Property bags are collections of properties for a given .Net object type that you can use to access and set data for an instance of an object of that type.
A property bag for a given type is a companion object that enables efficient data traversal algorithms based on instances of that type. By default, Unity uses reflection to generate the property bag for a type. This reflective approach offers convenience and occurs lazily only once per type when a property bag hasn’t been registered yet.
To enhance performance, you can opt-in to code generation by tagging the type with [Unity.Properties.GeneratePropertyBag]. Additionally, to activate code generation, you must tag the assembly with [assembly: Unity.Properties.GeneratePropertyBagsForAssembly]. Code-generated property bags are automatically registered when the domain is loaded.
In both the reflection and code-generation scenarios, the property bag generates properties for the following:
[SerializeField], [SerializeReference], or [CreateProperty]
[Unity.Properties.CreateProperty]
The property bag doesn’t generate a property for public, private, or internal fields tagged with [DontCreateProperty].
A generated property is read-only if the field is read-only or the property only has a getter.
You can also use [Unity.Properties.CreateProperty(ReadOnly = true)] to make a generated property read-only.
Creating properties in the property bag using serialization attributes for convenience is not always the preferred approach. Unity’s serialization system can only operate on fields and auto-properties, which makes it challenging to achieve validation or propagate changes effectively.
The following example combines the Unity serialization system with the Unity Properties system:
using UnityEngine;
using Unity.Properties;
public class MyBehaviour : MonoBehaviour
{
// Serializations go through the field, but we don't want to create a property for it.
[SerializeField, DontCreateProperty]
private int m_Value;
// For the property bag, use the property instead of the field. This ensures that
// the value stays within the appropriate bounds.
[CreateProperty]
public int value
{
get => m_Value;
set => m_Value = value;
}
// This is a similar example, but for an auto-property.
[field: SerializeField, DontCreateProperty]
[CreateProperty]
public float floatValue { get; set; }
}
Unlike the Unity serialization system, the properties within a property bag don’t qualify as value types with [SerializeField]. Instead, struct types are recognized as value types, whereas class types are recognized as references.
In Unity serialization, although polymorphism is supported, you must use the [SerializeReference] attribute to explicitly opt in. Otherwise, instances are serialized as value types. It’s worth noting that UnityEngine.Object types are an exception to this rule, as they’re automatically serialized as reference types.
Unity Properties uses .Net reflection to create property bags and properties that are strongly typed, which can introduce performance overhead the first time you request a property bag for a given container type.
When you create properties for field members through reflection, these properties may allocate garbage in IL2CPP builds. This allocation occurs due to the direct use of System.Reflection.FieldInfo, which leads to unavoidable boxing.
To avoid reflection, you can code-generate property bags during compilation. However, be aware that this optimization may lead to longer compilation times. To enable code generation for an assembly, tag the assembly with [Unity.Properties.GeneratePropertyBagsForAssemblyAttribute] and tag individual types with [Unity.Properties.GeneratePropertyBagAttribute]. To enable the property bag to access internal, and private fields and properties, make the type partial.