Kentico Category Filter

CategoryFilter.ascx

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="CategoryFilter.ascx.cs" Inherits="CMSGlobalFiles_CategoryFilter" %>
<asp:CheckBoxList ID="Categories" runat="server" />
<asp:Button ID="btnFilter" Text="Filter" OnClick="btnFilter_Click" runat="server" />

CategoryFilter.ascx.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web.UI.WebControls;
using CMS.CMSHelper;
using CMS.Controls;
using CMS.GlobalHelper;
using CMS.SettingsProvider;
using CMS.SiteProvider;

public partial class CMSGlobalFiles_CategoryFilter : CMSAbstractDataFilterControl
{
    protected override void OnInit(EventArgs e)
    {
        SetupControl();
        base.OnInit(e);
    }

    protected void btnFilter_Click(object sender, EventArgs e)
    {
        SetFilter();
    }

    private void SetFilter()
    {
        // Extract category ids from selected categories
        IEnumerable<string> categoryIds = from ListItem item in Categories.Items where item.Selected select item.Value;

        if (!categoryIds.Any()) return; // If no categoies where selected, then return

        // Query database for selected categories
        InfoDataSet<CategoryInfo> categories = CategoryInfoProvider.GetCategories(String.Format("CategoryID IN ({0})", String.Join(",", categoryIds)), null);

        List<string> conditions = new List<string>();
        
        // Build a "where" condition for each one of the selected categories
        foreach (CategoryInfo category in categories.Items)
            conditions.Add(CategoryInfoProvider.GetCategoryDocumentsWhereCondition(category, true));

        // Join the where conditions with an "OR" statement to include documents that belong to any of the selected categories.
        // You can use an "AND" statement to limit documents that belong to all of the selected categories
        WhereCondition = conditions.Join(" OR ");

        // Raise the filter changed event
        RaiseOnFilterChanged();
    }

    private void SetupControl()
    {
        if (StopProcessing) return;
        if (RequestHelper.IsPostBack()) return;

        // Bind categories to check box list
        InfoDataSet<CategoryInfo> categories = CategoryInfoProvider.GetCategories(CMSContext.CurrentSiteID);

        Categories.DataSource = categories;
        Categories.DataTextField = "CategoryDisplayName";
        Categories.DataValueField = "CategoryID";
        Categories.DataBind();
    }
}