Hi Szymon,

Why are you adding an unbound field. Why not simply just use the Country bound field directly. With this there will be no need to specify BindingPath. You can use XamComboEditor to directly edit field of a custom data type, in your case Country type.

The following is what I just tried out with a sample project. It has a DataItem class which has a Country property of type Country. Both of these are defined below. XamComboEditor is used to edit the Country field directly and it all works. The style for the XamComboEditor is in code below as well. XAML portion simply defines a XamDataGrid and doesn't have any other code.

public void OnLoaded( object sender, EventArgs e )
{
    List<DataItem> dataSource = new List<DataItem>( );
    DataItem item;

    item = new DataItem( );
    item.Country = new Country( "a", "AA" );
    dataSource.Add( item );

    item = new DataItem( );
    item.Country = new Country( "b", "BB" );
    dataSource.Add( item );

    _dp.DataSource = dataSource;

    ComboBoxItemsProvider countriesIP = new ComboBoxItemsProvider( );
    countriesIP.Items.Add( new Country( "a", "AA" ) );
    countriesIP.Items.Add( new Country( "b", "BB" ) );
    countriesIP.DisplayMemberPath = "Name";

    FieldSettings settings = _dp.FieldLayouts[0].Fields["Country"].Settings;
    settings.EditorType = typeof( XamComboEditor );

    Style style = new Style( typeof( XamComboEditor ) );
    style.Setters.Add( new Setter( XamComboEditor.ValueTypeProperty, typeof( Country ) ) );
    style.Setters.Add( new Setter( XamComboEditor.ItemsProviderProperty, countriesIP ) );
    style.Seal( );

    settings.EditorStyle = style;
}

public class DataItem : System.ComponentModel.INotifyPropertyChanged
{
    private static int ID_COUNTER = 1;
    private int _id;
    private Country _country;

    public DataItem( )
    {
        _id = ID_COUNTER++;
    }

    public int Id
    {
        get
        {
            return _id;
        }
    }

    public Country Country
    {
        get
        {
            return _country;
        }
        set
        {
            if ( _country != value )
            {
                _country = value;

                if ( null != this.PropertyChanged )
                    this.PropertyChanged( this, new PropertyChangedEventArgs( "Country" ) );

                Debug.WriteLine( "New Value = " + _country.Name );
            }
        }
    }

    public event System.ComponentModel.PropertyChangedEventHandler PropertyChanged;
}

public class Country
{
    private string _shortName, _name;

    public Country( string shortName, string name )
    {
        _shortName = shortName;
        _name = name;
    }

    public string Name
    {
        get
        {
            return _name;
        }
    }

    public string ShortName
    {
        get
        {
            return _shortName;
        }
    }

    public override int GetHashCode( )
    {
        return _shortName.GetHashCode( ) ^ _name.GetHashCode( );
    }

    public override bool Equals( object obj )
    {
        Country c = obj as Country;

        return null != c && _shortName == c._shortName && _name == c._name;
    }
}

Hope this helps,

Sandip