Even with your example I cannot get the Combobox items to populate. I find the RadGrid implementation of the Combobox somewhat confusing.
public class ExternalItemsProvider
{
public static List<string> Items { get; } = new List<string> { "5", "7", "8", "4", };
}
<Page.Resources>
<local:ExternalItemsProvider x:Key="items"/>
<tdg:RadDataGrid Name="tdgClients" UserEditMode="Inline" Margin="30,5,30,0" Height="200" BorderBrush="LightGray" BorderThickness="1" >
<tdg:RadDataGrid.Columns>
<tdg:DataGridComboBoxColumn PropertyName="StatusId" ItemsSource="{Binding Items, Source=items}" />
</tdg:RadDataGrid.Columns>
</tdg:RadDataGrid>
I even tried connectiong to the combobox datasource directly in code with: (I'd rather do it this way than via datacontext)
List<string> cbItems = new List<string>();
foreach (Lookup lu in Statuses)
cbItems.Add(lu.LookupName);
((DataGridComboBoxColumn)tdgClients.Columns[0]).ItemsSource = cbItems;
No errors but still no drop list.