Content update - How to add an event handler using code (user story 1878475)

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dotnet-desktop-guide/net/wpf/events/how-to-add-an-event-handler-using-code.md

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+---
+title: "How to add an event handler using code"
+description: Learn how to add an event handler in code-behind for an element in Windows Presentation Foundation (WPF).
+ms.date: "02/01/2021"
+dev_langs:
+  - "csharp"
+  - "vb"
+helpviewer_keywords:
+  - "event handlers [WPF], adding"
+  - "XAML [WPF], adding event handlers"
+---
+<!-- The acrolinx score was 99 on 02/01/2021-->
+
+# How to add an event handler using code (WPF .NET)
+
+You can assign an event handler to an element in Windows Presentation Foundation (WPF) using markup or code-behind. Although it's customary to assign an event handler in Extensible Application Markup Language (XAML), sometimes you might have to assign an event handler in code-behind. For instance, use code when:
+
+- You assign an event handler to an element after the markup page that contains the element loads.
+- You add an element and assign its event handler after the markup page that will contain the element loads.
+- You define the element tree for your application entirely in code.
+
+[!INCLUDE [desktop guide under construction](../../includes/desktop-guide-preview-note.md)]
+
+## Syntax for event handler assignment
+
+C# supports event handler assignment using:
+
+- The `+=` operator, which is also used in the common language runtime (CLR) event handling model.
+- The <xref:System.Windows.UIElement.AddHandler%2A?displayProperty=nameWithType> method.
+
+VB supports event handler assignment using:
+
+- The [AddHandler](/dotnet/visual-basic/language-reference/statements/addhandler-statement) statement with the [AddressOf](/dotnet/visual-basic/language-reference/operators/addressof-operator) operator, which is also used in the CLR event handling model.
+- The [Handles](/dotnet/visual-basic/language-reference/statements/handles-clause) keyword in the event handler definition. For more information, see [Visual Basic and WPF event handling](/dotnet/desktop/wpf/advanced/visual-basic-and-wpf-event-handling?view=netframeworkdesktop-4.8&preserve-view=true).
+- The <xref:System.Windows.UIElement.AddHandler%2A?displayProperty=nameWithType> method, together with the `AddressOf` operator to reference the event handler.
+
+## Example
+
+The following example uses XAML to define a <xref:System.Windows.Controls.Button> named `ButtonCreatedByXaml` and to assign the `ButtonCreatedByXaml_Click` method as its <xref:System.Windows.Controls.Primitives.ButtonBase.Click> event handler. `Click` is a built-in routed event for buttons that derive from <xref:System.Windows.Controls.Primitives.ButtonBase>.
+
+:::code language="xaml" source="./snippets/how-to-add-an-event-handler-using-code/csharp/MainWindow.xaml" id="ButtonCreatedByXaml":::
+
+The example uses code-behind to implement the `ButtonCreatedByXaml_Click` and `ButtonCreatedByCode_Click` handlers, and to assign the `ButtonCreatedByCode_Click` handler to the `ButtonCreatedByCode` and `StackPanel1` elements. Event handler methods can only be implemented in code-behind.
+
+:::code language="csharp" source="./snippets/how-to-add-an-event-handler-using-code/csharp/MainWindow.xaml.cs" id="ButtonEventHandlers":::
+:::code language="vb" source="./snippets/how-to-add-an-event-handler-using-code/vb/MainWindow.xaml.vb" id="ButtonEventHandlers":::
+
+When `ButtonCreatedByXaml` is clicked and its event handler runs, `ButtonCreatedByXaml_Click` programmatically:
+
+1. Adds a new button named `ButtonCreatedByCode` to the already constructed XAML element tree.
+1. Specifies properties for the new button, such as the name, content, and background color.
+1. Assigns the `ButtonCreatedByCode_Click` event handler to `ButtonCreatedByCode`.
+1. Assigns the same `ButtonCreatedByCode_Click` event handler to `StackPanel1`.
+
+When `ButtonCreatedByCode` is clicked:
+
+1. The <xref:System.Windows.Controls.Primitives.ButtonBase.Click> routed event is raised on `ButtonCreatedByCode`.
+1. The `ButtonCreatedByCode_Click` event handler assigned to `ButtonCreatedByCode` is triggered.
+1. The `Click` routed event traverses up the element tree to `StackPanel1`.
+1. The `ButtonCreatedByCode_Click` event handler assigned to `StackPanel1` is triggered.
+1. The `Click` routed event continues up the element tree potentially triggering other `Click` event handlers assigned to other traversed elements.
+
+The `ButtonCreatedByCode_Click` event handler obtains the following information about the event that triggered it:
+
+- The [sender](xref:System.Windows.RoutedEventHandler) object, which is the element that the event handler is assigned to. The `sender` will be `ButtonCreatedByCode` the first time the handler runs, and `StackPanel1` the second time.
+- The <xref:System.Windows.RoutedEventArgs.Source?displayProperty=nameWithType> object, which is the element that originally raised the event. In this example, the `Source` is always `ButtonCreatedByCode`.
+
+> [!NOTE]
+> A key difference between a routed event and a CLR event is that a routed event traverses the element tree, whereas a CLR event occurs only on the `sender`. As a result, a routed event `sender` can be any traversed element in the element tree.
+
+For more information on how to create and handle routed events, see [How to create a custom routed event](/dotnet/desktop/wpf/advanced/how-to-create-a-custom-routed-event?view=netframeworkdesktop-4.8&preserve-view=true) and [Handle a routed event](/dotnet/desktop/wpf/advanced/how-to-handle-a-routed-event?view=netframeworkdesktop-4.8&preserve-view=true).
+
+## See also
+
+- [Routed events overview](/dotnet/desktop/wpf/advanced/routed-events-overview?view=netframeworkdesktop-4.8&preserve-view=true)
+- [XAML in WPF](/dotnet/desktop/wpf/advanced/xaml-in-wpf?view=netframeworkdesktop-4.8&preserve-view=true)

dotnet-desktop-guide/net/wpf/events/snippets/how-to-add-an-event-handler-using-code/csharp/App.xaml.cs

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+using System;
+using System.Collections.Generic;
+using System.Configuration;
+using System.Data;
+using System.Linq;
+using System.Threading.Tasks;
+using System.Windows;
+
+namespace CodeSampleCsharp
+{
+    /// <summary>
+    /// Interaction logic for App.xaml
+    /// </summary>
+    public partial class App : Application
+    {
+    }
+}

dotnet-desktop-guide/net/wpf/events/snippets/how-to-add-an-event-handler-using-code/csharp/AssemblyInfo.cs

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+using System.Windows;
+
+[assembly: ThemeInfo(
+    ResourceDictionaryLocation.None, //where theme specific resource dictionaries are located
+                                     //(used if a resource is not found in the page,
+                                     // or application resource dictionaries)
+    ResourceDictionaryLocation.SourceAssembly //where the generic resource dictionary is located
+                                              //(used if a resource is not found in the page,
+                                              // app, or any theme specific resource dictionaries)
+)]

dotnet-desktop-guide/net/wpf/events/snippets/how-to-add-an-event-handler-using-code/csharp/CodeSampleCsharp.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>WinExe</OutputType>
+    <TargetFramework>net6.0-windows</TargetFramework>
+    <UseWPF>true</UseWPF>
+    <NeutralLanguage>en-us</NeutralLanguage>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Update="Properties\Resources.Designer.cs">
+      <DesignTime>True</DesignTime>
+      <AutoGen>True</AutoGen>
+      <DependentUpon>Resources.resx</DependentUpon>
+    </Compile>
+  </ItemGroup>
+
+  <ItemGroup>
+    <EmbeddedResource Update="Properties\Resources.resx">
+      <Generator>ResXFileCodeGenerator</Generator>
+      <LastGenOutput>Resources.Designer.cs</LastGenOutput>
+    </EmbeddedResource>
+  </ItemGroup>
+
+</Project>

dotnet-desktop-guide/net/wpf/events/snippets/how-to-add-an-event-handler-using-code/csharp/MainWindow.xaml

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+<Window x:Class="CodeSampleCsharp.MainWindow"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        Title="How to add an event handler using code" Height="100" Width="300">
+
+    <!--<ButtonCreatedByXaml>-->
+    <StackPanel Name="StackPanel1">
+        <Button
+            Name="ButtonCreatedByXaml" 
+            Click="ButtonCreatedByXaml_Click"
+            Content="Create a new button with an event handler"
+            Background="LightGray">
+        </Button>
+    </StackPanel>
+    <!--</ButtonCreatedByXaml>-->
+</Window>

dotnet-desktop-guide/net/wpf/events/snippets/how-to-add-an-event-handler-using-code/csharp/MainWindow.xaml.cs

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+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Controls.Primitives;
+using System.Windows.Media;
+
+namespace CodeSampleCsharp
+{
+    /// <summary>
+    /// Interaction logic for MainWindow.xaml.
+    /// </summary>
+    public partial class MainWindow : Window
+    {
+        public MainWindow()
+        {
+            InitializeComponent();
+        }
+
+        //<ButtonEventHandlers>
+        // The click event handler for the existing button 'ButtonCreatedByXaml'.
+        private void ButtonCreatedByXaml_Click(object sender, RoutedEventArgs e)
+        {
+            // Create a new button.
+            Button ButtonCreatedByCode = new();
+
+            // Specify button properties.
+            ButtonCreatedByCode.Name = "ButtonCreatedByCode";
+            ButtonCreatedByCode.Content = "New button and event handler created in code";
+            ButtonCreatedByCode.Background = Brushes.Yellow;
+
+            // Add the new button to the StackPanel.
+            StackPanel1.Children.Add(ButtonCreatedByCode);
+
+            // Assign an event handler to the new button using the '+=' operator.
+            ButtonCreatedByCode.Click += new RoutedEventHandler(ButtonCreatedByCode_Click);
+
+            // Assign an event handler to the new button using the AddHandler method.
+            // AddHandler(ButtonBase.ClickEvent, new RoutedEventHandler(ButtonCreatedByCode_Click);
+
+            // Assign an event handler to the StackPanel using the AddHandler method.
+            StackPanel1.AddHandler(ButtonBase.ClickEvent, new RoutedEventHandler(ButtonCreatedByCode_Click));
+        }
+
+        // The Click event handler for the new button 'ButtonCreatedByCode'.
+        private void ButtonCreatedByCode_Click(object sender, RoutedEventArgs e)
+        {
+            string sourceName = ((FrameworkElement)e.Source).Name;
+            string senderName = ((FrameworkElement)sender).Name;
+
+            Debug.WriteLine($"Routed event handler attached to {senderName}, " +
+                $"triggered by the Click routed event raised by {sourceName}.");
+        }
+        //</ButtonEventHandlers>
+
+        // Debug output when ButtonCreatedByCode is clicked:
+        // Routed event handler attached to ButtonCreatedByCode, triggered by the Click routed event raised by ButtonCreatedByCode.
+        // Routed event handler attached to StackPanel1, triggered by the Click routed event raised by ButtonCreatedByCode.
+    }
+}