Entity Liveness

Is It Alive?

Every entity knows whether it's alive or not. This is fundamental to safely working with entities in fennecs!

Entity.Alive

The Alive property tells you whether an entity currently exists in its World.

var fox = world.Spawn();
Console.WriteLine(fox.Alive);  // true

fox.Despawn();
Console.WriteLine(fox.Alive);  // false

Code

var entity = world.Spawn();
Console.WriteLine($"{entity} alive: {entity.Alive}");
entity.Despawn();
Console.WriteLine($"{entity} alive: {entity.Alive}");

Output

E-00000001:00001 alive: True
E-00000001:00001 alive: False

Implicit Bool Conversion

Entities can be used directly in boolean contexts – they implicitly convert to their Alive state:

var entity = world.Spawn();

if (entity)  // Same as: if (entity.Alive)
{
    Console.WriteLine("Entity is alive!");
}

entity.Despawn();

if (!entity)  // Same as: if (!entity.Alive)
{
    Console.WriteLine("Entity is dead!");
}

How Identity Recycling Works

Entities use a generational index system (similar to a Sparse Set):

1. Each entity has an index (which slot it occupies)
2. Each entity has a generation (how many times that slot has been reused)
3. When despawned, the generation increments and the slot returns to the pool

This means:

• Entity IDs are always unique within a World's lifetime
• Stale entity handles can be detected (generation mismatch)
• Memory is reused efficiently

var first = world.Spawn();   // Index 1, Generation 1
first.Despawn();

var second = world.Spawn();  // Index 1, Generation 2 (reused slot!)

Console.WriteLine(first.Alive);   // false (generation mismatch)
Console.WriteLine(second.Alive);  // true

Deferred Operations & Liveness

Entities Stay "Alive" During Deferred Mode

When inside a Stream runner or while holding a World lock, structural changes (including despawns) are deferred. The entity will report as Alive until the deferred operations are applied!

var stream = world.Query<Health>().Stream();

stream.For((in entity, ref health) =>
{
    entity.Despawn();
    
    // Still reports alive during deferred execution!
    Console.WriteLine(entity.Alive);  // true
});

// After runner completes, despawns are applied
// Now the entities would report as dead

Manual World Locking

You can manually defer operations using a World lock:

using var worldLock = world.Lock();

foreach (var entity in query)
{
    if (Random.Shared.NextSingle() >= 0.5f)
    {
        entity.Despawn();
    }
    
    // Entity still "Alive" while lock is held
    if (entity.Alive) 
    {
        Console.WriteLine("Dead Fox Walking!");
    }
}

// Lock disposed here - despawns are applied

Best Practices

Check Before Operating on Stored Entities

If you store entity references (in lists, dictionaries, components), check liveness before using them:

// Stored entity reference
private Entity _target;

public void Update()
{
    if (!_target.Alive)
    {
        _target = FindNewTarget();
    }
    
    // Safe to use now
    ref var pos = ref _target.Ref<Position>();
}

Don't Store Entity References Long-Term (If Possible)

Queries and Streams give you fresh, valid entity references each iteration. Prefer these over storing entities when possible.

Handle Dead Entities Gracefully

public void DamageEntity(Entity target, int damage)
{
    if (!target.Alive)
    {
        Console.WriteLine("Target already dead!");
        return;
    }
    
    ref var health = ref target.Ref<Health>();
    health.Value -= damage;
    
    if (health.Value <= 0)
    {
        target.Despawn();
    }
}

Quick Reference

Property/Pattern	Description
entity.Alive	Returns true if entity exists in world
if (entity)	Implicit bool conversion to Alive
Deferred mode	Entity reports Alive until lock released
Generational ID	Prevents stale handle collisions