Question Is Entity.Index safe to use as the ID of an entity?

Would there be cases where two entities have the same Index value?

 

To ensure 100% uniqueness you'd need to have both Index & Version or "Entity" as a key.
From code:

Code (CSharp):

1.         /// <summary>
2.         /// The ID of an entity.
3.         /// </summary>
4.         /// <value>The index into the internal list of entities.</value>
5.         /// <remarks>
6.         /// Entity indexes are recycled when an entity is destroyed. When an entity is destroyed, the
7.         /// EntityManager increments the version identifier. To represent the same entity, both the Index and the
8.         /// Version fields of the Entity object must match. If the Index is the same, but the Version is different,
9.         /// then the entity has been recycled.
10.         /// </remarks>
11.         public int Index;
12.  
13.         /// <summary>
14.         /// The generational version of the entity.
15.         /// </summary>
16.         /// <remarks>The Version number can, theoretically, overflow and wrap around within the lifetime of an
17.         /// application. For this reason, you cannot assume that an Entity instance with a larger Version is a more
18.         /// recent incarnation of the entity than one with a smaller Version (and the same Index).</remarks>
19.         /// <value>Used to determine whether this Entity object still identifies an existing entity.</value>
20.         public int Version;

Just Index will mean any reused entity will match.

 

In the same world no two entities can have same index. But in multiple worlds yes. However, as mentioned by previous poster, entities version is used also, to identify, if it is new unique entity, when is reused.

So if you destroy let's say lots of projectiles in a small game, over duration of game, you may be using for example only first 1000 indexes for entities (not guaranteed however). So the index of entities is not incremented indefinitely. But version of entity will, after each new entity is created and is reusing entity.

Side note:
Entity version always start from 1, and can not be less.
ECB when instantiating entities, uses negative index of entities. After ECB playback, next valid (positive) index of entity is assigned.

 

If you're sure that two Entities exist (created & not destroyed), then they would have different indices.

Otherwise, they might have the same index.