<TelerikTreeView Data="@FlatData"
                 OnItemClick="@OnItemClickHandler"
                 SelectionMode="@TreeViewSelectionMode.Multiple"
                 SelectedItems="@SelectedItems"
                 SelectedItemsChanged="((IEnumerable<object> items) => SelectedItemsChangedHandler(items))">
</TelerikTreeView>

@code {
    public IEnumerable<TreeItem> SelectedItems { get; set; } = new List<TreeItem>();

    public List<TreeItem> FlatData { get; set; }

    //Handle the SelectedItemsChanged event and do not execute any logic in it, so you can override the built-in selection.
    void SelectedItemsChangedHandler(IEnumerable<object> data)
    {
    }

    //Handle the OnItemClick to modify the SelectedItems collection
    async Task OnItemClickHandler(TreeViewItemClickEventArgs args)
    {
        var currItem = args.Item as TreeItem;

        if (SelectedItems.Any(x => x.Id == currItem.Id))
        {
            SelectedItems = SelectedItems.Where(x => x.Id != currItem.Id);
        }
        else
        {
            SelectedItems = SelectedItems.Concat(new[] { currItem });
        }
    }

    #region Data Generation

    protected override void OnInitialized()
    {
        LoadFlatData();
    }

    private void LoadFlatData()
    {
        List<TreeItem> items = new List<TreeItem>();

        items.Add(new TreeItem()
        {
            Id = 1,
            Text = "Project",
            ParentId = null,
            HasChildren = true,
            Icon = "folder",
            Expanded = true
        });

        items.Add(new TreeItem()
        {
            Id = 2,
            Text = "Design",
            ParentId = 1,
            HasChildren = true,
            Icon = "brush",
            Expanded = true
        });
        items.Add(new TreeItem()
        {
            Id = 3,
            Text = "Implementation",
            ParentId = 1,
            HasChildren = true,
            Icon = "folder",
            Expanded = true
        });

        items.Add(new TreeItem()
        {
            Id = 4,
            Text = "site.psd",
            ParentId = 2,
            HasChildren = false,
            Icon = "psd",
            Expanded = true
        });
        items.Add(new TreeItem()
        {
            Id = 5,
            Text = "index.js",
            ParentId = 3,
            HasChildren = false,
            Icon = "js"
        });
        items.Add(new TreeItem()
        {
            Id = 6,
            Text = "index.html",
            ParentId = 3,
            HasChildren = false,
            Icon = "html"
        });
        items.Add(new TreeItem()
        {
            Id = 7,
            Text = "styles.css",
            ParentId = 3,
            HasChildren = false,
            Icon = "css"
        });

        FlatData = items;
    }
    #endregion

    #region Data Models
    public class GridDataModel
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public int WorkingOn { get; set; }
    }

    public class TreeItem
    {
        public int Id { get; set; }
        public string Text { get; set; }
        public int? ParentId { get; set; }
        public bool HasChildren { get; set; }
        public string Icon { get; set; }
        public bool Expanded { get; set; }
    }
    #endregion
}