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
.
PropertyVisitor
to create a property visitorDid you find this page useful? Please give it a rating:
Thanks for rating this page!
What kind of problem would you like to report?
Thanks for letting us know! This page has been marked for review based on your feedback.
If you have time, you can provide more information to help us fix the problem faster.
Provide more information
You've told us this page needs code samples. If you'd like to help us further, you could provide a code sample, or tell us about what kind of code sample you'd like to see:
You've told us there are code samples on this page which don't work. If you know how to fix it, or have something better we could use instead, please let us know:
You've told us there is information missing from this page. Please tell us more about what's missing:
You've told us there is incorrect information on this page. If you know what we should change to make it correct, please tell us:
You've told us this page has unclear or confusing information. Please tell us more about what you found unclear or confusing, or let us know how we could make it clearer:
You've told us there is a spelling or grammar error on this page. Please tell us what's wrong:
You've told us this page has a problem. Please tell us more about what's wrong:
Thank you for helping to make the Unity documentation better!
Your feedback has been submitted as a ticket for our documentation team to review.
We are not able to reply to every ticket submitted.