Hello,

It has been a while since you have made your post, in case you still need support I will be glad to assist you further. I suppose the other community members can benefit from this answer as well. I have been looking into your post and I created a sample project for you where the functionality you want is achieved. Basically after changing the DataSource of the XamComboEditor I iterated trough the Records and make the Cell with the Editor to enter and exit edit mode and I handled the EditModeEnding event, so I can change the Editor’s Value to the updated one.

Feel free to write me if you have further questions.

Here's the source code:

MainWindow.xaml

<Window x:Class="XamGridTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
        xmlns:igd="http://infragistics.com/DataPresenter" 
        xmlns:Editors="http://infragistics.com/Editors" 
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <ControlTemplate TargetType="{x:Type Editors:XamComboEditor}" x:Key="ComboEditorTemplate">
            <ComboBox ItemsSource="{TemplateBinding ItemsSource}"
                      SelectedItem="{TemplateBinding SelectedItem}"
                      DisplayMemberPath="{TemplateBinding DisplayMemberPath}" />
                      
        </ControlTemplate>
    </Window.Resources>
    <DockPanel>
        <!-- Editor for item labels -->
        <ListBox DockPanel.Dock="Left" Width="200" x:Name="lstLabelEditor" ItemsSource="{Binding ItemCollection}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <TextBox Text="{Binding ItemLabel, Mode=TwoWay}" />
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
        
        <!-- Xam data binding -->
        <igd:XamDataGrid x:Name="xdgDataGrid" DataSource="{Binding Records}"
                            FieldLayoutInitialized="OnFieldLayoutInitialized">
            <igd:XamDataGrid.FieldLayoutSettings>
                <igd:FieldLayoutSettings AutoGenerateFields="False" />
            </igd:XamDataGrid.FieldLayoutSettings>
        </igd:XamDataGrid>
    </DockPanel>
</Window>

MainWindow.xaml.cs

using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Windows;
using System.Windows.Data;
using Infragistics.Windows.DataPresenter;
using Infragistics.Windows.DataPresenter.Events;
using Infragistics.Windows.Editors;

namespace XamGridTest
{
    public partial class MainWindow
    {
        private readonly MainViewModel _mainViewModel = new MainViewModel();

        public MainWindow()
        {
            InitializeComponent();
            DataContext = _mainViewModel;
        }

        private void OnFieldLayoutInitialized(object sender, FieldLayoutInitializedEventArgs e)
        {
            var layout = e.FieldLayout;
            layout.Fields.Clear();

            // Field using XamComboEditor's default template
            var comboField1 = new UnboundField
                                  {
                                      DataType = typeof (ItemViewModel),
                                      Label="Default template",
                                      Binding = new Binding("RecordItem") {Mode = BindingMode.TwoWay}
                                  };
            layout.Fields.Add(comboField1);
            comboField1.Settings.EditorType = typeof(XamComboEditor);
            Style style1 = new Style(typeof(XamComboEditor));
            {
                style1.Setters.Add(new Setter(XamComboEditor.ItemsSourceProperty, _mainViewModel.ItemCollection));
                style1.Setters.Add(new Setter(XamComboEditor.DisplayMemberPathProperty, "ItemLabel"));
                comboField1.Settings.EditorStyle = style1;
            }

            // Field using XamComboEditor with custom template
            var comboField2 = new UnboundField
                                  {
                                      DataType = typeof (ItemViewModel),
                                      Label = "Custom template",
                                      Binding = new Binding("RecordItem") {Mode = BindingMode.TwoWay}
                                  };
            layout.Fields.Add(comboField2);
            comboField2.Settings.EditorType = typeof(XamComboEditor);
            Style style2 = new Style(typeof (XamComboEditor));
            {
                style2.Setters.Add(new Setter(XamComboEditor.ItemsSourceProperty, _mainViewModel.ItemCollection));
                style2.Setters.Add(new Setter(XamComboEditor.DisplayMemberPathProperty, "ItemLabel"));
                style2.Setters.Add(new Setter(TemplateProperty, FindResource("ComboEditorTemplate")));
                comboField2.Settings.EditorStyle = style2;
            }
        }
    }

    // Main view model
    public class MainViewModel
    {
        private readonly ObservableCollection _itemCollection =
            new ObservableCollection
                {
                    new ItemViewModel{ItemLabel="Default item 1"},
                    new ItemViewModel{ItemLabel="Default item 2"},
                    new ItemViewModel{ItemLabel="Default item 3"},
                };
        public ObservableCollection ItemCollection
        {
            get { return _itemCollection; }
        }

        private readonly ObservableCollection _records;
        public ObservableCollection Records
        {
            get { return _records; }
        }

        public MainViewModel()
        {
            _records = new ObservableCollection
                           {
                               new RecordViewModel{RecordItem = _itemCollection[0]},
                               new RecordViewModel{RecordItem = _itemCollection[1]},
                               new RecordViewModel{RecordItem = _itemCollection[2]},
                               new RecordViewModel{RecordItem = _itemCollection[0]},
                               new RecordViewModel{RecordItem = _itemCollection[1]},
                               new RecordViewModel{RecordItem = _itemCollection[2]},
                           };
        }
    }

    // Record view model
    public class RecordViewModel : INotifyPropertyChanged
    {
        private ItemViewModel _recordValue;
        public ItemViewModel RecordItem
        {
            get { return _recordValue; }
            set { _recordValue = value;
                if (PropertyChanged != null)
                    PropertyChanged(this, new PropertyChangedEventArgs("RecordItem"));
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }

    // Item view model
    public class ItemViewModel : INotifyPropertyChanged
    {
        private string _itemLabel;
        public string ItemLabel
        {
            get { return _itemLabel; }
            set
            {
                _itemLabel = value;
                if (PropertyChanged != null)
                    PropertyChanged(this, new PropertyChangedEventArgs("ItemLabel"));
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }
}