<TelerikButton OnClick="@UncheckAll">Uncheck all</TelerikButton>

<TelerikTreeView Data="@TreeViewData"
                 CheckBoxMode="@TreeViewCheckBoxMode.Multiple"
                 @bind-CheckedItems="@CheckedItems"
                 CheckParents="true"
                 CheckChildren="true" />

@code {

    public IEnumerable<object> CheckedItems { get; set; } = new List<object>();

    public IEnumerable<TreeItem> TreeViewData { get; set; }

    private async Task UncheckAll()
    {
        // workaround start
        CheckedItems = TreeViewData;
        await Task.Delay(1);
        // workaround end
        CheckedItems = new List<object>();
    }

    protected override void OnInitialized()
    {
        LoadFlatData();

        var precheckedItem = TreeViewData.Where(x => x.Id == 2);

        CheckedItems = new List<object>(precheckedItem);
    }

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

        TreeViewData = items;
    }

    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; }
    }
}