GridOptions.MultiSelect | GridOptions.RowSelect

void SetupTree()
{
    this.treeTest.Options = GridOptions.MultiSelect | GridOptions.RowSelect;

    TreeViewColumn col1 = new TreeViewColumn("Description");
    col1.ReadOnly = true;   //no edit
    this.treeTest.Columns.Items.Add(col1);

    Column col2 = new Column("Value");
    col2.ReadOnly = true;   //no edit
    this.treeTest.Columns.Items.Add(col2);

    for (int i = 0; i < 10; i++)
    {
        TreeViewNode node = new TreeViewNode(new string[] { "New Node", "On" });
        this.treeTest.Rows.Items.Add(node);
    }
}

bool this._bMultiSelectChanged = false;

void tree_KeyDown(object sender, KeyEventArgs e)
{
    if (!e.Control && !e.Shift &&
        ((this.Options & GridOptions.MultiSelect) == GridOptions.MultiSelect) &&
        this.Rows.SelectedNodes.GetLength(0) > 0)
    {
        switch (e.KeyCode)
        {
            case System.Windows.Forms.Keys.Up:
            case System.Windows.Forms.Keys.Down:
                TreeViewNode node = (TreeViewNode) this.Rows.SelectedNodes[0];

                this.Options = this.Options & ~GridOptions.MultiSelect;

                node.Selected = true;
                this._bMultiSelectChanged = true;
                break;
        }
    }
}

void tree_KeyUp(object sender, KeyEventArgs e)
{
    if (this._bMultiSelectChanged)
    {
        this.Options = this.Options | GridOptions.MultiSelect;
        this._bMultiSelectChanged = false;
    }
}