sample_drag/DragAndDropSample/MainWindow.axaml.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Threading.Tasks;
using Avalonia.Controls;
using Avalonia.Input;

namespace DragAndDropSample;

public partial class MainWindow : Window
{
    private const string CustomFormat = "application/xxx-avalonia-controlcatalog-custom";
    class ItemListBox
    {
        public string Name { get; set; }
    }

    private List<ItemListBox> right_listbox =
    [
        new ItemListBox { Name = "123" },
        new ItemListBox { Name = "321" },
        new ItemListBox { Name = "222" },
        new ItemListBox { Name = "555" }
    ];
    private ObservableCollection<ItemListBox> left_listbox =
    [
        new ItemListBox { Name = "666" },
        new ItemListBox { Name = "777" },
        new ItemListBox { Name = "111" },
        new ItemListBox { Name = "333" }
    ];
    public MainWindow()
    {
        InitializeComponent();
        RightBox.ItemsSource = right_listbox;
        LeftBox.ItemsSource = left_listbox;
        DnDSetup(
            d =>
            {
                
                d.Set(CustomFormat, RightBox.SelectedItem as ItemListBox ?? throw new InvalidOperationException());
                return Task.CompletedTask;
            },
            DragDropEffects.Move);
    }

    private void DnDSetup(Func<DataObject, Task> factory, DragDropEffects effects)
    {
        async void DoDrag(object sender, PointerPressedEventArgs e)
        {
            var dragData = new DataObject();
            await factory(dragData);
            var result = await DragDrop.DoDragDrop(e, dragData, effects);
        }

        async void Drop(object sender, DragEventArgs e)
        {
            var sample  = e.Data as DataObject;
            left_listbox.Add(sample.Get(CustomFormat) as ItemListBox);
        }
        
        RightBox.PointerPressed += DoDrag;
        AddHandler(DragDrop.DropEvent, Drop);
    }
}