@using System.Collections.ObjectModel

<TelerikButton OnClick="@AddItem">Add Item</TelerikButton>

<TelerikButton OnClick="@RemoveItem">Remove Item</TelerikButton>

<p>
    TreeViewCheckedItems Count: @TreeViewCheckedItems.Count()
    <br />
    <label><TelerikCheckBox @bind-Value="@ApplyWorkaround" /> Apply Workaround</label>
</p>

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

@code {
    private IEnumerable<object> TreeViewCheckedItems { get; set; } = new List<TreeItem>();
    private ObservableCollection<object> TreeViewData { get; set; } = new ObservableCollection<object>();

    private int LastId { get; set; }
    private bool ApplyWorkaround { get; set; }

    private void AddItem()
    {
        TreeViewData.Add(
            new TreeItem
            {
                Id = ++LastId,
                Text = $"Item {LastId}",
            });

        if (ApplyWorkaround)
        {
            TreeViewCheckedItems = new List<object>(TreeViewCheckedItems);
        }

    }

    private void RemoveItem()
    {
        if (TreeViewData.Count > 1)
        {
            TreeViewData.Remove(TreeViewData.Last());

            if (ApplyWorkaround)
            {
                TreeViewCheckedItems = new List<object>(TreeViewCheckedItems);
            }
        }
    }

    protected override void OnInitialized()
    {
        TreeViewData = new ObservableCollection<object>() {
            new TreeItem()
            {
                Id = ++LastId,
                Text = $"Item {LastId}"
            }
        };

        TreeViewCheckedItems = new List<object>() { TreeViewData.First() };
    }
    public class TreeItem
    {
        public int Id { get; set; }
        public int? ParentId { get; set; }
        public string Text { get; set; } = string.Empty;
        public bool HasChildren { get; set; }
    }
}

System.NullReferenceException: Object reference not set to an instance of an object.
   at Telerik.Blazor.Components.Common.Trees.TelerikTreeDataSource.GetTreeItems(IEnumerable`1 data, Int32 level, String parentIndex)

@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; }
    }
}

<TelerikTreeView Data="@FlatData"
                 OnExpand="@LoadChildren"
                 CheckBoxMode="@TreeViewCheckBoxMode.Multiple"
                 CheckChildren="true"
                 CheckParents="true"
                 @bind-CheckedItems="@CheckedItems"
                 @bind-ExpandedItems="@ExpandedItems">
</TelerikTreeView>

@code {
    List<TreeItem> FlatData { get; set; } = new List<TreeItem>();

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

    async Task LoadChildren(TreeViewExpandEventArgs args)
    {
        TreeItem currItem = args.Item as TreeItem;

        if (args.Expanded && !FlatData.Any(x => x.ParentId == currItem.Id))
        {
            if (currItem.Id == 1)
            {
                FlatData.Add(new TreeItem()
                {
                    Id = 4,
                    Text = "Child 1 of Parent 1",
                    ParentId = 1,
                    HasChildren = false
                });

                FlatData.Add(new TreeItem()
                {
                    Id = 5,
                    Text = "Child 2 of Parent 1",
                    ParentId = 1,
                    HasChildren = false
                });
            }
        }
    }

    protected override void OnInitialized()
    {
        FlatData = LoadFlat();
    }

    private List<TreeItem> LoadFlat()
    {
        FlatData.Add(new TreeItem()
        {
            Id = 1,
            Text = "Parent 1",
            ParentId = null,
            HasChildren = true
        });

        return FlatData;
    }

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

<TelerikTreeView @ref="@TreeViewRef"
                 Data="@FlatData"
                 @bind-ExpandedItems="@ExpandedItems"
                 SelectionMode="@TreeViewSelectionMode.Multiple"
                 SelectedItems="@SelectedItems"
                 SelectedItemsChanged="@((IEnumerable<object> items) => SelectedItemsHandler(items))" />

<TelerikButton OnClick="@ExpandAll">Expand All</TelerikButton>
<TelerikButton OnClick="@CollapseAll">Collapse All</TelerikButton>

@code {
    public TelerikTreeView TreeViewRef { get; set; }
    public IEnumerable<TreeItem> FlatData { get; set; }
    public IEnumerable<object> SelectedItems { get; set; } = new List<object>();
    public IEnumerable<object> ExpandedItems { get; set; } = new List<object>();

    async Task ExpandAll()
    {
        ExpandedItems = FlatData.Where(x => x.HasChildren == true);

        await Task.Delay(1);
        TreeViewRef.Rebind();
    }

    void CollapseAll()
    {
        ExpandedItems = new List<object>();
        SelectedItems = new List<object>();
    }

    void SelectedItemsHandler(IEnumerable<object> items)
    {
        SelectedItems = items;
    }
    protected override async void OnInitialized()
    {
        FlatData = LoadFlat();
    }

    int TreeLevels { get; set; } = 3;
    int ItemsPerLevel { get; set; } = 3;
    int IdCounter { get; set; } = 1;
    List<TreeItem> LoadFlat()
    {
        List<TreeItem> items = new List<TreeItem>();
        PopulateTreeItems(items, null, 1);
        return items;
    }
    void PopulateTreeItems(List<TreeItem> items, int? parentId, int level)
    {
        for (int i = 1; i <= ItemsPerLevel; i++)
        {
            var itemId = IdCounter++;
            items.Add(new TreeItem()
            {
                Id = itemId,
                Text = $"Level {level} Item {i} Id {itemId}",
                ParentId = parentId,
                HasChildren = level < TreeLevels
            });
            if (level < TreeLevels)
            {
                PopulateTreeItems(items, itemId, level + 1);
            }
        }
    }

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