---
name: csharp-signals
description: Use when implementing signals in C# — [Signal] delegates, EmitSignal patterns, async signal awaiting, and event-driven architecture
---

# Signals in C# (Godot 4.x)

This skill is **C# only**. For general C# conventions and project setup, see the **csharp-godot** skill. Godot signals in C# require a different mental model from GDScript: delegates declared with `[Signal]`, strongly-typed `+=`/`-=` connections, and mandatory disconnection in `_ExitTree()`. All examples target Godot 4.x with no deprecated APIs.

> **Related skills:** **csharp-godot** for C# conventions and project setup, **event-bus** for global signal hub architecture, **component-system** for signal-based component communication.

---

## 1. Signal Declaration

Signals are declared as `public delegate void` with the `[Signal]` attribute inside a `partial class` that extends a Godot type. The delegate name **must** end with `EventHandler` — Godot strips that suffix to produce the signal name exposed to the engine.

```csharp
using Godot;

public partial class Player : CharacterBody2D
{
    // Signal name in engine: "HealthChanged"
    [Signal] public delegate void HealthChangedEventHandler(int current, int maximum);

    // Signal name in engine: "Died"
    [Signal] public delegate void DiedEventHandler();

    // Signal name in engine: "ItemCollected"
    [Signal] public delegate void ItemCollectedEventHandler(string itemName);
}
```

**Naming rules:**

| Delegate name                   | Engine signal name  |
|---------------------------------|---------------------|
| `HealthChangedEventHandler`     | `HealthChanged`     |
| `DiedEventHandler`              | `Died`              |
| `ItemCollectedEventHandler`     | `ItemCollected`     |
| `PlayerSpawnedEventHandler`     | `PlayerSpawned`     |

Omitting the `EventHandler` suffix compiles without error but registers no Godot signal — the signal will not appear in the editor and `EmitSignal` will throw at runtime.

**Parameter type constraints:** Signal parameters must be Godot-marshallable types: `int`, `float`, `bool`, `string`, `Vector2`, `Vector3`, `Color`, `GodotObject` subclasses, `GodotDictionary`, `GodotArray`. Plain C# classes, structs, and generics are not allowed as parameters.

---

## 2. Emitting Signals

Use `EmitSignal(SignalName.SignalName, args...)`. The `SignalName` nested class is auto-generated by the Godot source generators at build time — one static string constant per declared signal.

```csharp
using Godot;

public partial class Player : CharacterBody2D
{
    [Signal] public delegate void HealthChangedEventHandler(int current, int maximum);
    [Signal] public delegate void DiedEventHandler();
    [Signal] public delegate void ItemCollectedEventHandler(string itemName);

    [Export] public int MaxHealth { get; set; } = 100;
    private int _currentHealth;

    public override void _Ready()
    {
        _currentHealth = MaxHealth;
    }

    public void TakeDamage(int amount)
    {
        _currentHealth = Mathf.Clamp(_currentHealth - amount, 0, MaxHealth);

        // Type-safe emission — SignalName.HealthChanged is a generated constant.
        EmitSignal(SignalName.HealthChanged, _currentHealth, MaxHealth);

        if (_currentHealth == 0)
            EmitSignal(SignalName.Died);
    }

    public void CollectItem(string itemName)
    {
        EmitSignal(SignalName.ItemCollected, itemName);
    }
}
```

`EmitSignal` validates argument count and types at runtime in debug builds. Passing the wrong number of arguments raises an error immediately, making bugs easy to locate.

---

## 3. Connecting Signals

### The `+=` operator (preferred)

```csharp
using Godot;

public partial class HudLayer : CanvasLayer
{
    private Player _player;

    public override void _Ready()
    {
        _player = GetNode<Player>("../Player");

        // Connect with += — mirrors C# event syntax.
        _player.HealthChanged += OnHealthChanged;
        _player.Died          += OnDied;
        _player.ItemCollected += OnItemCollected;
    }

    private void OnHealthChanged(int current, int maximum)
    {
        GetNode<ProgressBar>("HealthBar").Value = (double)current / maximum * 100.0;
        GetNode<Label>("HealthLabel").Text = $"{current} / {maximum}";
    }

    private void OnDied()
    {
        GetNode<Control>("DeathScreen").Show();
    }

    private void OnItemCollected(string itemName)
    {
        GetNode<Label>("PickupLabel").Text = $"Picked up: {itemName}";
    }
}
```

### Lambda connections

Use lambdas for one-off, short-lived responses. Store the lambda in a field if you need to disconnect it later.

```csharp
// Anonymous lambda — cannot be disconnected by reference later.
_player.Died += () => GetNode<AudioStreamPlayer>("DeathSound").Play();

// Stored lambda — can be disconnected.
private Action<string> _onItemCollected;

public override void _Ready()
{
    _onItemCollected = (itemName) =>
    {
        _collectCount++;
        UpdateCollectDisplay();
    };
    _player.ItemCollected += _onItemCollected;
}

public override void _ExitTree()
{
    _player.ItemCollected -= _onItemCollected;
}
```

### Connecting in `_Ready()`

Always connect inside `_Ready()`. The node's references are resolved and the scene tree is available at that point. Connecting in the constructor or field initializers may fail because Godot node infrastructure is not yet initialised.

---

## 4. Disconnecting Signals

Unlike GDScript (which auto-cleans on `queue_free`), **C# must disconnect explicitly** in `_ExitTree()` — otherwise the listener delegate keeps the GodotObject alive past disposal, causing leaks. Use `signal -= handler` mirroring how you `+= handler` in `_Ready()`.

> See [references/disconnecting.md](references/disconnecting.md) for the `-=` cleanup pattern, why C# differs from GDScript, and a SafeDisconnect helper.

---

## 5. Awaiting Signals

`await ToSignal(node, SignalName.X)` pauses until the signal fires. Returns `Variant[]` of the signal's args. Add a timeout with `Task.WhenAny` or a `CancellationTokenSource` to avoid permanent hangs.

> See [references/awaiting-signals.md](references/awaiting-signals.md) for basic await, return-value extraction, and both timeout patterns.

---

## 6. Custom Signal Patterns

Three patterns: **typed event args** (wrap signal payload in a Resource subclass), **static event bus** (use C# `static event` as a pure-C# alternative when both peers are C#), **generic signal helper** (one helper per `SignalName.X` to reduce repetition).

> See [references/custom-signal-patterns.md](references/custom-signal-patterns.md) for full code on each pattern.

---

## 7. Connecting GDScript Signals from C#

When the signal is declared in a GDScript node and the listener is C#, use `node.Connect("signal_name", new Callable(this, MethodName.Handler))`. The `MethodName` generated symbol still works for C# methods.

> See [references/connecting-gdscript-signals.md](references/connecting-gdscript-signals.md) for the full pattern with both string-name and `MethodName` variants.

---

## 8. Common Mistakes

| Mistake | Symptom | Fix |
|---|---|---|
| Forgetting `EventHandler` suffix on the delegate | Compiles fine; signal does not appear in editor; `EmitSignal` fails at runtime with "signal not found" | Rename to `XxxEventHandler` |
| Wrong parameter types or count in `EmitSignal` | Runtime error in debug build: "Signal parameter mismatch" | Match `EmitSignal` args exactly to the delegate signature |
| Connecting to a freed object | `ObjectDisposedException` or silent crash on next signal emission | Guard with `IsInstanceValid` before connecting; store reference safely |
| Not disconnecting in `_ExitTree()` | Memory leak; crash on next signal emission after node is freed | Add `_ExitTree()` override with matching `-=` for every `+=` |
| Passing wrong argument count to `EmitSignal` | Debug-build error: "Expected N arguments, got M" | Count signal delegate parameters and match exactly |
| Using static C# events for cross-language signals | GDScript cannot observe the event; no error, just silent non-delivery | Use Godot `[Signal]` for any signal GDScript must receive |
| Double-connecting the same handler | Handler fires twice per emission; subtle logic bugs | Check if already connected, or use `ConnectFlags.OneShot`; never call `+=` twice for the same method |
| Awaiting a signal in `_Ready()` before the tree is ready | `NullReferenceException` or signal never resolves because the source node is not yet in the tree | Defer with `await ToSignal(GetTree(), SceneTree.SignalName.ProcessFrame)` first, or move the await to a method called after `_Ready()` |

---

## 9. Checklist

- [ ] Every `[Signal]` delegate name ends with `EventHandler`
- [ ] `EmitSignal` uses `SignalName.XxxName` constants (not raw strings)
- [ ] `EmitSignal` argument count and types match the delegate signature exactly
- [ ] All `+=` connections are in `_Ready()` (not in constructors or field initializers)
- [ ] Every `+=` has a matching `-=` in `_ExitTree()`
- [ ] `IsInstanceValid` guards are in place where the signal source may be freed before the subscriber
- [ ] Lambdas stored in fields if disconnection is needed later
- [ ] `async` signal awaits are not blocking `_Ready()` directly; deferred if necessary
- [ ] Cross-language signals (GDScript emitter, C# subscriber) use `Connect("snake_case_name", Callable.From<T>(Method))`
- [ ] Static C# events used only for pure-C# communication paths
- [ ] `RefCounted`-derived wrapper classes used for complex signal payloads, not plain C# classes or structs