C#

DataTable ResultSet = new DataTable();

csCOSApp.ActiveProject.dsDataStore.Tables[0].Select("ProductId > 0").CopyToDataTable(ResultSet, LoadOption.PreserveChanges, delegate { });
if (ResultSet.Rows.Count > 0)
{
    LoadProducts(ResultSet, true);
}

csCOSApp.ActiveProject.dsDataStore.Tables[0].Select("ProductId = 0").CopyToDataTable(ResultSet, LoadOption.PreserveChanges, delegate { });
if (ResultSet.Rows.Count > 0)
{
    LoadParts(ResultSet, true);
}

private void btnLoadData_Click(object sender, EventArgs e)
{
      this.treeView1.Nodes.Clear();
      
      this._dtEmployees = this.GetEmployees();
      this.FillHieraricalData(this._dtEmployees, this.treeView1, "LastName", "EmployeeID", "ReportsTo");
      this.treeView1.ExpandAll();
}

private DataTable GetEmployees()
{
      DataTable dt = new DataTable();

      using (SqlConnection con = new SqlConnection(Properties.Settings.Default.NorthwindConnectionString))
      {
        SqlCommand cmd = con.CreateCommand();
        cmd.CommandText = "select * from employees";
        con.Open();
        dt.Load(cmd.ExecuteReader());
        con.Close();
      }

      return dt;
}       

private void FillHieraricalData(DataTable dt, TreeView treeView, string infoColumnName, 
      string pkColumnName, string parentColumnName)
{
      TreeNode node = null;
      foreach (DataRow row in dt.Rows)
      {
        node = new TreeNode(row[infoColumnName].ToString());
        node.Tag = row[pkColumnName];

        if (row[parentColumnName] != DBNull.Value)
        {
          List<TreeNode> foundNodes = new List<TreeNode>();

          // search over treeView nodes to found parent of row (if exists)
          SearchNodeInTreeView(treeView.Nodes, Convert.ToInt32(row[parentColumnName]), foundNodes);
          if (foundNodes.Count > 0)
            foundNodes[0].Nodes.Add(node);
        }
        else // if row parent does not exists, then it's parent itself (or it's root), it should add to level 0 of treeView
          treeView.Nodes.Add(node);
      }
}

// this method search in treeView.Nodes to find specific nodes base on 'value'
private void SearchNodeInTreeView(TreeNodeCollection nodesToSearch, int value, List<TreeNode> foundNodes)
{
      for (int i = 0; i < nodesToSearch.Count; i++)
      {
        if (Convert.ToInt32(nodesToSearch[i].Tag) == value)
          foundNodes.Add(nodesToSearch[i]);

        //Recursively search in the child nodes        
        SearchNodeInTreeView(nodesToSearch[i].Nodes, value, foundNodes);
      }
}

private void btnSave_Click(object sender, EventArgs e)
{
      this.UpdateDataRowFromUI(this._currentRow);
      this._dtEmployees.AcceptChanges();

      this.treeView1.Nodes.Clear();
      this.FillHieraricalData(this._dtEmployees, this.treeView1, "LastName", "EmployeeID", "ReportsTo");
      this.treeView1.ExpandAll();
}

private void UpdateDataRowFromUI(DataRow row)
{
      row["LastName"] = txtLastName.Text;
      row["FirstName"] = txtFirstName.Text;
      row["ReportsTo"] = cmbReportsTo.SelectedValue;
}