Entity

The Entity is the main data structure when working with an ECS.

An Entity has a unique identity - Id - and acts as a container for components, tags, script and child entities. An EntityStore is a container of entities and used to create entities with store.CreateEntity().

public static void CreateEntity()
{
    var store = new EntityStore();
    store.CreateEntity();
    store.CreateEntity();
    
    foreach (var entity in store.Entities) {
        Console.WriteLine($"entity {entity}");
    }
    // > entity id: 1  []       Info:  [] entity has no components
    // > entity id: 2  []
}

Entities can be deleted with entity.DeleteEntity(). Variables of type Entity mimic the behavior of reference types. Using an entity method on a deleted entity throws a NullReferenceException. To handled this case use entity.IsNull.

public static void DeleteEntity()
{
    var store   = new EntityStore();
    var entity  = store.CreateEntity();
    entity.DeleteEntity();
    var isDeleted = entity.IsNull;
    Console.WriteLine($"deleted: {isDeleted}");     // > deleted: True
}

Entities can be disabled. Disabled entities are excluded from query results by default. To include disabled entities in a query result use query.WithDisabled().

public static void DisableEntity()
{
    var store   = new EntityStore();
    var entity  = store.CreateEntity();
    entity.Enabled = false;
    Console.WriteLine(entity);                      // > id: 1  [#Disabled]
    
    var query    = store.Query();
    Console.WriteLine($"default - {query}");        // > default - Query: []  Count: 0
    
    var disabled = store.Query().WithDisabled();
    Console.WriteLine($"disabled - {disabled}");    // > disabled - Query: []  Count: 1
}

Entity example code is part of the unit tests see: Tests/ECS/Examples.

When tying out the examples use a debugger to check entity state changes while stepping throw the code.

Screenshot: Entity state - enables browsing the entire store hierarchy.

Examples showing typical use cases of the Entity API

EntityStore

An EntityStore is a container for entities running as an in-memory database. It is highly optimized for efficient storage fast queries and event handling. In other ECS implementations this type is typically called World.

The store enables to

• create entities

• modify entities - add / remove components, tags, scripts and child entities

• query entities with a specific set of components or tags

• subscribe events like adding / removing components, tags, scripts and child entities

Multiple stores can be used in parallel and act completely independent from each other. The example shows how to create a store. Mainly every example will start with this line.

public static void CreateStore()
{
    var store = new EntityStore();
}

Component

Components are structs used to store data on entities. Multiple components with different types can be added / removed to / from an entity. If adding a component using a type already stored in the entity its value gets updated.

[ComponentKey("my-component")]
public struct MyComponent : IComponent {
    public int value;
}

public static void AddComponents()
{
    var store   = new EntityStore();
    var entity  = store.CreateEntity();
    
    // add components
    entity.AddComponent(new EntityName("Hello World!"));// EntityName is build-in
    entity.AddComponent(new MyComponent { value = 42 });
    Console.WriteLine($"entity: {entity}");             // > entity: id: 1  "Hello World!"  [EntityName, Position]
    
    // get component
    Console.WriteLine($"name: {entity.Name.value}");    // > name: Hello World!
    var value = entity.GetComponent<MyComponent>().value;
    Console.WriteLine($"MyComponent: {value}");         // > MyComponent: 42
    
    // Serialize entity to JSON
    Console.WriteLine(entity.DebugJSON);
}

Result of entity.DebugJSON:

{
    "id": 1,
    "components": {
        "name": {"value":"Hello World!"},
        "my-component": {"value":42}
    }
}

Unique entity

Add a UniqueEntity component to an entity to mark it as a "singleton" with a unique string id. The entity can than be retrieved with EntityStore.GetUniqueEntity() to reduce code coupling. It enables access to a unique entity without the need to pass an entity by external code.

public static void GetUniqueEntity()
{
    var store   = new EntityStore();
    store.CreateEntity(new UniqueEntity("Player"));     // UniqueEntity is build-in
    
    var player  = store.GetUniqueEntity("Player");
    Console.WriteLine($"entity: {player}");             // > entity: id: 1  [UniqueEntity]
}

Info Since version 3.0.0 there is more flexible and performant alternative available by using a Component Index. It supports defining a custom IIndexedComponent<> type and have several advantages:

• The unique key can be of any type - e.g. int, Guid, enum, string, ... . The key of unique entities is always a string.

• The storage of a Component Index is optimized for low memory footprint and fast lookup.

• Additional fields can be added to an IIndexedComponent<> type.

Tag

Tags are structs similar to components - except they store no data. They can be utilized in queries similar as components to restrict the amount of entities returned by a query. If adding a tag using a type already attached to the entity the entity remains unchanged.

public struct MyTag1 : ITag { }
public struct MyTag2 : ITag { }

public static void AddTags()
{
    var store   = new EntityStore();
    var entity  = store.CreateEntity();
    
    // add tags
    entity.AddTag<MyTag1>();
    entity.AddTag<MyTag2>();
    Console.WriteLine($"entity: {entity}");     // > entity: id: 1  [#MyTag1, #MyTag2]
    
    // get tag
    var tag1 = entity.Tags.Has<MyTag1>();
    Console.WriteLine($"tag1: {tag1}");         // > tag1: True
}

Clone / Copy entity

The methods Entity.CloneEntity() and Entity.CopyEntity(Entity target) are used to copy all components and tags from one entity to another.

• CloneEntity() creates a new entity having the same components and tags as the original entity.

• CopyEntity(Entity target) copy all components and tags to the given target entity. The target entity can be in the same or in a different store.

The example creates a new entity with the same components and tags as the original entity.

public static void CloneEntity()
{
    var store   = new EntityStore();
    var entity  = store.CreateEntity(new Position(1,2,3), Tags.Get<MyTag1>());
    
    var clone = entity.CloneEntity();
    // the cloned entity have the same components and tags as the original entity.
}

CopyEntity(Entity target) can be used to copy a subset or all entities of one store to another store. The entities in the target store will have the same entities ids as in the original store.

public struct NetTag : ITag { }

public static void CopyEntities()
{
    var store       = new EntityStore();
    var targetStore = new EntityStore();
        
    store.CreateEntity(new Position(1,1,1));                     // 1
    store.CreateEntity(new Position(2,2,2), Tags.Get<NetTag>()); // 2
    store.CreateEntity(new Position(3,3,3));                     // 3
    store.CreateEntity(new Position(4,4,4), Tags.Get<NetTag>()); // 4
    store.CreateEntity(new Position(5,5,5));                     // 5
        
    // Query will copy only entities [2, 4] having a NetTag
    var query = store.Query().AnyTags(Tags.Get<NetTag>());
    foreach (var entity in query.Entities) {
        // preserve same entity ids in target store
        if (!targetStore.TryGetEntityById(entity.Id, out Entity targetEntity)) {
            targetEntity = targetStore.CreateEntity(entity.Id);
        }
        entity.CopyEntity(targetEntity);
    }
    // target store contains two entities [2, 4] with same components and tags as in the original store
} 

Script

Scripts are similar to components and can be added / removed to / from entities. Scripts are classes and can also be used to store data. Additional to components they enable adding behavior in the common OOP style.

In case dealing only with a few thousands of entities Scripts are fine. If dealing with a multiple of 10.000 components should be used for efficiency / performance.

public class MyScript : Script { public int data; }

public static void AddScript()
{
    var store   = new EntityStore();
    var entity  = store.CreateEntity();
    
    // add script
    entity.AddScript(new MyScript{ data = 123 });
    Console.WriteLine($"entity: {entity}");             // > entity: id: 1  [*MyScript]
    
    // get script
    var myScript = entity.GetScript<MyScript>();
    Console.WriteLine($"data: {myScript.data}");        // > data: 123
}

Scripts enable to override their Start() and Update().

public class MyScript : Script
{
    public override void Start()  { }
    public override void Update() { }
}

These methods need to be called manually. The ECS has no build mechanism to execute these methods. The ECS provide only access to all scripts added to entities of an EntityStore. E.g. Executing Update() of all scripts in a store use:

foreach (var scripts in store.EntityScripts) {
    foreach (var script in scripts) {
        script.Update();
    }
}

Hierarchy

A typical use case in an Game or Editor is to build up a hierarchy of entities. To add an entity as a child to another entity use Entity.AddChild(). In case the added child already has a parent it gets removed from the old parent. The children of the added (moved) entity remain being its children. If removing a child from its parent all its children are removed from the hierarchy.

public static void AddChildEntities()
{
    var store   = new EntityStore();
    var root    = store.CreateEntity();
    var child1  = store.CreateEntity();
    var child2  = store.CreateEntity();
    
    // add child entities
    root.AddChild(child1);
    root.AddChild(child2);
    
    Console.WriteLine($"entities: {root.ChildEntities}"); // > entities: Count: 2
}

Archetype

An Archetype defines a specific set of components and tags for its entities. At the same time it is also a container of entities with exactly this combination of components and tags.

Explanation An Archetype instance corresponds to an SQL TABLE where its components are the counterpart of the table rows. In contrast to tables archetypes are created automatically on demand. A relational database requires to create tables upfront.

The following comparison shows the difference in modeling types in ECS vs OOP.

ECS - Composition	OOP - Polymorphism
Inheritance ECS does not utilize inheritance. It prefers composition over inheritance.	Common OPP is based on inheritance. Likely result: A god base class responsible for everything. 😊
Code coupling Data lives in components - behavior in systems. New behaviors does not affect existing code.	Data and behavior are both in classes. New behaviors may add dependencies or side effects.
Storage An Archetype is also a container of entities.	Organizing containers is part of application code.
Changing a type Supported by adding/removing tags or components.	Type is fixed an cannot be changed.
Component access / visibility Having a reference to an EntityStore enables unrestricted reading and changing of components.	Is controlled by access modifiers: public, protected, internal and private.
Example
// No base class Animal in ECS struct Dog : ITag { } struct Cat : ITag { } var store = new EntityStore(); var dogType = store.GetArchetype(Tags.Get<Dog>()); var catType = store.GetArchetype(Tags.Get<Cat>()); WriteLine(dogType.Name); // [#Dog] dogType.CreateEntity(); catType.CreateEntity(); var dogs = store.Query().AnyTags(Tags.Get<Dog>()); var all = store.Query().AnyTags(Tags.Get<Dog, Cat>()); WriteLine($"dogs: {dogs.Count}"); // dogs: 1 WriteLine($"all: {all.Count}"); // all: 2	class Animal { } class Dog : Animal { } class Cat : Animal { } var animals = new List<Animal>(); var dogType = typeof(Dog); var catType = typeof(Cat); WriteLine(dogType.Name); // Dog animals.Add(new Dog()); animals.Add(new Cat()); var dogs = animals.Where(a => a is Dog); var all = animals.Where(a => a is Dog or Cat); WriteLine($"dogs: {dogs.Count()}"); // dogs: 1 WriteLine($"all: {all.Count()}"); // all: 2
Performance
Runtime complexity O() of queries for specific types O(size of result set)	O(size of all objects)
Memory layout Continuous memory in heap - high hit rate of L1 cache.	Randomly placed in heap - high rate of L1 cache misses.
Instruction pipelining Minimize conditional branches in update loops. Process multiple components at once using SIMD.	Virtual method calls prevent branch prediction.

JSON Serialization

The entities stored in an EntityStore can be serialized as JSON using an EntitySerializer.

Note Currently serialization / deserialization only support struct fields - properties not. See Issue #28. Component and relation types are required to be struct's.

Writing the entities of a store to a JSON file is done with WriteStore(). Reading the entities of a JSON file into a store with ReadIntoStore().

Following attributes can be used to customize JSON serialization

• [ComponentKey("data")] - the attributed component struct uses "data" as component key

• [Ignore] - the attributed component field is ignored

• [Serialize("n")] - the attributed component field uses "n" as JSON key

[ComponentKey("data")]  // use "data" as component key in JSON
struct DataComponent : IComponent
{
    [Ignore]            // field is ignored in JSON
    public int          temp;
    
    [Serialize("n")]    // use "n" as field key in JSON 
    public string       name;
}

public static void JsonSerialization()
{
    var store = new EntityStore();
    store.CreateEntity(new EntityName("hello JSON"));
    store.CreateEntity(new Position(1, 2, 3));
    store.CreateEntity(new DataComponent{ temp = 42, name = "foo" });

    // --- Write store entities as JSON array
    var serializer = new EntitySerializer();
    var writeStream = new FileStream("entity-store.json", FileMode.Create);
    serializer.WriteStore(store, writeStream);
    writeStream.Close();

    // --- Read JSON array into new store
    var targetStore = new EntityStore();
    serializer.ReadIntoStore(targetStore, new FileStream("entity-store.json", FileMode.Open));

    Console.WriteLine($"entities: {targetStore.Count}"); // > entities: 3
}

The JSON content of the file "entity-store.json" created with serializer.WriteStore()

[{
    "id": 1,
    "components": {
        "name": {"value":"hello JSON"}
    }
},{
    "id": 2,
    "components": {
        "pos": {"x":1,"y":2,"z":3}
    }
},{
    "id": 3,
    "components": {
        "data": {"n":"foo"}
    }
}]