Question

Context: I am using EF 5 and trying to bind two entities to a WPF form. It works fine with simples property-to-textbox binding, but I’d like to bind a navigation property object to a combobox. I am learning WPF so this may be obvious.

I have two EF entities:

Product { int Id, string Name, Category Category }

and

Category { int Id, string Description }

I made a WPF form to create and edit products, it is something like this (simplified):

        <Window.Resources>
            <CollectionViewSource x:Key="productViewSource" d:DesignSource="{d:DesignInstance my:product, CreateList=True}" />
            <CollectionViewSource x:Key="categoryViewSource" d:DesignSource="{d:DesignInstance my:category, CreateList=True}" />
        </Window.Resources>
        <Grid DataContext="{StaticResource productViewSource}">
            <Label Content="Id" Height="28" HorizontalAlignment="Left" Margin="6,6,0,0" Name="label1" VerticalAlignment="Top" />
            <TextBox Height="23" HorizontalAlignment="Left" Margin="61,8,0,0" Name="txtId" Text="{Binding Path=Id, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}" VerticalAlignment="Top" Width="56" IsEnabled="False" />
            <Label Content="Name" Height="28" HorizontalAlignment="Left" Margin="6,38,0,0" Name="label2" VerticalAlignment="Top" />
            <TextBox Text="{Binding Path=Name, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}" Height="23" HorizontalAlignment="Left" Margin="61,40,0,0" Name="txtNome" VerticalAlignment="Top" Width="183" IsEnabled="False" />

            <Label Content="Category" Height="28" HorizontalAlignment="Left" Margin="6,129,0,0" Name="label5" VerticalAlignment="Top" />
            <ComboBox Height="23" HorizontalAlignment="Left" Margin="72,132,0,0" Name="comboCategory" VerticalAlignment="Top" Width="172" 
                      DisplayMemberPath="Description" 
                      SelectedValuePath="Id" 
                      SelectedValue="{Binding Path=Category, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                      ItemsSource="{Binding Path=categoryViewSource, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}}">
            </ComboBox>
         </Grid>

The code to load the two View Sources is this:

private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        context = new DBEntities();

        var productViewSource = ((System.Windows.Data.CollectionViewSource)(this.FindResource("productViewSource")));
        context.Product.Include(p => p.Category);
        context.Product.Load();
        ProductViewSource.Source = context.Product.Local;

        var categoryViewSource = ((System.Windows.Data.CollectionViewSource)(this.FindResource("categoryViewSource")));
        categoryViewSource.Source = context.Category.ToList();
    }

When the window loads the two textboxes work, but the combobox is always empty.

Probally this is what is wrong, but I can’t seem to find a way to make it work.

ItemsSource="{Binding Path=categoryViewSource, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}}"

Answer 1

This is wrong:

ItemsSource="{Binding Path=categoryViewSource, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}}">

What you’re telling WPF here is that you want the Property categoryViewSource from the first Window it encounters upwards in the Visual Tree. Of course the Window Class doesn’t have such property.

It should be:

ItemsSource="{StaticResource categoryViewSource}">

which tells WPF you want to look up a Resource with that name (key)

Edit: You’d better create a ViewModel to hold your data:

public class ViewModel: INotifyPropertyChanged
{
     public List<Category> Categories {get;set;} //Don't forget INotifyPropertyChanged!!

     //... other properties
}

private void Window_Loaded(object sender, RoutedEventArgs e)
{
    context = new DBEntities();

    var vm = new ViewModel() { Categories = context.Category.ToList(); }

    DataContext = vm;
}

XAML:

<ComboBox ItemsSource="{Binding Categories}"/>

Remove the Static Resources and remove all references to them.

Edit 2:

The SelectedValuePath And SelectedValue Properties work together. The SelectedValuePath is telling the ComboBox “Evaluate this property inside each item”, and the SelectedValue property is the actual value you expect to find in that property, so:

<ComboBox SelectedValuePath="Id" SelectedValue="{Binding Category}"/>

Is telling WPF that you have some int property called Category in your DataContext class which you expect to contain a value matching the Id of one of your entities.

What you actually need to do here is:

ViewModel:

public Category Category {get;set;} //Don't forget INotifyPropertyChanged!!!

XAML:

<ComboBox SelectedItem="{Binding Category}"/>

and remove both SelectedValuePath and SelectedValue.