C# Oledb update data in application

Question

So i have got a form in which there are two sections one to add and one to delete. But the problem is when i add data and after which when i check the combobox associated with delete section i can see all previous data except the one which i just added. So i need some kind of solution to refresh and re get everything into combobox as soon as i click Add button but no luck the latest data wont show.

Code for add button:

private void btnAddSubj_Click(object sender, EventArgs e)
    {
        OleDbDataReader cmd = ad.select("SELECT TOP 1 ID FROM subjects WHERE ID = " + int.Parse(txtSubjID.Text));
        if (cmd.Read())
        {
            MessageBox.Show("Subject ID you entered is taken, please select a different one");
        }
        else
        {
            ad.insert("insert into subjects (`ID`,`subjectName`) values(" + int.Parse(txtSubjID.Text) + ",'" + txtSubjName.Text + "')");

        }
        populateComboBoxSubjName();
    }

Here ad is associated to class which i created and contains all the methods for insert,select,delete,update.

code for populateComboBoxSubjName:

private void populateComboBoxSubjName()
    {
        comboBoxSubjName.Items.Clear();
        OleDbDataReader cmd = ad.select("SELECT * FROM subjects");

        while (cmd.Read())
        {
            for (int f = 0; f < cmd.FieldCount; f+=2)
            {
                string data = (cmd.GetValue(f).ToString() + "-" + cmd.GetValue(f + 1).ToString());
                comboBoxSubjName.Items.Add(data);
            }
        }
    }

Button Delete Code:

private void btnDeleteSubj_Click(object sender, EventArgs e)
    {
        string selected = this.comboBoxSubjName.GetItemText(this.comboBoxSubjName.SelectedItem);
        string[] idToDelete = selected.Split('-');
        ad.delete("DELETE FROM subjects WHERE ID=" + int.Parse(idToDelete[0]));
    }

Answer 1

I was trying to perform sql queries using another class (i created one and coded all the queries and connections with different methods for each like insert,delete,select) so i wouldnt need to write whole command again and again but for some reason it didnt work. I had to write every command again and again and that worked perfectyl.