Dependency Property Datacontext

Question

I have a usercontrol, and there is a Datacontext set for it. This usercontrol contains also a Dependency-Property. Now, i want simply bind to this property.

I think the problem has something to do with the wrong datacontext.

The dependency-Property in my usercontrol (called TimePicker) looks like this:

public TimeSpan Time
    {
        get { return (TimeSpan)GetValue(TimeProperty); }
        set
        {
            SetValue(TimeProperty, value);
            OnPropertyChanged();
        }
    }

    public static readonly DependencyProperty TimeProperty = DependencyProperty.Register("Time", typeof (TimeSpan), typeof (TimePicker));

I try to use it like this:

<upDownControlDevelopement:TimePicker Grid.Row="1" Time="{Binding Path=TimeValue}" />

When i do this i get the following binding error:

System.Windows.Data Error: 40 : BindingExpression path error: 'TimeValue' property not found on 'object' ''TimePicker' (Name='TimePickerControl')'. BindingExpression:Path=TimeValue; DataItem='TimePicker' (Name='TimePickerControl'); target element is 'TimePicker' (Name='TimePickerControl'); target property is 'Time' (type 'TimeSpan')

Any help would be highly appreciated

Greetings Michael

PS: you can download the code at here

Answer 1

Although this has now been solved there seems to be some, in my opinion, inappropriate use of the DataContext.

When developing a custom reusable control, you should not set DataContext at all. What the DataContext will be, that is for the user of the control to decide, not for the developer. Consider the following common pattern of code:

<Grid DataContext="{Binding Data}">
    <TextBox Text="{Binding TextValue1}" />
    <!-- Some more controls -->
</Grid>

Notice that here, you are using the Grid control. The developer of the control (in this case, the WPF team), didn't touch the DataContext at all - that is up to you. What does it mean for you as a control developer? Your DependencyProperty definition is fine, but you shouldn't touch the DataContext. How will you then bind something inside your control to the DependencyProperty value? A good way is using a template (namespaces omitted):

<MyTimePicker>
    <MyTimePicker.Template>
        <ControlTemplate TargetType="MyTimePicker">
            <!-- Stuff in your control -->
            <TextBlock Text="{TemplateBinding Time}" />
            <TextBox Text="{Binding Time, RelativeSource={RelativeSource TemplatedParent}}" />
        </ControlTemplate>
    <MyTimePicker.Template>
</MyTimePicker>

Note that TemplateBinding is always one-way only, so if you need any editing at all, you need to use normal binding (as you can see on the TextBox in the example).

This only means that the TextBlock/Box inside your control will get its Time value from your custom control itself, ignoring any DataContext you might have set.

Then, when you use the control, you do it like this (added to my first example):

<Grid DataContext="{Binding Data}">
    <TextBox Text="{Binding TextValue1}" />
    <!-- Some more controls -->
    <MyTimePicker Time="{Binding TimeValue}" />
</Grid>

What just happened here is that the MyTimePicker does not have DataContext set anywhere at all - it gets it from the parent control (the Grid). So the value goes like this: Data-->(binding)-->MyTimePicker.Time-->(template binding)-->TextBlock.Text.

And above all, avoid doing this in the constructor of your custom control:

public MyTimePicker()
{
    InitializeComponent();
    DataContext = this;
}

This will override any DataContext set in XAML, which will make binding a huge pain (because you'll have to always set Source manually). The previous example would not work, and this wouldn't work either:

<MyTimePicker DataContext="{Binding Data}" Time="{Binding TimeValue}" />

You would think this is OK, but the DataContext will be resolved in the InitializeComponent() call, so the value will be immediately overwritten. So the binding to TimeValue will look for it in the control instead (which will, of course, fail).

Just don't touch the DataContext when developing a control and you'll be fine.