When you have a form, where you use a dropdown box as a lookup selector and after you have selected a new value you want to get some more data out of the lookup table and not only the key value, then you could do this like this. In this example we have a lookup table result set called getBestartsResult. 

In Visual Studio this will look like as follows:

protected async System.Threading.Tasks.Task DropdownBestellartChange(dynamic args)
        {
        selectedBestart = getBestartsResult.Where(a => a.spr_gruppe == args && a.sprkz == Security.User.SprKZ).FirstOrDefault();
            if (selectedBestart != null)
            {
                aagfakt.bestart = selectedBestart.BESTART1;
            }
        }

We use args (in radzen it is called event) from event DropDownBestellartChange to select the whole record from resultset getBestartsResult. Because in our db model we have multilanguage support we also have to use the extra where clause. This is not relevant for now but only for explanation of this code example.

a.sprkz == Security.User.SprKZ