Getting erro in Dropdown control selectedindexchanged event?

in my web application when i select a dropdown control which is giving error like. Cannot have multiple items selected in a DropDownList. This is my code...

    try
    {             
        ddlState.Items.Clear();
        ddlState.Enabled = true;
        ListItem li1 = new ListItem();
        li1.Value = "0";
        li1.Text = "Select State";
        ddlState.Items.Add(li1);
        clsStates.Countryid = int.Parse(ddlCountry.SelectedValue.ToString());
        DataSet ds = clsStates.selectStateBl();
        for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
        {
            li1 = new ListItem();
            li1.Text = ds.Tables[0].Rows[i]["statename"].ToString();
            li1.Value = ds.Tables[0].Rows[i]["stateid"].ToString();
            ddlState.DataTextField = "statename";
            ddlState.DataValueField = "stateid";
            ddlState.Items.Add(li1);
        }
    }
    catch
    {

    }

this is binding the countries code...

     try
    {

        ListItem li = new ListItem();

        DataSet ds = clsCountrys.selectCountryB();

        for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
        {
            li = new ListItem();
            li.Text = ds.Tables[0].Rows[i]["countryname"].ToString();
            li.Value = ds.Tables[0].开发者_StackOverflow社区Rows[i]["countryid"].ToString();
            if (li.Value == Session["usercountry"].ToString())
                li.Selected = true;
            ddlCountry.DataTextField = "countryname";
            ddlCountry.DataValueField = "countryid";
            ddlCountry.Items.Add(li);
        }
    }
    catch
    {

    }

and this is state binding to dropdown control..

     try
    {
        ddlState.Items.Clear();
        ddlState.Enabled = true;
        ddlState.ClearSelection();
        ListItem li1 = new ListItem();
        li1.Value = "0";
        li1.Text = "Select State";
        ddlState.Items.Add(li1);
        clsStates.Countryid = int.Parse(ddlCountry.SelectedValue.ToString());
        DataSet ds = clsStates.selectStateBl();
        for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
        {

            li1 = new ListItem();
            li1.Text = ds.Tables[0].Rows[i]["statename"].ToString();
            li1.Value = ds.Tables[0].Rows[i]["stateid"].ToString();
            if (Session["userstate"].ToString() == li1.Value)
                li1.Selected = true;
            ddlState.DataTextField = "statename";
            ddlState.DataValueField = "stateid";
            ddlState.Items.Add(li1);
        }
    }
    catch
    {

    }

Make use of ddlState.ClearSelection() method to clear default selection.

Check this post : ASP.NET Tips: DropDownList.ClearSelection() to avoid "Cannot have multiple items selected in DropDownList"

EDIT

  try
    {
        ddlState.ClearSelection();
        ddlState.Items.Clear();
        ddlState.Enabled = true;

        ListItem li1 = new ListItem();
        li1.Value = "0";
        li1.Text = "Select State";
        ddlState.Items.Add(li1);
        clsStates.Countryid = int.Parse(ddlCountry.SelectedValue.ToString());
        DataSet ds = clsStates.selectStateBl();
        for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
        {

            li1 = new ListItem();
            li1.Text = ds.Tables[0].Rows[i]["statename"].ToString();
            li1.Value = ds.Tables[0].Rows[i]["stateid"].ToString();

            ddlState.DataTextField = "statename";
            ddlState.DataValueField = "stateid";
            ddlState.Items.Add(li1);

        }

          //code updated by pranay rana
           ddlState.ClearSelection();
              ListItem li = ddlState.Items.FindByValue(Session["userstate"].ToString());
                if (li != null)
                {
                    li.Selected = true;
                }
           //code updated by pranay rana
    }
    catch
    {

    }