C# Database: combobox value base on another combobox. How?

Question

I have an application that display some info from access 2016 database. - Form2 will display data from studentInfo table so Form2 will contain studentIDTextBox studentNameComboBox classComboBox levelComboBox studentDateDateTimePicker nationalityComboBox cityComboBox provinceComboBox blockTextBox...etc levelComboBox: SelectedValue: studentInfoBindingSource - Level DataSource: studentLevelBindingSource DisplayMember: Level ValueMember: LevelID classComboBox : SelectedValue: studentInfoBindingSource - Class DataSource: studentClassBindingSource DisplayMember: Class ValueMember: ClassID Now my question is: Without effect the info has been display i want if i add a new item ( record ) to the database and when i choice a specific Level from levelComboBox it will change a list of classes within the selected Level in classComboBox and vice versa. How i can do that ? I try to do that by using the code below but it is not good: private void provinceComboBox_SelectedIndexChanged( object sender EventArgs e) { var cityQuary = from displayCity in student1516DataSet.city where displayCity.ProvinceName == provinceComboBox.Text select displayCity; cityComboBox.DataSource = cityQuary.AsDataView(); }

Fahad Aldaferi · Answer

ThanksHanif Hefaz & Karthik Elumalai. Now the code work fine :) private void provinceComboBox_SelectedIndexChanged( object senderEventArgse) { if (provinceComboBox.SelectedIndex>-1) { varcityQuary= fromdisplayCity in student1516DataSet.city wheredisplayCity.ProvinceName==provinceComboBox.SelectedValue.ToString() selectdisplayCity; cityComboBox.DataSource=cityQuary.AsDataView(); } }

Karthik Elumalai · Answer

Hi fahad aldaferi I have personally posted the code snippet for select the dropdown values based on the other dropdown with simple example below. http://www.c-sharpcorner.com/code/3066/simple-web-service-method-for-generating-country-state-and-city-in-dropdo.aspx You can slightly change the as per your requirement and it will work fine.Hope it will be helpful kindly let me know your thoughts thanks karthik

Hanif Hefaz · Answer

That great it worked. by the way i just taught it may be related with references.

Fahad Aldaferi · Answer

HiHanif Hefaz I think it is not related to references you just said

Hanif Hefaz · Answer

Have you used assembly references? try something like this using.system.configuration or google the related reference i think

Fahad Aldaferi · Answer

I have question How i can do the vice versa. Now i have a previous code which work fine with a little change on it like the code below so i want the user when he choice the city the app will directly assign the right Province only if provinceComboBox is empty. private void provinceComboBox_SelectedIndexChanged( object sender EventArgs e) { if ( provinceComboBox.SelectedIndex != -1 ) { var cityQuary = from displayCity in student1516DataSet.city where displayCity.ProvinceName == provinceComboBox.SelectedValue.ToString() select displayCity; cityComboBox.DataSource = cityQuary.AsDataView(); } } I try this code with cityComboBox but it display some errors says  An exception of type 'System.NullReferenceException' occurred in schoolManagments.exe but was not handled in user code Additional information: Object reference not set to an instance of an object.  private void cityComboBox_SelectedIndexChanged( object sender EventArgs e) { if (provinceComboBox.SelectedIndex == -1) { var proviceQuary = from displayProvice in student1516DataSet.city where displayProvice.CityID == cityComboBox.SelectedValue.ToString() select displayProvice; provinceComboBox.DataSource = proviceQuary.AsDataView(); } }

Hanif Hefaz · Answer

follow these links i found them useful. 1- https://www.youtube.com/watch?v=rdAqkg6bGT0 2- https://www.youtube.com/watch?v=I_wwlZofCgk 3- https://www.safaribooksonline.com/library/view/.../ch01s03.html 4- www. c-sharpcorner .com/uploadfile/mahesh/ combobox -in-C-Sharp/ 5- www. c-sharpcorner .com/forums/get-a- combobox - value -from-one-from-to- other -form Hope this will help you.