combo binding

What you have to do is. Load only Class dropdown on page load.

 private void Form1_Load(object sender, EventArgs e)
        {
           
                OleDbConnection con = new OleDbConnection(@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Application.StartupPath + "\\school1.mdb");                 
           
         
            OleDbDataAdapter dadpter = new OleDbDataAdapter("select * from tblclass", con);
            DataSet dset = new DataSet();
            dadpter.Fill(dset);
                       comboBox1.DisplayMember= "classname"; // to display roll no. in combobox
            comboBox1.ValueMember = "classcode"; // to store name as value of combobox for selected roll no.
           
            comboBox1.DataSource = dset.Tables[0];
       
        }


Then In the selectedindex changed event of combobox 1, load the subject dropdown for the selected class.

 private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            {
                textBox2.Text = comboBox1.SelectedValue.ToString();
              
                OleDbConnection con1 = new OleDbConnection(@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Application.StartupPath + "\\school1.mdb");
                OleDbDataAdapter dadpter1 = new OleDbDataAdapter("select * from tblsubject where classcode= "+Convert.ToInt32(comboBox1.SelectedValue), con1);
                DataSet dset1 = new DataSet();
                dadpter1.Fill(dset1);
                comboBox2.DisplayMember = "subjectname"; // to display roll no. in combobox
                comboBox2.ValueMember = "subjectcode"; // to store name as value of combobox for selected roll no.
                comboBox2.DataSource = dset1.Tables[0];
               
            }

If you see the query , i have a where condition to fetch only subjects for the selected class.
Then In the selectedindex changed event of combobox 2, load the branch dropdown for the selected class and selected subject..

Like this.

 private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
        {
            textBox3.Text = comboBox2.SelectedValue.ToString();


          
            OleDbConnection con2 = new OleDbConnection(@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Application.StartupPath + "\\school1.mdb");
            OleDbDataAdapter dadpter2 = new OleDbDataAdapter("select * from tblbranch where  classcode= " + Convert.ToInt32(comboBox1.SelectedValue) + " and subjectcode= "+Convert.ToInt32(comboBox2.SelectedValue) , con2);
            DataSet dset2 = new DataSet();
            dadpter2.Fill(dset2);
            comboBox4.DataSource = dset2.Tables[0];

            comboBox4.DisplayMember = "branchname"; // to display roll no. in combobox
            comboBox4.ValueMember = "branchcode"; //
        }


.....

It goes like this.

Hope this would help you.Please let us know if you face difficulties