Please check this
Design:
<asp:DropDownList ID="ddlYear" AutoPostBack="true" runat="server" OnSelectedIndexChanged="ddlYear_SelectedIndexChanged"></asp:DropDownList>
<asp:DropDownList ID="ddlMonth" runat="server"></asp:DropDownList>
Code:
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
ddlYear.DataSource = Enumerable.Range(DateTime.Now.Year, 5);
ddlYear.DataBind();
ddlYear.Items.Insert(0,"Please select");
}
}
protected void ddlYear_SelectedIndexChanged(object sender, EventArgs e)
{
int remainingmonth = Convert.ToInt32(ddlYear.SelectedItem.Text) == DateTime.Today.Year ? DateTime.Today.Month : 1;
ddlMonth.Items.Clear();
for (int i = remainingmonth; i <= 12; i++)
{
ddlMonth.Items.Add(new System.Web.UI.WebControls.ListItem(DateTimeFormatInfo.CurrentInfo.GetMonthName(i), i.ToString()));
}
}