Htmx/docs/Components/Checkbox.md

# Checkbox

A styled checkbox with an optional text label. Use it in forms when you want the user to opt in or out of something — "Remember me", "I agree to the terms", or selecting items in a list.

---

## Quick example

```csharp
new Checkbox(
    id:    "newsletter",
    label: "Subscribe to newsletter",
    name:  "newsletter")
```

---

## All the options

```csharp
public Checkbox(
    string id,
    string label    = "",
    string name     = "",
    string value    = "true",
    bool   @checked = false)
```

| Parameter | What it does |
|---|---|
| `id` | The element id. Also used by the `<label for="...">` so clicking the label toggles the box. |
| `label` | Visible text next to the checkbox. Leave empty for a standalone checkbox with no label. |
| `name` | Form field name — required if you want the value submitted with the form. |
| `value` | The string that gets submitted when the box is checked. Defaults to `"true"`. |
| `checked` | Pre-tick the checkbox on render. |

---

## Real-world examples

### Terms and conditions on a registration form

```csharp
new Checkbox(
    id:    "terms",
    label: "I agree to the terms of service",
    name:  "terms",
    value: "accepted")
```

Reading it on the server:

```csharp
public record Command(
    [property: FromForm] string? Terms = null  // null when unchecked, "accepted" when checked
);

bool agreedToTerms = command.Terms == "accepted";
```

> **Important:** HTML forms only submit checkboxes that are *checked*. An unchecked checkbox sends nothing — the field is simply absent. Always use a nullable string (`string?`) or give it a default of `null`.

### Remember me (pre-ticked by default)

```csharp
new Checkbox(
    id:      "remember",
    label:   "Remember me",
    name:    "rememberMe",
    checked: true)
```

### Multiple checkboxes in a preferences form

```csharp
new Checkbox(id: "email-alerts",   label: "Email alerts",    name: "emailAlerts")
new Checkbox(id: "sms-alerts",     label: "SMS alerts",      name: "smsAlerts")
new Checkbox(id: "weekly-summary", label: "Weekly summary",  name: "weeklySummary")
```

Server-side:

```csharp
public record Command(
    [property: FromForm] string? EmailAlerts   = null,
    [property: FromForm] string? SmsAlerts     = null,
    [property: FromForm] string? WeeklySummary = null
);
```

---

## How it works

Checkbox renders a native `<input type="checkbox">` styled with `accent-primary` so the checkmark colour matches your theme's primary colour. The label is a separate `<label for="{id}">` element — clicking anywhere on the label text toggles the checkbox.

---

## Complete page example

**`Templates/PreferencesPage.htmx`**
```html
<div class="max-w-md mx-auto py-10">
  <h1 class="text-2xl font-bold mb-6">Preferences</h1>
  <form method="post" action="/preferences">
    $$AntiforgeryToken$$
    <div class="space-y-4 mb-6">
      $$NewsletterCheck$$
      $$MarketingCheck$$
      $$RememberCheck$$
    </div>
    $$SaveBtn$$
  </form>
  $$SuccessAlert$$
</div>
```

**`Templates/PreferencesPage.htmx.cs`**
```csharp
namespace Htmx.ApiDemo.Templates;

public sealed class PreferencesPage : PreferencesPageBase
{
    private readonly IHtmxComponent _newsletter;
    private readonly IHtmxComponent _marketing;
    private readonly IHtmxComponent _remember;
    private readonly IHtmxComponent _save;
    private readonly IHtmxComponent _success;
    private readonly byte[] _afToken;

    public PreferencesPage(
        IAntiforgery af,
        HttpContext ctx,
        UserPrefs? prefs = null,
        bool saved = false)
    {
        var tokens = af.GetAndStoreTokens(ctx);
        _afToken = $"""<input type="hidden" name="{tokens.FormFieldName}" value="{tokens.RequestToken}">""".ToUtf8Bytes();

        _newsletter = new Components.Checkbox(
            id: "newsletter", label: "Receive newsletter",
            name: "newsletter", @checked: prefs?.Newsletter ?? false);
        _marketing  = new Components.Checkbox(
            id: "marketing",  label: "Receive marketing emails",
            name: "marketing", @checked: prefs?.Marketing ?? false);
        _remember   = new Components.Checkbox(
            id: "remember",   label: "Keep me signed in",
            name: "remember",  @checked: prefs?.Remember ?? false);
        _save    = new Components.Button("Save preferences", type: "submit");
        _success = saved
            ? new Components.Alert(title: "Preferences saved.")
            : HtmxEmpty.Instance;
    }

    protected override void RenderAntiforgeryToken(HtmxRenderContext ctx) => ctx.Writer.WriteUtf8(_afToken);
    protected override void RenderNewsletterCheck(HtmxRenderContext ctx)  => _newsletter.Render(ctx.Next());
    protected override void RenderMarketingCheck(HtmxRenderContext ctx)   => _marketing.Render(ctx.Next());
    protected override void RenderRememberCheck(HtmxRenderContext ctx)    => _remember.Render(ctx.Next());
    protected override void RenderSaveBtn(HtmxRenderContext ctx)          => _save.Render(ctx.Next());
    protected override void RenderSuccessAlert(HtmxRenderContext ctx)     => _success.Render(ctx.Next());
}
```

**POST handler**
```csharp
[Handler]
[MapPost("/preferences")]
public static partial class PostPreferencesHandler
{
    public record Command(
        [property: FromForm] string? Newsletter = null,
        [property: FromForm] string? Marketing  = null,
        [property: FromForm] string? Remember   = null);

    private static Task<IResult> HandleAsync(
        [AsParameters] Command cmd, HttpContext ctx, IAntiforgery af, CancellationToken ct)
    {
        var prefs = new UserPrefs(
            Newsletter: cmd.Newsletter != null,
            Marketing:  cmd.Marketing  != null,
            Remember:   cmd.Remember   != null);
        // Persist prefs…
        return ctx.WriteHtmxPage(new PreferencesPage(af, ctx, prefs, saved: true), title: "Preferences");
    }
}

public record UserPrefs(bool Newsletter, bool Marketing, bool Remember);
```

**`AppJsonSerializerContext.cs`**
```csharp
[JsonSerializable(typeof(PostPreferencesHandler.Command), TypeInfoPropertyName = "PreferencesCommand")]
```