Janus GridEX . How to fill combobox with datasource

Question

I have the version Janus GridEX Control v3.5.

 

 As datasource I have implemented a dictionary:

Dictionary<Guid, Vehicle> vehicles= new Dictionary<Guid, Vehicle>();

 

I have the following source code in my project:

 

gridEX1.DropDowns["VehicleDropDown"].DataSource = vehicles.Values.ToList();
gridEX1.DropDowns["VehicleDropDown"].DisplayMember = "licenseplatenumber"; //value

 

Unfortunately, no license plate numbers are displayed in the dropdown list.
What am I doing wrong?