Create a stored procedure including all the sql select queries.
create procedure testProc
as
begin
select Name from tblEmpMaster
select eId from tblLeave
end
and then add that to a single data adapter & use it as u want.
private void button11_Click(object sender, EventArgs e)
{
connectDb DBC = new connectDb();
SqlConnection con = DBC.connectToLocalDb();
con.Open();
SqlCommand cmd = new SqlCommand("testProc", con);
cmd.CommandType = CommandType.StoredProcedure;
DataTable dt = new DataTable();
SqlDataAdapter adapter = new SqlDataAdapter(cmd);
adapter.Fill(dt);
comboBox4.DataSource = dt;
comboBox4.DisplayMember = "Name";
con.Close();
}