WPF DataContext继承混乱

Question

I have UserControl with two DependencyProperties . I'm trying to understand how DataContext works here, because it seems inconsistent for me and it's probably somethin obvious that I'm missing.

My user control:

<UserControl x:Class="XYZ.MyControl" xmlns:oxy="clr-namespace:OxyPlot.Wpf;assembly=OxyPlot.Wpf" ...>
    <Grid x:Name="RootGrid">
        <oxy:Plot Title="{Binding ChartTitle}" x:Name="PlotxName" IsLegendVisible="False">
                .
                .
                .
            <oxy:LineSeries Title="test" ItemsSource="{Binding Items}" DataFieldX="X" DataFieldY="Y" MarkerType="Circle">
                <oxy:LineSeries.LabelFormatString>{1}</oxy:LineSeries.LabelFormatString>
                <oxy:LineSeries.XAxisKey>X1</oxy:LineSeries.XAxisKey>
            </oxy:LineSeries>
        </oxy:Plot>
    </Grid>
</UserControl>

My code behind:

public partial class MyControl: UserControl
{

    #region ChartTitle
    public string ChartTitle
    {
        get { return (string)GetValue(ChartTitleProperty); }
        set { SetValue(ChartTitleProperty, value); }
    }

    public static readonly DependencyProperty ChartTitleProperty =
        DependencyProperty.Register("ChartTitle", typeof(string), typeof(MFPlot), new PropertyMetadata(""));
    #endregion

    #region Items
    public ObservableCollection<Item> Items
    {
        get { return (ObservableCollection<Item>)GetValue(ItemsProperty); }
        set
        {
            SetValue(ItemsProperty, value);
        }
    }

    public static readonly DependencyProperty ItemsProperty =
        DependencyProperty.Register("Items", typeof(ObservableCollection<Item>), typeof(MFPlot), new PropertyMetadata(new ObservableCollection<Item>(), ItemsPropertyChanged));
        .
        .
        .
    #endregion

    public MyControl()
    {
        SetValue(ItemsProperty, new ObservableCollection<Item>()); //overwrite default collection to avoid using the same one by all MyControls
        InitializeComponent();
        RootGrid.DataContext = this; //parent element
    }
}

How I use this control:

<Page xmlns:local="clr-namespace:XYZ" ...>
    <local:MyControl Items="{Binding SubItemsTre}" ChartTitle="TEST TITLE"/>
</Page>

With this setup, RootGrid DataContext (set as per last line of code behind) should be inherited by it's descendants, right? This works for Title property of <oxy:Plot> , it displays correctly "TEST TITLE". However, it doesn't work for ItemsSource property of <oxy:LineSeries> . It's DataContext is still the same as MyControl 's parent ie Page, not the same as RootGrid . I have wasted a lot of time on this and found out that I have to add DataContext="{Binding}" to <oxy:Plot> . Why is that? Why one element seems to inherit it's parent DataContext while the other doesn't? Is it because <oxy:LineSeries> is child of <oxy:Plot> , not RootGrid ?

Answer 1

It's true that DataContext generally is inherhitted by the parent, with the exception beeing items in an itemscontrol, which typically has the datamodel/entity it should present as datacontext.

<Page xmlns:local="clr-namespace:XYZ" ...>
     <Page.DataContext>
         <local:MyControl/> <!-- One way of setting dc... -->
     </Page.DataContext>
    <local:MyControl Items="{Binding SubItemsTre}" ChartTitle="TEST TITLE"/>
</Page>

Another option is to set the datacontext manually when navigating to the page, or setting it through a ViewModelLocator. Generally your should not start setting dc from the code, or on elements, if it isn't a really really good reason for it. The bad thing about setting dc like above, is that you cannnot reuse your control with a different dc ofc.

BTW this code:

public static readonly DependencyProperty ItemsProperty =DependencyProperty.Register("Items", typeof(ObservableCollection<Item>), typeof(MFPlot), new PropertyMetadata(new ObservableCollection<Item>(), ItemsPropertyChanged));

Allocating like this directly in the DC will actually make it static (new ObservableCollection() in the DO definition).

Your ctor is a bit fishy too:

public MyControl()
{
    // Bad pie!!
    SetValue(ItemsProperty, new ObservableCollection<Item>()); //overwrite default collection to avoid using the same one by all MyControls
    InitializeComponent();
    RootGrid.DataContext = this; //parent element
}

public MyControl()
{
    InitializeComponent();
    Initialize(); // control is not yet loaded...
}    

protected void Initialize()
{ 
    Items = new ObservableCollection<Item>();
}

Personally, I would have moved all properties etc into a separate viewmodel(using INotifyPropertyChanged rather than beeing a DO), and set that as DataContext. I found an example here, which is kind of related

Hope it helps,

Cheers,

Stian