Cannot have multiple items selected in a DropDownList

Question

I have two dropdownlist and a button. I used the breakpoint in my project and everything is working fine. But as soon I am getting out of the function of the button this is

Answer 1

A previous answer mentions ddl.SelectedItem = "parameter"; as an option. However, SelectedItem is readonly.

Using ddl.SelectedValue = "value" will also resolve the OP's error.