xaml-csharp-development-skill-for-avalonia

WinForms TreeView (Lazy Loading, Selection, Check State) to Avalonia

Table of Contents

1. Scope and APIs
2. Tree Mapping
3. Conversion Example
4. C# Equivalent
5. Troubleshooting

Scope and APIs

Primary WinForms APIs:

• TreeView, TreeNode
• BeforeExpand / AfterSelect
• CheckBoxes and node check state updates

Primary Avalonia APIs:

• TreeView + TreeDataTemplate
• TreeView.SelectedItem, TreeView.AutoScrollToSelectedItem
• TreeViewItem.Expanded event for lazy-load triggers

Tree Mapping

WinForms	Avalonia
TreeNode hierarchy	hierarchical view-model with Children
BeforeExpand async population	TreeViewItem.Expanded handler + EnsureChildrenLoadedAsync()
AfterSelect	SelectedItem binding + command/view-model state updates
built-in check boxes	CheckBox inside TreeDataTemplate for two-state or three-state logic

Conversion Example

WinForms C#:

treeView1.CheckBoxes = true;
treeView1.BeforeExpand += (_, e) => LoadChildrenIfNeeded(e.Node);
treeView1.AfterSelect += (_, e) => detailsPanel.Bind(e.Node.Tag);

Avalonia XAML:

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:vm="using:MyApp.ViewModels"
             x:DataType="vm:ExplorerTreeViewModel">
  <TreeView ItemsSource="{CompiledBinding Roots}"
            AutoScrollToSelectedItem="True"
            SelectedItem="{CompiledBinding SelectedNode, Mode=TwoWay}">
    <TreeView.ItemTemplate>
      <TreeDataTemplate x:DataType="vm:TreeNodeViewModel"
                        ItemsSource="{CompiledBinding Children}">
        <StackPanel Orientation="Horizontal" Spacing="6">
          <CheckBox IsThreeState="True"
                    IsChecked="{CompiledBinding IsChecked, Mode=TwoWay}" />
          <TextBlock Text="{CompiledBinding Name}" />
        </StackPanel>
      </TreeDataTemplate>
    </TreeView.ItemTemplate>
  </TreeView>
</UserControl>

C# Equivalent

using Avalonia.Controls;
using Avalonia.Interactivity;

var tree = new TreeView
{
    ItemsSource = viewModel.Roots,
    AutoScrollToSelectedItem = true
};

tree.AddHandler(TreeViewItem.ExpandedEvent, (sender, e) =>
{
    if (e.Source is TreeViewItem { DataContext: TreeNodeViewModel node })
    {
        _ = node.EnsureChildrenLoadedAsync();
    }
}, RoutingStrategies.Bubble);

Troubleshooting

1. Expanded nodes lose state after refresh.
   • use stable node identities and update existing child collections in place.
2. Lazy loading causes duplicate children.
   • guard EnsureChildrenLoadedAsync() with an idempotent IsLoaded flag.
3. Check-state propagation is inconsistent.
   • keep parent/child check computation in view model, not per-control event handlers.

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