public enum TreeListSelectedRowsExportMode

@rendermode InteractiveServer
@inject SpaceObjectDataProvider SpaceObjectDataProvider

<DxTreeList @ref="TreeList" Data="TreeListData" ChildrenFieldName="Satellites">
    <Columns>
        <DxTreeListSelectionColumn Width="60px" AllowSelectAll="true" />
        <DxTreeListDataColumn FieldName="Name" />
        <DxTreeListDataColumn FieldName="TypeOfObject" Caption="Type" />
        <DxTreeListDataColumn FieldName="Mass10pow21kg" Caption="Mass, kg" DisplayFormat="N2" />
        <DxTreeListDataColumn FieldName="MeanRadiusInKM" Caption="Radius, km" DisplayFormat="N2"/>
    </Columns>
    <ToolbarTemplate>
        <DxToolbar>
            <DxToolbarItem Text="Export to XLSX" Click="ExportXlsx_Click" BeginGroup="true" />
        </DxToolbar>
    </ToolbarTemplate>
</DxTreeList>

@code {
    ITreeList TreeList { get; set; }
    object TreeListData { get; set; }

    protected override async Task OnInitializedAsync() {
        TreeListData = SpaceObjectDataProvider.GenerateData();
    }
    async Task ExportXlsx_Click() {
        await TreeList.ExportToXlsxAsync("ExportResult", new TreeListXlExportOptions() {
            ExportSelectedRowsOnly = true,
            SelectedRowsExportMode = TreeListSelectedRowsExportMode.KeepHierarchy
        });
    }
}

using System.Collections.Generic;

public class SpaceObject {
    public string Name { get; set; }
    public double MeanRadiusInKM { get; set; }
    public double Volume10pow9KM3 { get; set; }
    public double Mass10pow21kg { get; set; }
    public double Density { get; set; }
    public double SurfaceGravity { get; set; }
    public string TypeOfObject { get; set; }
    public List<SpaceObject> Satellites { get; set; }
    public SpaceObject(
        string name,
        double meanRadiusInKM,
        double volume10pow9KM3,
        double mass10pow21kg,
        double density,
        double surfaceGravity,
        string typeOfObject,
        List<SpaceObject> satellites = null
    ) {
        Name = name;
        MeanRadiusInKM = meanRadiusInKM;
        Volume10pow9KM3 = volume10pow9KM3;
        Mass10pow21kg = mass10pow21kg;
        Density = density;
        SurfaceGravity = surfaceGravity;
        TypeOfObject = typeOfObject;
        Satellites = satellites ?? new List<SpaceObject>();
    }
}

public class SpaceObjectDataProvider {
    public List<SpaceObject> GenerateData() {
        return new List<SpaceObject>() {
            new SpaceObject("Sun", 696000, 1412000000, 1989100000, 1.409, 274.0, "Star", new List<SpaceObject>() {
                new SpaceObject("Mercury", 2439.7, 60.83, 330.2, 5.43, 3.7, "Planet"),
                new SpaceObject("Venus", 6051.8, 928.43, 4868.5, 5.24, 8.872, "Planet"),
                new SpaceObject("Earth", 6371.0, 1083.21, 5973.6, 5.515, 9.78033, "Planet", new List<SpaceObject> () {
                    new SpaceObject("Moon", 1737.1, 21.958, 73.5, 3.3464, 1.625, "Satellite")
                }),
                new SpaceObject("Mars", 3390.0, 163.18, 641.85, 3.94, 3.7, "Planet"),
                new SpaceObject("Jupiter", 69911, 1431280, 1898600, 1.33, 24.79, "Planet", new List<SpaceObject>() {
                    new SpaceObject("Ganymede", 2631.2, 76.30, 148.2, 1.936, 1.428, "Satellite"),
                    new SpaceObject("Callisto", 2410.3, 58.65, 107.6, 1.83, 1.23603, "Satellite"),
                    new SpaceObject("Io", 1821.5, 25.32, 89.3, 3.528, 1.797, "Satellite"),
                    new SpaceObject("Europa", 1561, 15.93, 48, 3.01, 1.316, "Satellite"),
                }),
                new SpaceObject("Saturn", 58232, 827130, 568460, 0.70, 10.445, "Planet", new List<SpaceObject>() {
                    new SpaceObject("Titan", 2576, 71.52, 134.5, 1.88, 1.354, "Satellite"),
                    new SpaceObject("Rhea", 764.4, 1.87, 2.3166, 1.23, 0.26, "Satellite"),
                    new SpaceObject("Iapetus", 736, 1.55, 1.9739, 1.08, 0.223, "Satellite"),
                    new SpaceObject("Dione", 561.6, 0.73, 1.096, 1.48, 0.232, "Satellite"),
                    new SpaceObject("Tethys", 533, 0.624, 0.6173, 1.15, 0.145, "Satellite"),
                    new SpaceObject("Enceladus", 252.1, 0.067, 0.108, 1.61, 0.111, "Satellite"),
                    new SpaceObject("Mimas", 198.3, 0.033, 0.03749, 1.15, 0.06363, "Satellite")
                }),
                new SpaceObject("Uranus", 25362, 68340, 86832, 1.30, 8.87, "Planet", new List<SpaceObject>() {
                    new SpaceObject("Titania", 788.9, 2.06, 3.526, 1.72, 0.378, "Satellite"),
                    new SpaceObject("Oberon", 761.4, 1.85, 3.014, 1.63, 0.347, "Satellite"),
                    new SpaceObject("Umbriel", 584.7, 0.84, 1.2, 1.4, 0.234, "Satellite"),
                    new SpaceObject("Ariel", 578.9, 0.81, 1.35, 1.67, 0.269, "Satellite"),
                    new SpaceObject("Miranda", 235.8, 0.055, 0.0659, 1.20, 0.07910375, "Satellite"),
                }),
                new SpaceObject("Neptune", 24622, 62540, 102430, 1.76, 11.15, "Planet", new List<SpaceObject>() {
                    new SpaceObject("Triton", 1353.4, 10.38, 21.5, 2.061, 0.782, "Satellite"),
                    new SpaceObject("Proteus", 210, 0.038, 0.050, 1.3, 0.0666, "Satellite"),
                }),
                new SpaceObject("Eris", 1170, 7, 16.7, 2.25, 0.662, "Dwarf planet"),
                new SpaceObject("Pluto", 1153, 7.15, 13.105, 2.0, 0.61, "Dwarf planet"),
                new SpaceObject("Makemake", 710, 1.8, 3, 2.0, 0.4, "Dwarf planet"),
                new SpaceObject("Haumea", 575, 1.3, 4.006, 3, 0.44, "Dwarf planet"),
                new SpaceObject("Ceres", 475, 0.437, 0.95, 2.08, 0.27, "Dwarf planet"),
                new SpaceObject("Pallas", 266, 0.078, 0.211, 2.8, 0.2, "Asteroid"),
                new SpaceObject("Vesta", 264.6, 0.078, 0.262, 3.42, 0.251, "Asteroid")
            })
        };
    }
}

// ...
builder.Services.AddSingleton<SpaceObjectDataProvider>();