---
title: Components and bundles
description: Declare components, tags, and bundles.
sidebar:
  order: 1
---

```csharp
using Archevolt;

[Component, TrackChanges]
public readonly record struct Position(float X);

[Component]
public readonly record struct Velocity(float X);

[Component]
public readonly struct Frozen;

[Bundle]
public readonly partial record struct Particle(Position Position, Velocity Velocity);
```

## Components

Components are unmanaged value types. The generator rejects reference-containing components and components that implement `IDisposable` (diagnostic [`AV002`](/reference/diagnostics)). Native pointers and external handles remain the application's responsibility.

A component with no instance fields is a **tag**. Tags support membership and tracked change filters, but have no readable or writable value column. Test for one with `Has<T>`.

Prefer `readonly record struct` components. A writable `ref` still lets a system replace the whole value with a `with` expression. Other unmanaged structs are supported when their layout or update style is useful.

Add `[TrackChanges]` to enable [added and changed stamps](/guides/change-tracking).

## Bundles

A bundle describes insertion into an entity. Bundles are unmanaged partial structs. Their generated `Descriptor` and `IBundle<T>` implementation flatten nested bundles, and a repeated component type is an error. A bundle does not provide entity identity.

Spawn a bundle with `world.Spawn<TBundle>(in bundle)` or `Commands.Spawn<TBundle>(in bundle)`. `InsertBundle` adds or replaces bundle members and preserves unrelated components.

Bundles used by spawn commands must be included in a [module](/guides/modules) explicitly.