Respond to an Event

There are a number of ways you can implement actions in an Avalonia application. On this page, you will see how to use one of the simplest: how to write event handling code for a button click.

To start, you will write a button click event handler that does not interact with any of the other controls.

Code-Behind

XAML files can have C# source files associated with it referred to as by "code-behind". Code-behind is used to access named controls and handle events for its associated XAML. When using an IDE, you can find this file in the Solution Explorer as a sub-item of the .axaml file.

To change the code-behind for MainWindow:

Open the MainWindow.axaml.cs file

You should see C# code like this:

using Avalonia.Controls;

namespace GetStartedApp.Views;

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }
}

The partial class MainWindow corresponds to the Window created by Avalonia as a result of the XAML you already have. The namespace and class name must be the same in both XAML and code-behind. You can find this class name in the root XAML tag:

<UserControl x:Class="GetStartedApp.Views.MainWindow"
    ...>
</UserControl>

To add an event handler for the Button, follow this procedure:

Locate the MainWindow constructor in the code-behind file (see above instructions).
After the constructor, add the following code:

private void Button_OnClick(object? sender, RoutedEventArgs e)
{
    Debug.WriteLine("Click!");
}

This will require the following using statements:

using Avalonia.Interactivity;
using System.Diagnostics;

Switch to the XAML file and locate the <Button> tag.
Enter the Click attribute at the end of the tag, as follows:

<Button Grid.Row="2"  Grid.Column="1" Click="Button_OnClick">Calculate</Button>

Run the app in Debug mode and click the Calculate button.

Rider
Visual Studio

You should see the result on the Output window for Debug, like this:

On the next page, you will see how to use code-behind to read and change the properties of Avalonia controls at runtime.

Code-Behind​

Code-Behind