using (SqlConnection con = new SqlConnection(strcon))
{
con.Open();
SqlCommand cm = new SqlCommand("select culumnname from tablename", con);
using (SqlDataReader dr = cm.ExecuteReader())
{
if (dr.HasRows)
{
DataTable dt = new DataTable();
dt.BeginLoadData();
dt.Load(dr);
dt.EndLoadData();
ComboBox cmb = new ComboBox();
cmb.DataSource = dt;
}
}