fetch dropdownlist selected value in NeedDatasource event of radgrid in asp.net ajax web application

I want to fetch value from dropdownlist when selected. On that selection of value I am firing a simple select query ie.

"select * from Category where cat_name ='" + dropdownlistselectedvalue + "'"

here is the code of needDatasource

protected void rdgridview_NeedDataSource(object sender, Telerik.Web.UI.GridNeedDataSourceEventArgs e) { if (!e.IsFromDetailTable) {
//I want to fetch the dropdownlist selected value here ie

          string dpvalue = DropDownList1.SelectedValue.ToString();

      string strqry = "select * from Categories where Category_Name = '"+ dpvalue +"'";

        rdgridview.DataSource = getDataTable(strqry);            
    }
}

But I am not able to fetch the value from the dropdownlist selected value in needDatasource method. How shall I fetch the value or fire any event of dropdownlist control?

Thanks in advance