RookieRick is right, using DataGridTemplateColumn instead of DataGridComboBoxColumn gives a much simpler XAML.
Moreover, putting the CompanyItem list directly accessible from the GridItem allows you to get rid of the RelativeSource.
IMHO, this give you a very clean solution.
XAML:
<DataGrid AutoGenerateColumns="False" ItemsSource="{Binding GridItems}" >
<DataGrid.Resources>
<DataTemplate x:Key="CompanyDisplayTemplate" DataType="vm:GridItem">
<TextBlock Text="{Binding Company}" />
</DataTemplate>
<DataTemplate x:Key="CompanyEditingTemplate" DataType="vm:GridItem">
<ComboBox SelectedItem="{Binding Company}" ItemsSource="{Binding CompanyList}" />
</DataTemplate>
</DataGrid.Resources>
<DataGrid.Columns>
<DataGridTextColumn Binding="{Binding Name}" />
<DataGridTemplateColumn CellTemplate="{StaticResource CompanyDisplayTemplate}"
CellEditingTemplate="{StaticResource CompanyEditingTemplate}" />
</DataGrid.Columns>
</DataGrid>
View model:
public class GridItem
{
public string Name { get; set; }
public CompanyItem Company { get; set; }
public IEnumerable<CompanyItem> CompanyList { get; set; }
}
public class CompanyItem
{
public int ID { get; set; }
public string Name { get; set; }
public override string ToString() { return Name; }
}
public class ViewModel
{
readonly ObservableCollection<CompanyItem> companies;
public ViewModel()
{
companies = new ObservableCollection<CompanyItem>{
new CompanyItem { ID = 1, Name = "Company 1" },
new CompanyItem { ID = 2, Name = "Company 2" }
};
GridItems = new ObservableCollection<GridItem> {
new GridItem { Name = "Jim", Company = companies[0], CompanyList = companies}
};
}
public ObservableCollection<GridItem> GridItems { get; set; }
}