---
title: Resources and local state
description: Share singleton data between systems and keep per-system state.
sidebar:
  order: 6
---

```csharp
[Resource, TrackChanges]
public readonly record struct FrameTime(float DeltaSeconds);

[Resource(OwnerThread = true)]
public readonly record struct WindowHandle(nint Value);
```

## System parameters

| Parameter | Capability |
| --- | --- |
| `scoped Res<T>` | Readonly access to a required unmanaged resource. |
| `scoped ResMut<T>` | Writable access to a required unmanaged resource. |
| `scoped OptionalRes<T>` | Readonly access to a registered resource that may be absent. `HasValue` guards `Value`. |
| `scoped ManagedRes<T>` | Access to a managed resource class, exclusively on the owner thread. |
| `scoped Local<T>` | Mutable `Value` belonging to this system instance, initially `default(T)`. |

## Presence and registration

Required resources must be present at binding and before each execution. An optional resource still requires type registration. `Res<T>` and `OptionalRes<T>` expose `IsAdded` and `IsChanged` when [tracking](/guides/change-tracking) is enabled.

## Owner-thread affinity

`[Resource(OwnerThread = true)]` keeps an unmanaged resource's system on the owner thread. Disjoint worker work may overlap that system. Managed resources and reference-containing locals make their system exclusive.

## Ownership

| Call | Ownership |
| --- | --- |
| `world.InsertResource(value)` | The world borrows the value. |
| `world.InsertOwnedResource(value)` | Transfers ownership of a managed `IDisposable` value to the world. |
| `world.RemoveResource<T>()` | Removes and disposes an owned value once. |
| `world.TakeResource<T>()` | Removes a value and transfers its ownership to the caller. |
| `world.GetResource<T>()` | Returns the resource value. |

Replacing an owned resource disposes the previous one once.

## Local state

Each repeated registration and each bound schedule has independent locals, query change cursors, and message-reader cursors.