.NET MAUI Custom Handlers

Decision: Customize Existing vs. Create New

| Scenario | Approach | |---|---| | Change how a built-in control looks/behaves on one platform | Customize — use AppendToMapping / PrependToMapping | | Need the change on only some instances of a control | Customize — subclass the control + type-check in mapper | | Need a completely new cross-platform control with native backing | Create new handler with partial classes |

⚠️ Prefer AppendToMapping over ModifyMapping. ModifyMapping replaces the default mapper action entirely — if the framework adds behaviour in a future release, your override silently drops it.

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Gotchas & Common Mistakes

Mapper customizations are global

Every instance of the control is affected. Guard with a subclass check for instance-specific behaviour:

// ❌ Removes borders from EVERY Entry in the app
EntryHandler.Mapper.AppendToMapping("NoBorder", (handler, view) =>
{
#if ANDROID
    handler.PlatformView.Background = null;
#endif
});

// ✅ Only affects BorderlessEntry instances
EntryHandler.Mapper.AppendToMapping("NoBorder", (handler, view) =>
{
    if (view is not BorderlessEntry) return;
#if ANDROID
    handler.PlatformView.Background = null;
#endif
});

Unsubscribe native events in HandlerChanging

Failing to remove native event handlers causes memory leaks because the native view may outlive the managed wrapper.

// ❌ Subscribes but never unsubscribes — leaks
entry.HandlerChanged += (s, e) =>
{
#if ANDROID
    ((Entry)s!).Handler!.PlatformView.As<Android.Widget.EditText>()!
        .FocusChange += OnNativeFocusChange;
#endif
};

// ✅ Pair subscribe in HandlerChanged with unsubscribe in HandlerChanging
entry.HandlerChanged += OnHandlerChanged;
entry.HandlerChanging += OnHandlerChanging;

Partial class name/namespace mismatch

Namespace and class name must match exactly across the shared handler file and every platform file. A mismatch silently creates separate classes — no compiler error, just a handler that does nothing on that platform.

Conditional using placement

The using PlatformView = ... aliases must be at the top of the shared handler file (not the platform files) so the ViewHandler<TControl, TPlatformView> base-class generic resolves correctly per platform.

// ✅ Top of Handlers/VideoPlayerHandler.cs
#if ANDROID
using PlatformView = Android.Widget.VideoView;
#elif IOS || MACCATALYST
using PlatformView = AVKit.AVPlayerViewController;
#elif WINDOWS
using PlatformView = Microsoft.UI.Xaml.Controls.MediaPlayerElement;
#endif

Missing CreatePlatformView()

Each platform partial must override CreatePlatformView(). Omitting it produces a compile error — but the error message points at the base class, not your handler, making it confusing to debug.

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Mapper Method Selection

| Method | Risk | Use when | |---|---|---| | AppendToMapping | Low — runs after default | Adding behaviour without breaking defaults | | PrependToMapping | Low — runs before default | Setting initial state that the default can override | | ModifyMapping | ⚠️ High — replaces default | You intentionally want to suppress the framework's mapper logic |

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PropertyMapper vs. CommandMapper

| Mapper | Purpose | Pattern | |---|---|---| | PropertyMapper | Sync a bindable property to the native view | Runs whenever the property value changes | | CommandMapper | Fire-and-forget action from control → handler | Runs once per invocation, no return value |

⚠️ Don't put property sync logic in CommandMapper — it won't re-run when the property changes, leading to stale native views.

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Checklist — New Handler

• [ ] Cross-platform control inherits View (or appropriate base)
• [ ] Shared handler file has conditional using PlatformView = ... aliases
• [ ] Handler inherits ViewHandler<TControl, PlatformView>
• [ ] PropertyMapper maps every bindable property
• [ ] Each platform partial overrides CreatePlatformView()
• [ ] Namespace + class name identical across all partial files
• [ ] Handler registered in MauiProgram.cs via ConfigureMauiHandlers
• [ ] Native event subscriptions cleaned up in HandlerChanging