godot-csharp

The full skill.

Original instructions from the publisher’s SKILL.md

# Godot C# / .NET (4.x)

Write Godot game code in C#: node subclasses, the engine lifecycle, exports, signals as
events, and GDScript interop. Targets **Godot 4.7 (.NET / C#)** and **.NET 8**.

## When to use

- Use when scripting a Godot game in C# (`.cs` + `.csproj`), translating GDScript idioms
  to C#, exposing `[Export]` fields, or wiring `[Signal]` delegates and GetNode<T>.

**When *not* to use:** GDScript-specific syntax → `godot-gdscript`; engine concepts that
are language-neutral (scenes, physics, animation) → the relevant `godot-*` skill. You need
the **Godot .NET build** + the .NET SDK installed; the standard build can't run C#.

## Core workflow

1. **Use the Godot .NET editor build** and install the matching .NET 8 SDK. Creating the first C#
   script generates a `.csproj`/`.sln`. Build with the editor or `dotnet build`.
2. **Every node script is a `partial` class** extending a Godot type (the source generator
   relies on `partial`). The file/class name should match the node script.
3. **Override lifecycle methods in PascalCase** with `double` delta: `_Ready()`,
   `_Process(double delta)`, `_PhysicsProcess(double delta)`.
4. **Expose tunables with `[Export]`**; they show in the Inspector like GDScript `@export`.
5. **Declare signals as `[Signal]` delegates** named `XxxEventHandler`; emit with
   `EmitSignal(SignalName.Xxx, ...)` and subscribe with the generated C# `event`.
6. **Get nodes with `GetNode<T>("Path")`** (or `%Unique`), and call into GDScript with
   `Call`/`Get`/`Set` when needed.

## Patterns

### 1. A node script: lifecycle, [Export], GetNode<T>

```csharp
using Godot;

public partial class Player : CharacterBody2D
{
    [Export] public float Speed = 200.0f;          // editable in the Inspector
    [Export] public float JumpVelocity = -400.0f;

    private const float Gravity = 1200.0f;
    private AnimatedSprite2D _sprite;

    public override void _Ready()
    {
        _sprite = GetNode<AnimatedSprite2D>("AnimatedSprite2D");
    }

    public override void _PhysicsProcess(double delta)
    {
        Vector2 v = Velocity;                       // Velocity is a property here
        if (!IsOnFloor())
            v.Y += Gravity * (float)delta;          // delta is double; cast for float math
        if (Input.IsActionJustPressed("jump") && IsOnFloor())
            v.Y = JumpVelocity;

        float dir = Input.GetAxis("move_left", "move_right");
        v.X = dir != 0 ? dir * Speed : Mathf.MoveToward(v.X, 0, Speed);

        Velocity = v;
        MoveAndSlide();                             // no args, like GDScript 4.x
    }
}
```

### 2. Signals as C# events

```csharp
using Godot;

public partial class Health : Node
{
    // Delegate name MUST end with "EventHandler"; generator creates the event + SignalName.
    [Signal] public delegate void HealthChangedEventHandler(int current, int max);

    private int _hp = 100;

    public void TakeDamage(int amount)
    {
        _hp = Mathf.Max(_hp - amount, 0);
        EmitSignal(SignalName.HealthChanged, _hp, 100);   // type-safe signal name
    }

    public override void _Ready()
    {
        HealthChanged += OnHealthChanged;            // subscribe like a normal C# event
    }

    private void OnHealthChanged(int current, int max) => GD.Print($"HP {current}/{max}");
}
```

### 3. Instancing a scene in C#

```csharp
public partial class Spawner : Node2D
{
    // Load once; PackedScene is the C# equivalent of preload's result.
    private readonly PackedScene _bullet = GD.Load<PackedScene>("res://bullet.tscn");

    public void Shoot(Vector2 at)
    {
        var b = _bullet.Instantiate<Node2D>();       // typed instantiate
        b.GlobalPosition = at;
        AddChild(b);
    }
}
```

### 4. Interop with GDScript nodes

```csharp
public override void _Ready()
{
    Node gd = GetNode("GDScriptNode");
    // Call a GDScript method and read/write its properties dynamically.
    gd.Call("take_damage", 10);
    int score = (int)gd.Get("score");
    gd.Set("score", score + 5);
    // Connect to a GDScript signal by name:
    gd.Connect("died", Callable.From(OnDied));
}

private void OnDied() => GD.Print("entity died");
```

## Pitfalls

- **Forgetting `partial`.** Without `partial`, the Godot source generator can't extend the
  class and `[Export]`/`[Signal]` break with confusing build errors.
- **Wrong method case/signature.** C# overrides are `_Ready`, `_Process(double)`,
  `_PhysicsProcess(double)` — PascalCase and `double` delta (GDScript uses snake_case and
  `float`). A mismatched name just won't be called.
- **`[Signal]` delegate naming.** It must end with `EventHandler`; the engine exposes the
  signal as the name without that suffix and generates `SignalName.X` and a C# `event`.
- **`GD.Print` vs `Console.WriteLine`.** Use `GD.Print`/`GD.PrintErr` to reach the Godot
  output panel; `Console` output may not appear.
- **Value-type structs.** `Vector2`, `Color`, `Transform2D` are structs — mutate a local
  copy (`var v = Velocity; v.X = ...; Velocity = v;`); editing `Velocity.X` directly won't
  compile/persist.
- **Needs the .NET build + SDK.** The non-.NET editor can't run C#; mismatched/missing
  .NET SDK causes build failures. Godot 4.7 targets .NET 8; check current platform
  export notes because Android and other AOT targets can require newer SDK tooling.
- **`QueueFree()` vs `Free()`** — same rules as GDScript; prefer `QueueFree()`. Disposed
  objects throw `ObjectDisposedException` if used after freeing.
- **Export to some platforms differs for .NET** (e.g. extra steps for web/mobile); check
  the .NET export notes for your target.

## References

- For export attribute variants (`[ExportGroup]`, ranges, typed arrays), async with
  `await ToSignal(...)`, `Godot.Collections` vs System collections, custom Resources in
  C#, and project/build setup, read `references/csharp-setup-and-interop.md`.

## Related skills

- `godot-gdscript` — the GDScript equivalents of these patterns.
- `godot-signals-groups` — signal/event architecture (language-neutral).
- `godot-resources` — data resources; the C# `[Export]` + `Resource` pattern.
- `unity-csharp-scripting` — C# in Unity, for developers coming from there.