<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; }
    }
}

<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; }
    }
}

crit: Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]
      Unhandled exception rendering component: Unknown edit type: 0
      Error: Unknown edit type: 0
          at e.applyEdits (https://localhost:44363/_framework/blazor.webassembly.js:1:15008)
          at e.updateComponent (https://localhost:44363/_framework/blazor.webassembly.js:1:12880)
          at Object.t.renderBatch (https://localhost:44363/_framework/blazor.webassembly.js:1:1704)
          at Object.window.Blazor._internal.renderBatch (https://localhost:44363/_framework/blazor.webassembly.js:1:34784)
          at _mono_wasm_invoke_js_unmarshalled (https://localhost:44363/_framework/wasm/dotnet.3.2.0.js:1:172099)
          at wasm_invoke_iiiiii (<anonymous>:wasm-function[3160]:0x9b33d)
          at icall_trampoline_dispatch (<anonymous>:wasm-function[5777]:0xfe711)
          at mono_wasm_interp_to_native_trampoline (<anonymous>:wasm-function[4607]:0xca81d)
          at ves_pinvoke_method (<anonymous>:wasm-function[3209]:0x9cd40)
          at interp_exec_method (<anonymous>:wasm-function[1120]:0x2598d)
Microsoft.JSInterop.JSException: Unknown edit type: 0
Error: Unknown edit type: 0
    at e.applyEdits (https://localhost:44363/_framework/blazor.webassembly.js:1:15008)
    at e.updateComponent (https://localhost:44363/_framework/blazor.webassembly.js:1:12880)
    at Object.t.renderBatch (https://localhost:44363/_framework/blazor.webassembly.js:1:1704)
    at Object.window.Blazor._internal.renderBatch (https://localhost:44363/_framework/blazor.webassembly.js:1:34784)
    at _mono_wasm_invoke_js_unmarshalled (https://localhost:44363/_framework/wasm/dotnet.3.2.0.js:1:172099)
    at wasm_invoke_iiiiii (<anonymous>:wasm-function[3160]:0x9b33d)
    at icall_trampoline_dispatch (<anonymous>:wasm-function[5777]:0xfe711)
    at mono_wasm_interp_to_native_trampoline (<anonymous>:wasm-function[4607]:0xca81d)
    at ves_pinvoke_method (<anonymous>:wasm-function[3209]:0x9cd40)
    at interp_exec_method (<anonymous>:wasm-function[1120]:0x2598d)
  at Microsoft.JSInterop.WebAssembly.WebAssemblyJSRuntime.InvokeUnmarshalled[T0,T1,T2,TResult] (System.String identifier, T0 arg0, T1 arg1, T2 arg2) <0x3ae01e8 + 0x00046> in <filename unknown>:0 
  at Microsoft.JSInterop.WebAssembly.WebAssemblyJSRuntime.InvokeUnmarshalled[T0,T1,TResult] (System.String identifier, T0 arg0, T1 arg1) <0x3ae0108 + 0x00014> in <filename unknown>:0 
  at Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer.UpdateDisplayAsync (Microsoft.AspNetCore.Components.RenderTree.RenderBatch& batch) <0x3ae0010 + 0x0001e> in <filename unknown>:0 
  at Microsoft.AspNetCore.Components.RenderTree.Renderer.ProcessRenderQueue () <0x387e448 + 0x000f2> in <filename unknown>:0

protected async Task OnItemClickHandler(TreeViewItemClickEventArgs e)
{
    var item = e.Item as TreeViewItem;
    item.Expanded = true;
}