UserControl DataContext Binding

Question

I have three projects in my solution:

• My main WPF Application which contains a MainWindow + MainViewModel
• UserControl Library with a UserControl (ConfigEditorView)
• UIProcess class with the ViewModel for the UserControl (ConfigEditorViewModel)

In my MainWindow I want to use the UserControl with the ViewModel of UIProcess.

First I set the UserControl in my MainWindow:

<TabItem Header="Editor">
   <Grid>
     <cel:ConfigEditorView DataContext="{Binding ConfEditModel, NotifyOnSourceUpdated=True, NotifyOnTargetUpdated=True, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
   </Grid>
</TabItem>

I don't know which of these properties I need here, so I put all together but it still doesn't work.

Then I've set this in my MainViewModel:

public ConfigEditorViewModel ConfEditModel { get; set; }

With simple method that is bound to a Button:

private void doSomething()
{
    ConfEditModel = new ConfigEditorViewModel("Hello World");
}

My ConfigEditorViewModel looks basically like this:

public class ConfigEditorViewModel : ViewModelBase
{
    private string _Description;
    public string Description
    {
        get
        {
            return _Description;
        }
        set
        {
            _Description = value;
            base.RaisePropertyChanged();
        }
    }

    public ConfigEditorViewModel(string t)
    {
        Description = t;
    }
}

The description is bound to a TextBox in my UserControl.

<TextBox Grid.Row="1" Grid.Column="1" Margin="0,0,0,10" Text="{Binding Description}"/>

When I start the application and click the Button the TextBox should contain "Hello World" but it's empty.

What I've done wrong?

Answer 1

i gave you a general answer:

within a "real(a usercontrol you wanna use with different viewmodels with different property names)" usercontrol you bind just to your own DependencyProperties and you do that with ElementName or RelativeSource binding and you should never set the DataContext within a UserControl.

 <UserControl x:Name="myRealUC" x:class="MyUserControl">
   <TextBox Text="{Binding ElementName=myRealUC, Path=MyOwnDPIDeclaredInMyUc, Path=TwoWay}"/>
 <UserControl>

if you do that you can easily use this Usercontrol in any view like:

<myControls:MyUserControl MyOwnDPIDeclaredInMyUc="{Binding MyPropertyInMyViewmodel}"/>

and for completeness: the Dependency Property

    public readonly static DependencyProperty MyOwnDPIDeclaredInMyUcProperty = DependencyProperty.Register(
    "MyOwnDPIDeclaredInMyUc", typeof(string), typeof(MyUserControl), new PropertyMetadata(""));

    public bool MyOwnDPIDeclaredInMyUc
    {
        get { return (string)GetValue(MyOwnDPIDeclaredInMyUcProperty); }
        set { SetValue(MyOwnDPIDeclaredInMyUcProperty, value); }
    }

Answer 2

Your view models (and, optionally, models) need to implement INotifyPropertyChanged.

Binding's aren't magic. There is no inbuilt mechanism that allows for code to be notified when a plain old property's value changes. You'd have to poll it in order to check to see if a change happened, which would be very bad, performance-wise.

So bindings will look at the objects they are bound against and see if they implement INotifyPropertyChanged and, if so, will subscribe to the PropertyChanged event. That way, when you change a property and fire the event, the binding is notified and updates the UI.

Be warned, you must implement the interface and use it correctly. This example says it's for 2010, but it works fine.