@page "/"
<TelerikTreeView Data="@FlatData"
                 CheckBoxMode="@TreeViewCheckBoxMode.Multiple"
                 CheckParents="@true"
                 CheckChildren="@true"
                 CheckOnClick="@false"
                 @bind-CheckedItems="@SelectedItems">
</TelerikTreeView>

@code {
    List<TreeItem> FlatData { get; set; }
    IEnumerable<object> SelectedItems { get; set; } = new List<object>();
    protected override async Task OnInitializedAsync()
    {
        await GenerateData();
        await SelectDefault();
    }

    async Task SelectDefault()
    {
        await Task.Delay(100);
        SelectedItems = FlatData.Where(data => data.Id == 2);
    }
#pragma warning disable
    async Task GenerateData()
    {
        FlatData = new List<TreeItem>();

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

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

    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; }
    }
}