this is base class for webpages you can remove System.Web.UI.Page class and add this for warpping query string strongly typed. check below code

• QueryStringModel : this property for accessing all property of query string
• GetQueryString() : this method for getting query string as string

using IAnswerable.Web.Core; 
namespace MyWebFormApplication
{
    public partial class _Default : IAnswerable.Web.WebPage.Page<ProductQueryString>
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            Response.Write(QueryStringModel.CategoryID);
            Response.Write(QueryStringModel.QueryText);
            Response.Write(QueryStringModel.PriceRange);
        }
        protected void btnSearch_Click(object sender, EventArgs e)
        {
            QueryStringModel.CategoryID = ddlCategory.SelectedValue;
            QueryStringModel.QueryText = txtSearch.Text;
            QueryStringModel.PriceRange = ddlPriceRange.SelectedValue;
        Response.Redirect("search.aspx?" + GetQueryString());
    }
}
public class ProductQueryString
{
    [Key("cat")]
    public string CategoryID { get; set; }

    [Key("q")]
    public string QueryText { get; set; }

    [Key("range")]
    public string PriceRange { get; set; }
}

}