comboBox所选值更改

Question

I'd like to give the customer an option to choose a city from COMBOBOX, and once the city's chosen, a list of that city's streets should be in COMBOBOX2. I tried the following code and I got an error, during the first run, maybe someone can explain this to me?

private void Search_by_Apartment_Load(object sender, EventArgs e)
    {
        List<Cities> city = DAL.cities();
        cmBxCity.DataSource = city;//Here he ran the second function, why?
        cmBxCity.DisplayMember = "city";
        cmBxCity.ValueMember = "cityID";
    }

    private void cmBxCity_SelectedIndexChanged(object sender, EventArgs e)
    {

        List<Streets> street = DAL.streets(Convert.ToInt32(cmBxCity.SelectedText));
        // List<Streets> street = DAL.streets(Convert.ToInt32(cmBxCity.SelectedValue));
        comBxStreet.DataSource = street;
        comBxStreet.DisplayMember = "street";
        //cmBxCity.ValueMember = "cityID";

    }

Answer 1

when you assign the DataSource of the cmBxCity control its selectedItem changes from nothing to one item and this triggers the event handler cmBxCity_SelectedIndexChanged .

in the question you are talking about COMBOBOX and COMBOBOX2 but in the code there is only one control which is: cmBxCity .

shouldn't you show the streets in the second control called: cmBxStreet ?

Answer 2

The SelectedIndexChanged event is fired whenever the selected index is changed programatically or by the user.

As Davide Pirsa points out, when you change the DataSource of cmBxCity, you are programatically changing the selected index, hence firing the 'cmBxCity.SelectedIndexChanged' event at this line:

cmBxCity.DataSource = city;//Here he ran the second function, why? 

One possible solution is to use the SelectionChangeCommitted event instead, which is only fired for changes made by the user.