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docs/debugger/hot-reload-metadataupdatehandler.md

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+---
+title: "Extend .NET Hot Reload"
+description: "Learn to use the MetadataUpdateHandler attribute to extend .NET Hot Reload to support additional scenarios."
+ms.date: 06/20/2023
+ms.topic: conceptual
+helpviewer_keywords:
+  - "Hot reload, MetadataUpdateHandler"
+  - ".NET Hot Reload, MetadataUpdateHandler"
+  - "MetadataUpdateHandler for .NET Hot Reload"
+author: mikejo5000
+ms.author: mikejo
+manager: jmartens
+ms.technology: vs-ide-debug
+monikerRange: '>= vs-2022'
+ms.workload:
+  - "multiple"
+---
+# Extend .NET Hot Reload using MetadataUpdateHandler (C#, Visual Basic)
+
+ [!INCLUDE [Visual Studio](~/includes/applies-to-version/vs-windows-only.md)]
+
+You can programmatically extend .NET Hot Reload support for additional scenarios that aren't typically supported, such as code changes that require clearing a cache or refreshing the UI. For example, to support hot reload with a JSON serializer, you need to clear its cache when a type is modified. For .NET MAUI developers, you may need to extend hot reload for edits/updates that don't trigger hot reload under normal conditions, such as editing a constructor, or an event handler for a UI element. You can use the <xref:System.Reflection.Metadata.MetadataUpdateHandlerAttribute> to refresh the application state, trigger a UI re-render, or perform similar actions.
+
+The type specified by this attribute should implement static methods matching the signature of one or more of the following:
+
+```csharp
+static void ClearCache(Type[]? updatedTypes)
+static void UpdateApplication(Type[]? updatedTypes)
+```
+
+`ClearCache` gives update handlers an opportunity to clear any caches that are inferred based on the application's metadata. After all `ClearCache` methods have been invoked, `UpdateApplication` is invoked for every handler that specifies one. You might use `UpdateApplication` to refresh the UI.
+
+## Example
+
+The following example shows a scenario for a .NET MAUI project that initially does not support hot reload, but then supports the feature after implementing `MetadataUpdateHandler`.
+
+### Test .NET Hot Reload
+
+1. Create a new .NET MAUI project in Visual Studio. Choose the **.NET MAUI App** project template.
+
+1. In *App.xaml.cs*, replace the code to create the MainPage with the following code:
+
+   ```csharp
+   //MainPage = new MainPage(); // Template default code
+   MainPage = new NavigationPage(new MainPage());
+   ```
+
+   Next, you implement a Build method to simplify a UI update in C#. This method sets the `ContentPage.Content` and is called in the page's `OnNavigatedTo`. The `OnNavigatedTo` event must be hosted within Shell or a NavigationPage.
+
+1. In *MainPage.xaml.cs*, replace the `MainPage` constructor code with the following code:
+
+   ```csharp
+   public MainPage()
+   {
+      InitializeComponent();
+      Build();
+   }
+
+   void Build() => Content =
+      new Label
+      {
+         Text = "First line\nSecond line"
+      };
+
+   protected override void OnNavigatedTo(NavigatedToEventArgs args)
+   {
+      base.OnNavigatedTo(args);
+      Build();
+   }
+   ```
+
+1. Press **F5** to start the app.
+
+1. After the page loads, change the label text in the C# code to something like: "First line\nSecond line\nThird line"
+
+1. Select the **Hot Reload** ![Screenshot of the Hot Reload button.](../debugger/media/vs-2022/hot-reload-icon.png) button.
+
+   The updated text does not display in the running app. There's no Hot Reload support for this scenario by default.
+
+   ![Screenshot of Hot Reload not working.](../debugger/media/vs-2022/hot-reload-metadataupdatehandler-example.png)
+
+### Add the MetadataUpdateHandler
+
+In a .NET MAUI app, you must do something to re-run C# UI code after you make a code change. If your UI code is written in C#, you could use the `UpdateApplication` method in `MetadataUpdateHandler` to reload the UI. To set this up, add *HotReloadService.cs* to your application using the following code.
+
+```csharp
+#if DEBUG
+[assembly: System.Reflection.Metadata.MetadataUpdateHandlerAttribute(typeof(YourAppNamespace.HotReloadService))]
+namespace YourAppNamespace { 
+    public static class HotReloadService
+    {
+        #pragma warning disable CS8632 // The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.
+        public static event Action<Type[]?>? UpdateApplicationEvent;
+        #pragma warning restore CS8632 // The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.
+
+        internal static void ClearCache(Type[]? types) { }
+        internal static void UpdateApplication(Type[]? types) {
+            UpdateApplicationEvent?.Invoke(types);
+        }
+    }
+}
+#endif
+```
+
+Make sure you replace `YourAppNamespace` with the namespace for the page you're targeting.
+
+Now, with the preceding code added, when you edit live code in Visual Studio, a metadata change occurs and the app dispatches the `UpdateApplicationEvent`. So, you need to add code to register the event and perform the UI update.
+
+> [!NOTE]
+> For this scenario, XAML Hot Reload must be enabled.
+
+In *MainPage.xaml.cs*, add code to register the `UpdateApplicationEvent` event handler in the `OnNavigatedTo` event.
+
+```csharp
+protected override void OnNavigatedTo(NavigatedToEventArgs args)
+    {
+        base.OnNavigatedTo(args);
+
+        Build();
+
+#if DEBUG
+        HotReloadService.UpdateApplicationEvent += ReloadUI;
+#endif
+    }
+```
+
+Unsubscribe the event handler in `OnNavigatedFrom` and then add code to handle the event and re-execute the call to `Build`.
+
+```csharp
+protected override void OnNavigatedFrom(NavigatedFromEventArgs args)
+   {
+   base.OnNavigatedFrom(args);
+
+#if DEBUG
+   HotReloadService.UpdateApplicationEvent -= ReloadUI;
+#endif
+    }
+
+private void ReloadUI(Type[] obj)
+{
+   MainThread.BeginInvokeOnMainThread(() =>
+   {
+      Build();
+   });
+}
+```
+
+Now, start the app. When you make a change to the label text in your C# code and hit the Hot Reload button, the UI gets refreshed!
+
+![Screenshot of Hot Reload working.](../debugger/media/vs-2022/hot-reload-metadataupdatehandler-example-works.png)
+
+## Additional resources
+
+- [C# UI and .NET Hot Reload](https://dev.to/davidortinau/c-ui-and-net-hot-reload-a-match-made-in-net-maui-243f)
+- [Supporting Hot Reload in your .NET application](https://www.meziantou.net/supporting-hot-reload-in-your-dotnet-application.htm)

docs/debugger/media/vs-2022/hot-reload-icon.png

@@ -110,8 +110,12 @@
       href: mixed-code-and-missing-information-in-the-call-stack-window.md
   - name: Live code editing
     items:
-    - name: Hot Reload (.NET/C++)
-      href: hot-reload.md
+    - name: Hot Reload
+      items:
+      - name: Hot Reload (.NET/C++)
+        href: hot-reload.md
+      - name: Extend Hot Reload (.NET)
+        href: hot-reload-metadataupdatehandler.md
     - name: XAML Hot Reload
       items:
       - name: Write and debug running XAML code