using (SqlConnection conn = new SqlConnection("server=127.0.0.1;database=northwind;user id=sa;password=sa;"))
{
    using (SqlDataAdapter adapter = new SqlDataAdapter("select top 10 ProductID,ProductName from Products", conn))
    {
        DataTable table = new DataTable();
        adapter.Fill(table);

        comboBox1.DataSource = table;
        comboBox1.DisplayMember = "ProductName";
        comboBox1.ValueMember = "ProductID";
    }
}