Dropdowndatagrid calulated field is case-sensitive in filter


I have add a calulated field using VS and C# code :


public partial class Baugruppe
public string ArtikelnrBezeichnung { get; set; }

Partial class:

public partial class BaugruppesController
	partial void OnBaugruppesRead(ref IQueryable<Models.Rootserver.Baugruppe> items)
		foreach (Models.Rootserver.Baugruppe item in items)
			//conate Artikelnr and Bezeichnung
			item.ArtikelnrBezeichnung = item.Artikelnr + "-" + item.Bezeichnung;

I can display this field (ArtikelnrBezeichnung) in a dropdowndatagrid when I set the Textproperty to this calulated field.

But now when I use the filter in the dropdowndatagrid the filter works case-sensitive, so upper and lower case is important for search results.

So my question is:
How to filter a calculated field in a dropdowndatagrid without case-sensitive?

You can try the solution from this thread:

the solution in this link does not work for me, when I click on the datadropdowngrid it says "no records to display"
(but there are records)

And when I try to search for something nothing happens and I just see the "wait-mousepointer":

What I have done now as a workaround is that I set the calculated field to lower-case:

//conate Artikelnr and Bezeichnung
item.ArtikelnrBezeichnung = (item.Artikelnr + "-" + item.Bezeichnung).ToLower() ;

So at least now filtering is possible but it does not look nice for the user.