to stop drop down list auto refresh

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C#

ASP.NET

C#4.0

Hi,
I am accessing values from the database to fill the drop down list.
now when i click on the show button or if second way i select the index of the list. the page becomes auto refresh and the first value of the list selects auto. the result displayed only for the first value in the drop down list each time. how i could solve it......

protected void ddlNames_SelectedIndexChanged(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        con = new SqlConnection("Data Source=KADSERVER;Initial Catalog=Kadsyscon_temp;User ID=sa;Password=syscon1234");
        con.Open();
        //DataSet ds = new DataSet();
        dtLogin = new DataTable();
        string str1 = "select Date,InTime ,OutTime from EmpAttendanceDetails WHERE EID=" + ddlNames.SelectedValue.ToString();
        Command = new SqlCommand(str1, con);
        Command.CommandType = CommandType.Text;
        SqlDataAdapter da = new SqlDataAdapter(Command);
        da.Fill(dtLogin);

        gvDetails.DataSource = dtLogin;
        gvDetails.DataBind();
    }
}

this is the code i have written. and i used the same code on the button click but the problem persists....