c# combobox selectedindexchanged events

If you are talking about cascading dropdownlist then have a look:

Selecting value in cascading dropdown list dynamically[^]
How to populate a dropdown list with another ddl[^]
Cascading DropDown - Country to state[^]

ASP.NET cascading drop-down-list (CSASPNETCascadingDropDownList)[^]
CascadingDropDown Demonstration[^]
Cascading DropDownList in ASP.NET MVC[^]
Creating Cascading DropDownLists in ASP.Net[^]
Using CascadingDropDown with a Database[^]

Hi ,
Check this

private void Form1_Load(object sender, EventArgs e)
        {
         DataTable dt  = new DataTable();
        using (SqlConnection Cn = new SqlConnection(Properties.Settings.Default.con))
        {
            using (SqlCommand Cmd = new SqlCommand("SELECT * FROM  Orders  where oid = @oid", Cn))
            {
                Cn.Open();            
               SqlDataAdapter  adpt = new SqlDataAdapter(Cmd);
                adpt.Fill(dt);
                comboBox1.DataSource = dt;
                comboBox1.DisplayMember="Orders";
                comboBox1.ValueMember="id";
            }
        }
     }
        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
        DataTable dt  = new DataTable();
        using (SqlConnection Cn = new SqlConnection(Properties.Settings.Default.con))
        {
            using (SqlCommand Cmd = new SqlCommand("SELECT * FROM  OrdersDetails  where oid = @oid", Cn))
            {
                Cn.Open();
                Cmd.Parameters.AddWithValue("@oid" ,Convert.ToInt32( comboBox1.SelectedValue));
               SqlDataAdapter  adpt = new SqlDataAdapter(Cmd);
                adpt.Fill(dt);
                comboBox2.DataSource = dt;
                comboBox2.DisplayMember="amount";
                comboBox2.ValueMember="id";
            }

        }
      
    }

Best Regards
M.Mitwalli

suppose we have 2 tables one containing records of student and other containing their subjects

we have 2 combo box one for students and other for their subjects

'''' giving value to first combobox

dataset ds = new dataset() ;
string querry = "select studentid,studentname from tbstudent";
sqlcommand cmd = new sqlcommand(querry,connection);
dataadapter da = new sqldataadapter(cmd);
combobox1.valuemember = "studentid";
combobox1.displaymember = "studentname";
combobox1.datasource = ds.tables(0);

now on combobox1 selected index event

dataset ds = new dataset() ;
 string querry = "select subjectid,subjectname from tbsubject where studentid = combobox1.selectedvalue";
 sqlcommand cmd = new sqlcommand(querry,connection);
 dataadapter da = new sqldataadapter(cmd);
 combobox2.valuemember = "subjectid";
 combobox2.displaymember = "subjectname ";
 combobox2.datasource = ds.tables(0);