Search with Dropdownlist

hi everyone, im maintaince a one webportal,

 

but i dont know this part how to fix 

 im posted code below, now on system just use a one date search, but i wantto use 2 dropdownlist lstYear and FirstYear ddlist, i wantto search between FirstYear  and LastYear, how can i do it ?

 

protected void btnSearch_Click(object sender, EventArgs e)
{
DocHelper.SearchConds conds = new DocHelper.SearchConds();

conds.Year = ConvertLib.ToInt(this.lstYear.SelectedValue, DefVal.Int);
conds.Keyword = this.txtKeyword.Text.Trim();

List countrySIds = new List();
foreach (var block in this._countryAreaBlocks)
{
countrySIds.AddRange(block.GetSeledCountrySIds());
}
conds.CountrySIds = countrySIds.ToArray();

List territorySIds = new List();
foreach (var block in this._territoryBlocks)
{
if (block.Checked)
{
territorySIds.Add(block.SId);
}
}
conds.TerritorySIds = territorySIds.ToArray();

Session[SessionDefine.SearchConds] = conds;

Response.Redirect(string.Format("{0}doc/index.aspx", SystemDefine.WebSiteRoot));
}