public bool GetNodeExpanded(
    Func<ITreeViewNodeInfo, bool> predicate
)

<DxTreeView @ref="@treeView" 
            AllowSelectNodes="true" 
            BeforeCollapse="@BeforeExpandCollapse" 
            BeforeExpand="@BeforeExpandCollapse" 
            SelectionChanged="@SelectionChanged">
    @* ... *@
</DxTreeView>

@code {
    DxTreeView treeView;

    protected void SelectionChanged(TreeViewNodeEventArgs e) {
        if (e.NodeInfo.IsLeaf && e.NodeInfo.Level == 0) {
            treeView.CollapseAll();
        }
        else if (!e.NodeInfo.IsLeaf) {
            Func<ITreeViewNodeInfo, bool> nodePredicate = n => string.Equals(n.Text, e.NodeInfo.Text);

            bool nodeExpanded = treeView.GetNodeExpanded(nodePredicate);
            if (!nodeExpanded) {
                treeView.CollapseAll();
            }
            treeView.SetNodeExpanded(nodePredicate, !nodeExpanded);
        }
    }

    protected void BeforeExpandCollapse(TreeViewNodeCancelEventArgs e) {
        e.Cancel = true;
    }
}