 bytes
    public int Health;       // 4 bytes
    public bool IsActive;    // 1 byte
    public byte Category;    // 1 byte
    // 2 bytes automatic trailing padding to 8-byte boundary
}

2.3 Zero-Allocation C# Allocation Rules

To achieve zero Garbage Collection (GC) spikes in Update():

1. No LINQ: Replaces all System.Linq queries with explicit loops or NativeArray operations.
2. Struct Enumerators: Implement custom GetEnumerator() returning value types for custom collections.
3. ArrayPool<T>: Utilize System.Buffers.ArrayPool<T>.Shared for transient array allocations.
4. Lambda Pre-allocation: Cache delegates/closures into static fields instead of passing inline lambdas.

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3. Frame Budget & Job System Optimizations

3.1 Frame Time Budget Allocation (60 FPS = 16.66ms / 120 FPS = 8.33ms)

Total Frame Budget (16.66ms @ 60 FPS)
├── Input & Game Logic (Update): 3.0ms
├── Physics Simulation (FixedUpdate): 4.0ms
├── ECS Job Execution (Parallel Worker Threads): 6.0ms
└── Rendering Prep & Command Buffers (LateUpdate): 3.66ms

3.2 Burst Compiler Guidelines

To ensure [BurstCompile] compatibility:

• Use only blittable primitives, value types (struct), and NativeContainer pointers.
• No managed object references (string, class, UnityEngine.Object).
• Set CompileSynchronously = true on critical hot paths to avoid fallback un-burstified execution during domain reloads.

3.3 Structural Change Avoidance via EntityCommandBuffer (ECB)

Executing structural changes (creating entities, destroying entities, adding/removing components) invalidates archetype chunks and forces a Sync Point, stalling all job worker threads.

Solution: Record structural commands into EntityCommandBuffer.ParallelWriter within jobs, then playback deferred commands on the main thread during designated system groups (EndSimulationEntityCommandBufferSystem).

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4. Production Code Examples

4.1 Production-Grade Burst-Compiled ECS System (ISystem)

A complete high-performance spatial translation update system for a projectile population:

using Unity.Burst;
using Unity.Collections;
using Unity.Entities;
using Unity.Mathematics;
using Unity.Transforms;

namespace GameEngine.Architecture.ECS
{
    public struct ProjectileData : IComponentData
    {
        public float3 Velocity;
        public float LifetimeRemaining;
        public float Damage;
    }

    [BurstCompile]
    public partial struct ProjectileUpdateSystem : ISystem
    {
        private EntityQuery _query;

        [BurstCompile]
        public void OnCreate(ref SystemState state)
        {
            _query = state.GetEntityQuery(
                ComponentType.ReadWrite<LocalTransform>(),
                ComponentType.ReadWrite<ProjectileData>()
            );
            state.RequireForUpdate(_query);
        }

        [BurstCompile]
        public void OnUpdate(ref SystemState state)
        {
            float deltaTime = state.WorldUnmanaged.Time.DeltaTime;
            
            var ecbSingleton = SystemAPI.GetSingleton<EndSimulationEntityCommandBufferSystem.Singleton>();
            EntityCommandBuffer ecb = ecbSingleton.CreateCommandBuffer(state.WorldUnmanaged);

            var job = new ProjectileMovementJob
            {
                DeltaTime = deltaTime,
                Ecb = ecb.AsParallelWriter()
            };

            state.Dependency = job.ScheduleParallel(state.Dependency);
        }

        [BurstCompile]
        private partial struct ProjectileMovementJob : IJobEntity
        {
            public float DeltaTime;
            public EntityCommandBuffer.ParallelWriter Ecb;

            // Executed in parallel across worker threads per chunk
            private void Execute(
                Entity entity,
                [EntityIndexInQuery] int sortIndex,
                ref LocalTransform transform,
                ref ProjectileData projectile)
            {
                transform.Position += projectile.Velocity * DeltaTime;
                projectile.LifetimeRemaining -= DeltaTime;

                if (projectile.LifetimeRemaining <= 0f)
                {
                    // Defer structural modification to ECB without stalling worker threads
                    Ecb.DestroyEntity(sortIndex, entity);
                }
            }
        }
    }
}

4.2 ScriptableObject Event Channel Pattern

A zero-allocation type-safe event broadcasting channel:

using System;
using UnityEngine;

namespace GameEngine.Architecture.Events
{
    [CreateAssetMenu(fileName = "FloatEventChannel", menuName = "Architecture/Events/Float Event Channel")]
    public class FloatEventChannelSO : ScriptableObject
    {
        private Action<float> _onEventRaised;

        public void RaiseEvent(float value)
        {
            _onEventRaised?.Invoke(value);
        }

        public void Subscribe(Action<float> action)
        {
            _onEventRaised += action;
        }

        public void Unsubscribe(Action<float> action)
        {
            _onEventRaised -= action;
        }
    }
}

4.3 High-Performance Zero-GC Array Pool & Job Processing

Processing non-ECS arrays without allocations using NativeArray and IJobParallelFor:

using Unity.Burst;
using Unity.Collections;
using Unity.Jobs;
using UnityEngine;

namespace GameEngine.Architecture.Jobs
{
    public class NativeBatchProcessor : MonoBehaviour
    {
        [SerializeField] private int elementCount = 100000;

        private NativeArray<Vector3> _positions;
        private NativeArray<Vector3> _velocities;

        private void Awake()
        {
            _positions = new NativeArray<Vector3>(elementCount, Allocator.Persistent);
            _velocities = new NativeArray<Vector3>(elementCount, Allocator.Persistent);
        }

        private void Update()
        {
            var job = new BatchPositionJob
            {
                DeltaTime = Time.deltaTime,
                Positions = _positions,
                Velocities = _velocities
            };

            JobHandle handle = job.Schedule(elementCount, 64);
            handle.Complete(); // Complete synchronously when required by rendering pipeline
        }

        private void OnDestroy()
        {
            if (_positions.IsCreated) _positions.Dispose();
            if (_velocities.IsCreated) _velocities.Dispose();
        }

        [BurstCompile]
        private struct BatchPositionJob : IJobParallelFor
        {
            public float DeltaTime;
            public NativeArray<Vector3> Positions;
            [ReadOnly] public NativeArray<Vector3> Velocities;

            public void Execute(int index)
            {
                Positions[index] += Velocities[index] * DeltaTime;
            }
        }
    }
}

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5. Anti-Patterns & Critical Pitfalls

❌ Anti-Pattern 1: Managed References in IComponentData

// WRONG: Adding managed types causes compiler errors or GC tracking overhead in ECS
public struct BadComponent : IComponentData
{
    public string TargetName; // Managed reference! Breaks Burst and memory layout
    public GameObject PrefabReference; // Managed reference!
}

// RIGHT: Use fixed-size byte buffers or Entity references
public struct GoodComponent : IComponentData
{
    public FixedString64Bytes TargetName;
    public Entity PrefabEntity;
}

❌ Anti-Pattern 2: Direct Structural Changes in Parallel Jobs

Calling EntityManager.AddComponent() or EntityManager.DestroyEntity() inside a parallel job causes dynamic archetype re-allocation, corrupting memory pointers read by concurrent threads. Always pass an EntityCommandBuffer.ParallelWriter.

❌ Anti-Pattern 3: Mid-Frame GC Allocs via String Concatenation & LINQ

// WRONG: Allocates string objects and closures every frame in Update
void Update()
{
    var activeEnemies = _enemies.Where(e => e.IsAlive).ToList();
    uiText.text = "Enemies: " + activeEnemies.Count;
}

// RIGHT: Reuse non-allocating string buffers or FixedString
private readonly char[] _scoreBuffer = new char[32];
void Update()
{
    int count = GetActiveEnemyCount();
    // Use StringBuilder or direct custom number formatters without heap allocation
}

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