如果combobox1和combobox2为true，则更新combobox3中的项目

Question

I want to add new items/change the items in cmbSection if cmbCourse, cmbYear and cmbSems met the requirements. I try this code but cmbSection won't add new items/change the items.

private void AddNewStudent_Load(object sender, EventArgs e)
    {
        if (cmbCourse.SelectedItem.Equals("BSIT"))
        {
            if (cmbYear.SelectedItem.Equals("1st"))
            {
                if (cmbSems.SelectedItem.Equals("1st"))
                {
                    cmbSection.Items.Add("IT101C");
                    cmbSection.Items.Add("IT102C");
                    cmbSection.Items.Add("IT103C");
                    cmbSection.Items.Add("IT104C");
                }
                else if (cmbSems.SelectedItem.Equals("2nd"))
                {
                    cmbSection.Items.Add("IT201C");
                    cmbSection.Items.Add("IT202C");
                    cmbSection.Items.Add("IT203C");
                    cmbSection.Items.Add("IT204C");
                }
            }
            else if (cmbYear.SelectedItem.Equals("2nd"))
            {
                if (cmbSems.SelectedItem.Equals("1st"))
                {
                    cmbSection.Items.Add("IT301C");
                    cmbSection.Items.Add("IT302C");
                    cmbSection.Items.Add("IT303C");
                    cmbSection.Items.Add("IT304C");
                }
                else if (cmbSems.SelectedItem.Equals("2nd"))
                {
                    cmbSection.Items.Add("IT401C");
                    cmbSection.Items.Add("IT402C");
                    cmbSection.Items.Add("IT403C");
                    cmbSection.Items.Add("IT404C");
                }
            }
        }
    }

Answer 1

I think you should use event in C#. You register event SelectedIndexChanged of cmbCourse ==> When cmbCourse changed selected index, you will load new data into cmbYear.
You register event SelectedIndexChanged of cmbYear==> When cmbYear changed selected index, you will load new data into cmbSems.

You register event SelectedIndexChanged of cmbSems==> When cmbSems changed selected index, you will load new data into cmbSection.

Answer 2

You are checking all the conditions when the form loads; I assume you want to check it when the user changes the option in one of the combo boxes, in which case, as tauitdnmd stated, SelectedIndexChanged is the event you should be using, not Form.Load . You can set the SelectedIndexChanged event for each ComboxBox to all be handled by a single method, ie, the one you have (although I strongly recommend a name change).

However, if you were to simply change which event is being handled, the use of Items.Add would result in the new options being added along side the old ones, quickly creating an enormously long and redundant drop-down. The solution to this is simply clearing each ComboBox in question before adding all the relevant options with Items.Clear() .

@Vulcronos: I created a test application with the same code, only with a button click as the event, and the SelectedItem.Equals() check appears to works fine (the validity of its use notwithstanding).