WPF / XAML绑定：使用实际的DataContext

Question

This is my XAML code:

<ComboBox Grid.Row="0" Margin="5" ItemsSource="{Binding Path=Clvm.Categories}">
</ComboBox>

<GridSplitter Grid.Row="0" Height="3" />

<DataGrid Grid.Row="1" AutoGenerateColumns="False" ItemsSource="{Binding Alvm.Artists}">
    <DataGrid.Columns>
        <DataGridTextColumn Header="ID" IsReadOnly="True" Binding="{Binding Id}" />
        <DataGridTextColumn Header="Name" Binding="{Binding Name}" />
        <DataGridTextColumn Header="Email" Binding="{Binding Email}" />
        <DataGridComboBoxColumn Header="Category" SelectedItemBinding="{Binding Category}"
                                ItemsSource="{Binding Path=Clvm.Categories}" />
    </DataGrid.Columns>
</DataGrid>

The ComboBox above is just for testing - I want to see if I can display all my Categories and it works well.

The DataGrid below is the control I really want. I want that Categories ComboBox as a column of the DataGrid. But the way I do it in the snippet obviously doesn't work as the data bindings for the columns are relative to the DataGrid's ItemsSource and not to the old DataContext. How can I switch it back for that binding?

The Control class:

public partial class ArtistManagementControl : UserControl {
    public ArtistManagementControl() {
        InitializeComponent();

        IDatabase db = new MYSQLDatabase("Server = localhost; Database = ufo; Uid = root;");
        SharedServices.Init(db);
        Alvm = new ArtistListViewModel(SharedServices.GetInstance().ArtistService, SharedServices.GetInstance().CategoryService);
        Clvm = new CategoryListViewModel(SharedServices.GetInstance().CategoryService);
        this.DataContext = this;
    }

    public ArtistListViewModel Alvm {
        get; private set;
    }

    public CategoryListViewModel Clvm {
        get; private set;
    }
}

The ViewModel for the categories:

public class CategoryListViewModel {
    private ICategoryService categoryService;

    public CategoryListViewModel(ICategoryService categoryService) {
        this.categoryService = categoryService;
        Categories = new ObservableCollection<CategoryViewModel>();
        UpdateCategories();
    }

    public ObservableCollection<CategoryViewModel> Categories {
        get; set;
    }

    public void UpdateCategories() {
        Categories.Clear();
        foreach(var cat in categoryService.GetAllCategories()) {
            Categories.Add(new CategoryViewModel(cat));
        }
    }
}

Answer 1

Since DataGridComboBoxColumn or any other supported data grid columns are not part of visual tree of datagrid so they don't inherit the DataContext of datagrid . Since, they don't lie in visual tree so any try to get DataContext using RelativeSource won't work.

Solution - You can create a proxy element to bind the data context of window; use that proxy element to bind the ItemsSource of DataGridComboBoxColumn .

<Grid>
  <Grid.Resources>
           <FrameworkElement x:Key="ProxyElement" DataContext="{Binding}"/>
   </Grid.Resources>
   <ContentControl Visibility="Collapsed" Content="{StaticResource ProxyElement}"></ContentControl>
    <DataGrid x:Name="datagrid" Grid.Row="1" AutoGenerateColumns="False" ItemsSource="{Binding Alvm.Artists}">
      <DataGrid.Columns>
        <DataGridTextColumn Header="ID" IsReadOnly="True" Binding="{Binding Id}" />
        <DataGridTextColumn Header="Name" Binding="{Binding Name}" />
        <DataGridTextColumn Header="Email" Binding="{Binding Email}" />
        <DataGridComboBoxColumn Header="Category" SelectedItemBinding="{Binding Category}"
                                ItemsSource="{Binding DataContext.Clvm.Categories, Source={StaticResource ProxyElement}}" />
      </DataGrid.Columns>
    </DataGrid>     
</Grid>

Answer 2

You can bind to the DataGrids DataContext by using an ElementName Binding:

<DataGrid x:Name="datagrid" Grid.Row="1" AutoGenerateColumns="False" ItemsSource="{Binding Alvm.Artists}">
    <DataGrid.Columns>
        <DataGridTextColumn Header="ID" IsReadOnly="True" Binding="{Binding Id}" />
        <DataGridTextColumn Header="Name" Binding="{Binding Name}" />
        <DataGridTextColumn Header="Email" Binding="{Binding Email}" />
        <DataGridComboBoxColumn Header="Category" SelectedItemBinding="{Binding Category}"
                                ItemsSource="{Binding Path=DataContext.Clvm.Categories, ElementName=datagrid}" />
    </DataGrid.Columns>
</DataGrid>

Note that you have to give the DataGrid a name for that to work.

---

Another option is to bind to an Ancestor . With this solution you dont have to give the DataGrid a name, but it is more complex:

<DataGridComboBoxColumn Header="Category" SelectedItemBinding="{Binding Category}"
      ItemsSource="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type DataGrid}}, Path=DataContext.Clvm.Categories}" />