Implementing Cascading Checklist Filtering in UI for ASP.NET AJAX Grid

protected void RadGrid1_NeedDataSource(object sender, GridNeedDataSourceEventArgs e)
{
    RadGrid1.DataSource = OrdersTable();
}

protected void RadGrid1_FilterCheckListItemsRequested(object sender, GridFilterCheckListItemsRequestedEventArgs e)
{
    string filterExpression = RadGrid1.MasterTableView.FilterExpression; // Get current grid filters
    string DataField = (e.Column as IGridDataColumn).GetActiveDataField(); // Column being filtered

    if (!string.IsNullOrEmpty(filterExpression))
    {
        DataTable filteredData = ApplyFilterToDataSource(filterExpression); // Filter the full dataset

        List<object> columnValues = filteredData
            .AsEnumerable()
            .Select(row => row[e.Column.UniqueName]) // Get values only from filtered rows
            .ToList();

        e.ListBox.DataSource = columnValues; // Bind filtered values to checklist
        e.ListBox.DataBind();
    }
    else
    {
        // No filter applied, show all distinct values
        e.ListBox.DataSource = OrdersTable().DefaultView.ToTable(true, DataField);
        e.ListBox.DataKeyField = DataField;
        e.ListBox.DataTextField = DataField;
        e.ListBox.DataValueField = DataField;
        e.ListBox.DataBind();
    }
}

private DataTable ApplyFilterToDataSource(string filterExpression)
{
    DataTable dt = OrdersTable(); // Start from the full dataset
    DataView dv = new DataView(dt);

    string rowFilter = ConvertRadGridFilterToRowFilter(filterExpression); // Convert LINQ-style to RowFilter

    try
    {
        dv.RowFilter = rowFilter; // Apply the filter to the DataView
    }
    catch
    {
        dv.RowFilter = ""; // Fallback in case of invalid filter
    }

    return dv.ToTable(); // Return filtered rows as DataTable
}

private string ConvertRadGridFilterToRowFilter(string filterExpression)
{
    string pattern = @"Convert\.ToString\(it\[""(?<col>[^""]+)""\]\)\s*=\s*""(?<val>[^""]*)"""; // Regex matches patterns like Convert.ToString(it["Column"]) = "Value"
    string result = Regex.Replace(filterExpression, pattern, m => $"{m.Groups["col"].Value} = '{m.Groups["val"].Value}'");  // Replace each match with: Column = 'Value'

    result = result.Replace("||", "OR").Replace("&&", "AND"); // Convert logical operators from RadGrid syntax to DataView syntax

    return result;
}

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

    dt.Columns.Add(new DataColumn("OrderID", typeof(int)));
    dt.Columns.Add(new DataColumn("OrderDate", typeof(DateTime)));
    dt.Columns.Add(new DataColumn("Freight", typeof(decimal)));
    dt.Columns.Add(new DataColumn("ShipName", typeof(string)));
    dt.Columns.Add(new DataColumn("ShipCountry", typeof(string)));

    dt.PrimaryKey = new DataColumn[] { dt.Columns["OrderID"] };

    for (int i = 0; i < 70; i++)
    {
        int index = i + 1;

        DataRow row = dt.NewRow();

        row["OrderID"] = index;
        row["OrderDate"] = new DateTime(DateTime.Now.Year, DateTime.Now.Month, DateTime.Now.Day, 0, 0, 0).AddHours(index);
        row["Freight"] = index * 0.1 + index * 0.01;
        row["ShipName"] = "Name " + index;
        row["ShipCountry"] = "Country " + index;

        dt.Rows.Add(row);
    }

    return dt;
}

<telerik:RadGrid ID="RadGrid1" runat="server" AllowPaging="True" Width="800px"
    AllowFilteringByColumn="true"
    FilterType="HeaderContext"
    EnableHeaderContextMenu="true"
    EnableHeaderContextFilterMenu="true"
    OnFilterCheckListItemsRequested="RadGrid1_FilterCheckListItemsRequested"
    OnNeedDataSource="RadGrid1_NeedDataSource">
    <MasterTableView AutoGenerateColumns="False" DataKeyNames="OrderID">
        <Columns>
            <telerik:GridBoundColumn DataField="OrderID" DataType="System.Int32" FilterCheckListEnableLoadOnDemand="true"
                FilterControlAltText="Filter OrderID column" HeaderText="OrderID"
                ReadOnly="True" SortExpression="OrderID" UniqueName="OrderID">
            </telerik:GridBoundColumn>
            <telerik:GridDateTimeColumn DataField="OrderDate" DataType="System.DateTime" FilterCheckListEnableLoadOnDemand="true"
                FilterControlAltText="Filter OrderDate column" HeaderText="OrderDate"
                SortExpression="OrderDate" UniqueName="OrderDate">
            </telerik:GridDateTimeColumn>
            <telerik:GridNumericColumn DataField="Freight" DataType="System.Decimal" FilterCheckListEnableLoadOnDemand="true"
                FilterControlAltText="Filter Freight column" HeaderText="Freight"
                SortExpression="Freight" UniqueName="Freight">
            </telerik:GridNumericColumn>
            <telerik:GridBoundColumn DataField="ShipName"
                FilterControlAltText="Filter ShipName column" HeaderText="ShipName" FilterCheckListEnableLoadOnDemand="true"
                SortExpression="ShipName" UniqueName="ShipName">
            </telerik:GridBoundColumn>
            <telerik:GridBoundColumn DataField="ShipCountry"
                FilterControlAltText="Filter ShipCountry column" HeaderText="ShipCountry" FilterCheckListEnableLoadOnDemand="true"
                SortExpression="ShipCountry" UniqueName="ShipCountry">
            </telerik:GridBoundColumn>
        </Columns>
    </MasterTableView>
</telerik:RadGrid>