Application.Current.MainWindow not available in Avalonia

[ShrutiJaiswal1494]

Hi,
I want to show a dialog window when a button is clicked inside one of my UserControl's ViewModel. For the button click event on the user control I have the following code but the Application.Current.MainWindow is throwing error as no definition is present for MainWindow. How can I get the owner's window of my control to pass it in ShowDiaglog() function below.

 public class UserControlView : UserControl
    {
        private readonly Button m_advancedFilterButton;
        public UserControlView ()
        {
            this.InitializeComponent();
            m_advancedFilterButton = this.FindControl<Button>("advancedFilterBtn");
            m_advancedFilterButton.Click += AdvancedFilterBtnOnClick;
        }

        private void InitializeComponent()
        {
            AvaloniaXamlLoader.Load(this);
        }

        public void AdvancedFilterBtnOnClick(object sender, RoutedEventArgs e)
        {
            var advanceFilterWindow = new AdvancedFilterWindow();
            advanceFilterWindow.ShowDialog(Application.Current.MainWindow);
        }
}

Code for my Dialog window:

Xaml file for view

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="xyz.AdvancedFilterWindow"
        Title="AdvancedFilterWindow"
        CanResize="True"
        WindowStartupLocation="CenterOwner" >
  <DockPanel>
    <TextBlock>Hi, This is the Dialog Box</TextBlock>
    <Button Name="applyBtn" Content="Apply"/>
  </DockPanel>
</Window>

Code behind:

public class AdvancedFilterWindow : Window
    {
        private readonly Button m_applyButton;
        public AdvancedFilterWindow()
        {
            AvaloniaXamlLoader.Load(this);
            m_applyButton = this.FindControl<Button>("applyBtn");
            m_applyButton.Click += ApplyButtonOnClick;
        }
        private void ApplyButtonOnClick(object sender, EventArgs e)
        {
            Close();
        }
    }

[maxkatz6]

Application.Current.MainWindow is an antipattern, and is not applicable for some applications types, where "main" window is not defined.
Also on some platforms there are no root windows at all: embed and mobile.
And your user control could be defined on non-"main" window.

What you can do, you can use VisualRoot property. If you are running your app from desktop environment, you can safely cast it to Window.

var currentWindow = (Window)myUserControl.VisualRoot;
var advanceFilterWindow = new AdvancedFilterWindow();
advanceFilterWindow.ShowDialog(currentWindow);

[maxkatz6]

Also another option is to use application lifetimes.
If you use "IClassicDesktopStyleApplicationLifetime", you will be able to get main window from there:
https://github.com/AvaloniaUI/Avalonia/wiki/Application-lifetimes

[ShrutiJaiswal1494]

Thanks. The first option worked for me. However, I did not understand how can I retrieve the main window from IClassicDesktopStyleApplicationLifetime

[maxkatz6]

if (Application.ApplicationLifetime is IClassicDesktopStyleApplicationLifetime lifetime)
{
    var mainWindow = lifetime.MainWindow;
}