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How to use a user control dependency property when setting its data context?

I've made a user control which contains a command, to be called in response to a certain event. This command is a dependency property. I want to use it in the main window like this:
<local:myUserControl Command="{Binding someCommand}"/>
The "myCommand" is the dependency property I created for this user control. And I bind it to a command of the view model of the main window ("someCommand").
The problem is that I am setting the datacontext of my usercontrol (I have a view model for it), and it seems to reset the "Command" to null… Here is the code-behind of my view model:
public partial class myUserControl : UserControl, ICommandSource
{
public myUserControl()
{
this.DataContext = new myViewModel();
InitializeComponent();
}
public ICommand Command
{
get { return (ICommand)GetValue(CommandProperty); }
set { SetValue(CommandProperty, value); }
}
public static readonly DependencyProperty CommandProperty =
DependencyProperty.Register("Command", typeof(ICommand), typeof(myUserControl), new PropertyMetadata(null));
public object CommandParameter
{
get { return (object)GetValue(CommandParameterProperty); }
set { SetValue(CommandParameterProperty, value); }
}
public static readonly DependencyProperty CommandParameterProperty =
DependencyProperty.Register("CommandParameter", typeof(object), typeof(myUserControl), new PropertyMetadata(0));
public IInputElement CommandTarget
{
get { return (IInputElement)GetValue(CommandTargetProperty); }
set { SetValue(CommandTargetProperty, value); }
}
public static readonly DependencyProperty CommandTargetProperty =
DependencyProperty.Register("CommandTarget", typeof(IInputElement), typeof(myUserControl), new PropertyMetadata(null));
private void TextBlock_MouseUp(object sender, MouseButtonEventArgs e)
{
Command.Execute(this.CommandParameter);
}
}
The code of my user control could be the Following:
<UserControl x:Class="myApp.myUserControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:myApp"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="800">
<Grid>
<TextBlock MouseUp="TextBlock_MouseUp">
</TextBlock>
</Grid>
</UserControl>
(I know that this element seems a bit silly (or useless), but I have simplified it to test what didn't worked and also in order to ask a rather simple question).
I have discovered that, if I comment the "this.DataContext = new myViewModel();" line, the binding to the command works perfectly. And when I uncomment this line and put a breakpoint in the "TextBlock_MouseUp", the "Command" property is equal to null...
Would there be a way to resolve this problem? I have some complicated code in my view model (so I'm quite forced to keep this line "this.DataContext = new myViewModel();"), and I am not sure I could find another solution than having a "Command" dependency property in my user control…
To be sure I give a maximum of informations, I have the following code in the view model of my main window:
public ICommand someCommand { get; set; }
//Constructor
public MainWindowViewModel()
{
this.someCommand = new RelayCommand((obj) => { return true; },
(obj) =>
{
//I put a breakpoint here
int dummy = 0;
});
}
(The RelayCommand class is a standard RelayCommand class, with a "Predicate" CanExecute and an "Action Execute).
I hope this question is not a duplicate… I have found several similar question, but they did not seem to answer mine...

I'm really sorry for this question which was in fact a bit silly. I hadn't understand very well what happens during a binding. I thought that this code line in the MainWindow…
<local:myUserControl Command="{Binding someCommand}"/>
…would have made an attempt to bind the UserControl's "Command" property to the "someCommand" of the datacontext of the MainWindow. In fact, as #elgonzo pointed out, the binding looks up in the UserControl's datacontext for the "someCommand" property (and not in the MainWindow's datacontext!!). Therefore, setting the UserControl's datacontext with this line…
this.DataContext = new myViewModel();
...was preventing the binding to be correctly done (since it looks for the "someCommand" property of the UserControl's datacontext, which is now "myViewModel", which does not contain "someCommand"...).
To fix this, I had to change the binding like this:
<local:myUserControl Command="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}},
Path=DataContext.someCommand}"/>
I've found this solution here: https://stackoverflow.com/a/1127964/11609068.
Maybe it is not the best way to do it (the "Path= DataContext. someCommand" make me think this, it doesn't seem very elegant), but it works. Another way to do it is to name the MainWindow (x:Name="someName"), so that the binding is a bit simpler:
<local:myUserControl Command="{Binding ElementName=someName, Path=DataContext.someCommand}"/>
Again, sorry and many thanks to #elgonzo.

Bind XAML element to UserControl using ElementName

I have the following scenario and hierarchy of XAML elements in my page:
<Page> ....
<StackPanel> ...
<Grid> ....
<StackPanel>
<uc:MyUserControl
ReferencedButton={Binding ElementName=RightButton} />
<Button x:Name="RightButton" Click="{x:Bind ViewModel.OpenFlyout}" Content="Clickme" />
</StackPanel>
......
Then the code behind from 'MyUserControl'
public UIElement ReferencedButton
{
get { return (UIElement)GetValue(ReferencedButtonProperty); }
set { SetValue(ReferencedButtonProperty, value); }
}
public static readonly DependencyProperty ReferencedButtonProperty =
DependencyProperty.Register(nameof(ReferencedButton), typeof(UIElement), typeof(MyUserControl), null);
So far so good, however I was expecting that in my code behind, the 'ReferencedButton' property would be filled with a reference to the 'RightButton' button. However it always returns null.
I even tried:
{Binding RelativeSource={RelativeSource Mode=TemplatedParent}, ElementName=RightButton}
I know it is possible to bind the element, because I got the example from a DevExpress component, but still without any success.
I am following the suggestions/rules from the following docs:
Binding ElementName
XAML Namescopes
p.s: I know that I can pass the reference to the button in my code behind however I would like to do this through XAML itself.

It turns out that I needed to use a PropertyChangedCallback to make it work. So the solution is as below:
public static readonly DependencyProperty ReferencedButtonProperty=
DependencyProperty.Register(nameof(ReferencedButton),
typeof(UIElement),
typeof(MyUserControl),
new PropertyMetadata(default(UIElement),
new PropertyChangedCallback(PlacementCallBack)));
and in the code behind of my control I can access and set the value by implementing the PlacementCallBack like this:
public static void PlacementCallBack(object sender, DependencyPropertyChangedEventArgs e)
{
var myuserControl = sender as MyUserControl;
myuserControl.ReferencedButton = e.NewValue as UIElement;
}
The object DependencyPropertyChangedEventArgs contains two propeties NewValue and OldValue, they hold the old and new values of the previous object set.

How to create a Dependency Property for Binding

I'm working on a "simple" case. I like to create a new custom control which implements a DependencyProperty. In the next step I like to create a binding for updating the properties in both directions. I've builded a simple sample for this case, but the binding doesn't seem to work. I've found a way for updating the DPControl's property by using the FrameworkPropertyMetadata, but I don't know whether it's also a good idea to use the OnPropertyChanged event.
HERE is my sample project:
My control contains simply a Label
<UserControl x:Class="WPF_MVVM_ListBoxMultiSelection.DPControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:WPF_MVVM_ListBoxMultiSelection"
mc:Ignorable="d" Height="84.062" Width="159.641">
<Grid Margin="0,0,229,268">
<Label Content="TEST" x:Name="label" Margin="0,0,-221,-102"/>
</Grid>
</UserControl>
and implement a custom dependency property. Currently, I have also implemented the PropertyChanged method for the FramePropertyMetadata and set in this method the label's content, but I like to get it work in both directions.
public partial class DPControl : UserControl
{
public DPControl()
{
InitializeComponent();
}
public string MyCustomLabelContent
{
get { return (string)GetValue(MyCustomLabelContentProperty);}
set
{
SetValue(MyCustomLabelContentProperty, value);
}
}
private static void OnMyCustomLabelContentPropertyChanged(DependencyObject source,
DependencyPropertyChangedEventArgs e)
{
DPControl control = (DPControl)source;
control.label.Content = e.NewValue;
}
public static readonly DependencyProperty MyCustomLabelContentProperty = DependencyProperty.Register(
"MyCustomLabelContent",
typeof(string),
typeof(DPControl),
new FrameworkPropertyMetadata(null,
OnMyCustomLabelContentPropertyChanged
)
);
I use this control simply in a Window by:
<local:DPControl MyCustomLabelContent="{Binding MyLabelContent, Mode=TwoWay}" Margin="72,201,286,34"/>
MyLabelContent is a property in the ViewModel, which has implemented also the INotifyPropertyChanged interface.
public class ViewModel_MainWindow:NotifyPropertyChanged
{
private string _myLabelContent;
public string MyLabelContent
{
get { return _myLabelContent; }
set { _myLabelContent = value;
RaisePropertyChanged();
}
}...
So how can I get it work: Using the binding feature with my new control on custom properties.

In your UserControl:
<Label
Content="{Binding MyCustomLabelContent, RelativeSource={RelativeSource AncestorType=UserControl}}"
x:Name="label" Margin="0,0,-221,-102"/>
And get rid of that property-changed callback. All you need is the Binding.
I like to get it work in both directions
To make the dependency property two-way by default:
public static readonly DependencyProperty MyCustomLabelContentProperty =
DependencyProperty.Register(
"MyCustomLabelContent",
typeof(string),
typeof(DPControl),
new FrameworkPropertyMetadata(null,
FrameworkPropertyMetadataOptions.BindsTwoWayByDefault)
);
I omitted the unnecessary property change handler.
It can't usefully be two-way now, because Label.Content can't generate its own value. If you want your UserControl to set the value in its codebehind, that's easy:
MyCustomLabelContent = "Some arbitrary value";
If you did the binding like I showed you, that will update the Label in the UserControl XAML as well as the viewmodel property bound to the UserControl's dependency property.
If you want the XAML to set it, you'll need to
Lastly, this:
Margin="0,0,-221,-102"
Is not a good way to do layout. WPF layout with Grid, StackPanel, etc. is much easier and more robust.

Why is this basic binding failing with an exception?

Can anyone explain why the binding on TagObject below code throws the following binding exception?
System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:Path=Value; DataItem=null; target element is 'TagObject' (HashCode=37895910); target property is 'Value' (type 'String')
My suspicion is its because TagObject itself isn't a subclass of FrameworkElement so it doesn't have a data context itself and thus doesn't know how to resolve the XAML binding.
To test, I changed the base type of TagObject to FrameworkElement and sure enough, the binding error went away, but Value still didn't change. My theory there is although the binding was now valid, TagObject wasn't part of the Visual Tree, therefore it didn't inherit its DataContext.
I also tried giving 'TextBlocka name, then specifying it as theElementNamein the binding, but that again threw a binding exception. In this case, my suspicion is that it can't find the named element becauseTagObject` still is not part of the visual tree, even with the base-class change above.
For the record, I do know a solution would be to simply hide that object creation behind a ValueConverter to wrap it for me, but I'm wondering if there's a XAML-only solution to address that binding on TagObject.
Here's the XAML:
<Window x:Class="Test.TestWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:test="clr-namespace:Test">
<Window.Resources>
<DataTemplate DataType="{x:Type test:DataObject}">
<TextBlock Text="{Binding Value}">
<TextBlock.Tag>
<test:TagObject Value="{Binding Value}" />
</TextBlock.Tag>
</TextBlock>
</DataTemplate>
</Window.Resources>
<ListBox x:Name="MainListBox" BorderThickness="0" />
</Window>
This doesn't work either...
<TextBlock x:Name="MyTextBlock" Text="Test">
<TextBlock.Tag>
<test:TargetObject Value="{Binding DataContext.Value, ElementName=MyTextBlock}" />
</TextBlock.Tag>
</TextBlock>
Here's the code:
using System.Windows;
using System.Collections.ObjectModel;
namespace Test
{
public partial class TestWindow : Window
{
public TestWindow()
{
InitializeComponent();
var sourceItems = new ObservableCollection<DataObject>();
for(int i = 1; i <= 10; i++)
sourceItems.Add(new DataObject() { Value = "Item " + i});
MainListBox.ItemsSource = sourceItems;
}
}
public class DataObject : DependencyObject
{
public static readonly DependencyProperty ValueProperty = DependencyProperty.Register(
"Value",
typeof(string),
typeof(DataObject),
new UIPropertyMetadata(null));
public string Value
{
get { return (string)GetValue(ValueProperty); }
set { SetValue(ValueProperty, value); }
}
}
public class TagObject : DependencyObject
{
public static readonly DependencyProperty ValueProperty = DependencyProperty.Register(
"Value",
typeof(string),
typeof(TagObject),
new UIPropertyMetadata(null));
public string Value
{
get { return (string)GetValue(ValueProperty); }
set { SetValue(ValueProperty, value); }
}
}
}

I recreated your sample-code in a VS2013 solution and replicated what you're seeing. The binding seems to be happening just fine, but yes - it is spitting out those annoying error messages. After some research, it appears that this is a known bug -- I see others complaining of it as well. The binding is working fine. The problem, as far as I can tell from others, is that, for ItemsSources, WPF is trying to optimize the evaluation of the styles and data-templates as it composes the visual tree, sometimes doing this before the binding is available on the actual elements. In your case, the TextBlock is already available, and it has a binding value, but the Tag property within it is composed before that and thus complains of a missing FrameworkElement (which, an instant later, is no longer missing).
I do not find this to be an encouraging sign from the WPF team, as this seems like a very simple scenario. Correct code should never be emitting warnings or errors.

wpf usercontrol data-binding using xaml

I have wfp form like that:
public partial class MediaPlayerControlMain : Window
{
MediaPlayerMain MediaPlayerMain;
public MediaPlayerControlMain()
{
MediaPlayerMain = new MediaPlayerMain();
InitializeComponent();
}
}
I have my user control (PlayList) that use MediaPlayerMain object.
That User Control have that:
public partial class PlayList : UserControl
{
public MediaPlayerMain MediaPlayer
{
get { return (MediaPlayerMain)GetValue(MediaPlayerProperty); }
set { SetValue(MediaPlayerProperty, value); }
}
public static readonly DependencyProperty MediaPlayerProperty =
DependencyProperty.Register(
"MediaPlayer", typeof(MediaPlayerMain), typeof(PlayList),
new FrameworkPropertyMetadata()
);
}
Is there the way to set MediaPlayer property using just xaml. I tried to use "{Binding ElementName=MediaPlayerMain}" but it seems to be that MediaPlayerMain haven't initialized yet. Although i initialized it before InitializeComponent() function. What am i doing wrong?. And what is the best option to pass this object to my user control?

public partial class MediaPlayerControlMain : Window,INotifyPropertyChanged
{
public MediaPlayerControlMain()
{
InitializeComponent();
MediaPlayerMain = new MediaPlayerMain();
}
private MediaPlayerMain mediaPlayerMain;
public MediaPlayerMain MediaPlayerMain
{
get { return mediaPlayerMain; }
set { mediaPlayerMain = value; Notify("MediaPlayerMain"); }
}
public event PropertyChangedEventHandler PropertyChanged;
private void Notify(string propName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propName));
}
}
"{Binding MediaPlayerMain RelativeSource={RelativeSource AncestorType=Window}}"
The issue is you are trying to bind the field not property.For binding source must be the property not field because binding system uses reflection and looks only for properties not for fields.I hope this will help.

You need to name your root element (the Window/UserControl itself) in the markup. Ie:
<Window x:Name="mediaPlayer"
....>
<TextBlock Text="{Binding SomeProperty,ElementName=mediaPlayer}"

I don't see you setting the DataContext anywhere, and I don't think you have an object named MediaPlayerMain in your XAML tree
When you write {Binding ElementName=MediaPlayerMain}, you are telling WPF to set the property equal to the XAML object in the Visual Tree that is named MediaPlayerMain
What you're probably looking for instead is to bind to a Property in the DataContext named MediaPlayerMain, in which case your binding would look like this:
<local:PlayList MediaPlayer="{Binding MediaPlayerMain}" />
But that won't work unless your DataContext is set to an object containing the property MediaPlayerMain, such as setting this.DataContext = this in your Window's constructor.
As an alternative, you can use ElementName in your binding to tell WPF to look up the property on an object in the Visual Tree instead of in the DataContext, such as your Window.
<local:MediaPlayerControlMain x:Name="MainWindow" ...>
<local:PlayList MediaPlayer="{Binding MediaPlayerMain, ElementName=MainWindow}" />
</local:MediaPlayerControlMain>
This will make your binding look for the property MediaPlayerMain in the XAML element named MainWindow, which is your MediaPlayerControlMain class.
If you're new to WPF's DataBinding, I actually have an article on my blog specifically for beginners that should help you understand better what it is and how it works: What is this "DataContext" you speak of?