public class TreeViewNodeEventArgs :
    EventArgs

<DxTreeView AllowSelectNodes="true" 
            SelectionChanged="@SelectionChanged">
    @* ... *@
</DxTreeView>

@code {
    string SelectedGroup = "none";

    void SelectionChanged(TreeViewNodeEventArgs e) {
        SelectedGroup = e.NodeInfo.Text;
        InvokeAsync(StateHasChanged);
    }
}