Getting Id from database

Issue a Select command on the database:

SELECT Id FROM MyTable WHERE NameOfValueColumn = value

Alternatively, create a class which contains the ID and the value you to display (among others) and override the ToString method. Make the overridden method return the string you want to display, and pass the ComboBox an array of instances. It will display the string, and you can get the original object instance from the SeletedItem property without further database access:

MyClass mc = MyComboBox.SelectedItem as MyClass;
Console.WriteLine(mc.Id);