Binding a WPF DataGridComboBoxColumn with MVVM

Question

I've looked at the answers to various questions, but haven't managed to map the content in the answers to the problem I'm attempting to solve. I've reduced it down to the following code (representative of the outcome I'm trying to achieve), and basically want to be able to render the Person.TitleId as its corresponding Title.TitleText when the row isn't being edited, and have the drop-down bound correctly so that it displays the TitleTexts in the drop-down and writes the associated TitleId back to the Person record when its changed.

In short, what do I put in my <DataGridComboBoxColumn> to achieve this?

App.xaml.cs

protected override void OnStartup(StartupEventArgs e)
{
    base.OnStartup(e);

    var viewModel = new ViewModels.MainWindowViewModel();
    var mainWindow = new MainWindow();
    mainWindow.DataContext = viewModel;
    mainWindow.ShowDialog();
}

MainWindow.xaml

<Grid>
    <DataGrid AutoGenerateColumns="False" ItemsSource="{Binding Path=Contacts}">
        <DataGrid.Columns>
            <DataGridComboBoxColumn Header="Title" SelectedItemBinding="{Binding Person}">
                <DataGridComboBoxColumn.ElementStyle>
                    <Style TargetType="ComboBox">
                        <Setter Property="ItemsSource" Value="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=DataContext.Titles}"/>
                        <Setter Property="IsReadOnly" Value="True"/>
                    </Style>
                </DataGridComboBoxColumn.ElementStyle>
                <DataGridComboBoxColumn.EditingElementStyle>
                    <Style TargetType="ComboBox">
                        <Setter Property="ItemsSource" Value="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=DataContext.Titles}"/>
                        <Setter Property="DisplayMemberPath" Value="TitleText" />
                    </Style>
                </DataGridComboBoxColumn.EditingElementStyle>
            </DataGridComboBoxColumn>
        </DataGrid.Columns>
    </DataGrid>
</Grid>

Person.cs

public class Person
{
    public int TitleId { get; set; }
    public string LastName { get; set; }
    public string FirstName { get; set; }
}

Title.cs

public struct Title
{
    public Title(int titleId, string titleText)
        : this()
    {
        TitleId = titleId;
        TitleText = titleText;
    }

    public string TitleText { get; private set; }
    public int TitleId { get; private set; }

    public static List<Title> GetAvailableTitles()
    {
        var titles = new List<Title>();

        titles.Add(new Title(1, "Mr"));
        titles.Add(new Title(2, "Miss"));
        titles.Add(new Title(3, "Mrs"));

        return titles;
    }
}

MainWindowViewModel.cs

public class MainWindowViewModel : ViewModelBase
{
    private ObservableCollection<Person> contacts;
    private List<Title> titles;

    public MainWindowViewModel()
    {
        titles = Title.GetAvailableTitles();

        Contacts = new ObservableCollection<Person>();
        Contacts.Add(new Person() { FirstName = "Jane", LastName = "Smith", TitleId = 2 });
    }

    public List<Title> Titles
    {
        get { return titles; }
    }

    public ObservableCollection<Person> Contacts
    {
        get { return contacts; }
        set
        {
            if (contacts != value)
            {
                contacts = value;
                this.OnPropertyChanged("Contacts");
            }
        }
    }
}

ViewModelBase.cs

public class ViewModelBase : INotifyPropertyChanged
{
    protected void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

Answer 1

Here is a working code. The key point here was to use SelectedValueBinding instead of SelecteItemBinding.

<DataGridComboBoxColumn Header="Title" 
                        SelectedValueBinding="{Binding TitleId}"
                        SelectedValuePath="TitleId"
                        DisplayMemberPath="TitleText"
                        >
    <DataGridComboBoxColumn.ElementStyle>
        <Style TargetType="ComboBox">
            <Setter Property="ItemsSource" Value="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=DataContext.Titles}"/>
        </Style>
    </DataGridComboBoxColumn.ElementStyle>
    <DataGridComboBoxColumn.EditingElementStyle>
        <Style TargetType="ComboBox">
            <Setter Property="ItemsSource" Value="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=DataContext.Titles}"/>
        </Style>
    </DataGridComboBoxColumn.EditingElementStyle>
</DataGridComboBoxColumn>

Answer 2

@SnowBear's answer worked well for me. But I want to clarify a detail of the binding.

In @Rob's example, both Title and Person classes use TitleID. Therefore, in @SnowBear's answer, in the binding:

SelectedValueBinding="{Binding TitleId}"

it wasn't immediately obvious to me which class and property was being bound.

Because the SelectedValueBinding attribute appeared on the DataGridComboBoxColumn, it is binding to the ItemsSource of the containing DataGrid. In this case the Contacts collection of Person objects.

In my case, the DataGrid's DataSource collection was attributed with a property that was named different from the ValuePath of the ComboBox's ItemSource collection. So my SelectedValueBinding's value was bound to a different property than the property named in the ComboBox's SelectedValuePath.