How do I use RelativeSource with WPF bindings and what are the different use-cases?
This question is related to
.net
wpf
xaml
data-binding
relativesource
The answer is
Some useful bits and pieces:
Here's how to do it mostly in code:
Binding b = new Binding();
b.RelativeSource = new RelativeSource(RelativeSourceMode.FindAncestor, this.GetType(), 1);
b.Path = new PropertyPath("MyElementThatNeedsBinding");
MyLabel.SetBinding(ContentProperty, b);
I largely copied this from Binding Relative Source in code Behind.
Also, the MSDN page is pretty good as far as examples go: RelativeSource Class
Don't forget TemplatedParent:
<Binding RelativeSource="{RelativeSource TemplatedParent}"/>
or
{Binding RelativeSource={RelativeSource TemplatedParent}}
In WPF RelativeSource binding exposes three properties to set:
1. Mode: This is an enum that could have four values:
a. PreviousData(
value=0): It assigns the previous value of thepropertyto the bound oneb. TemplatedParent(
value=1): This is used when defining thetemplatesof any control and want to bind to a value/Property of thecontrol.For example, define
ControlTemplate:
<ControlTemplate>
<CheckBox IsChecked="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Value, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
</ControlTemplate>
c. Self(
value=2): When we want to bind from aselfor apropertyof self.For example: Send checked state of
checkboxasCommandParameterwhile setting theCommandonCheckBox
<CheckBox ...... CommandParameter="{Binding RelativeSource={RelativeSource Self},Path=IsChecked}" />
d. FindAncestor(
value=3): When want to bind from a parentcontrolinVisual Tree.For example: Bind a
checkboxinrecordsif agrid,ifheadercheckboxis checked
<CheckBox IsChecked="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type iDP:XamDataGrid}}, Path=DataContext.IsHeaderChecked, Mode=TwoWay}" />
2. AncestorType: when mode is FindAncestor then define what type of ancestor
RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type iDP:XamDataGrid}}
3. AncestorLevel: when mode is FindAncestor then what level of ancestor (if there are two same type of parent in visual tree)
RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type iDP:XamDataGrid, AncestorLevel=1}}
Above are all use-cases for
RelativeSource binding.
I just posted another solution for accessing the DataContext of a parent element in Silverlight that works for me. It uses Binding ElementName.
Don't forget TemplatedParent:
<Binding RelativeSource="{RelativeSource TemplatedParent}"/>
or
{Binding RelativeSource={RelativeSource TemplatedParent}}
Here's a more visual explanation in the context of a MVVM architecture:
This is an example of the use of this pattern that worked for me on empty datagrids.
<Style.Triggers>
<DataTrigger Binding="{Binding Items.Count, RelativeSource={RelativeSource Self}}" Value="0">
<Setter Property="Background">
<Setter.Value>
<VisualBrush Stretch="None">
<VisualBrush.Visual>
<TextBlock Text="We did't find any matching records for your search..." FontSize="16" FontWeight="SemiBold" Foreground="LightCoral"/>
</VisualBrush.Visual>
</VisualBrush>
</Setter.Value>
</Setter>
</DataTrigger>
</Style.Triggers>
If an element is not part of the visual tree, then RelativeSource will never work.
In this case, you need to try a different technique, pioneered by Thomas Levesque.
He has the solution on his blog under [WPF] How to bind to data when the DataContext is not inherited. And it works absolutely brilliantly!
In the unlikely event that his blog is down, Appendix A contains a mirror copy of his article.
Please do not comment here, please comment directly on his blog post.
Appendix A: Mirror of blog post
The DataContext property in WPF is extremely handy, because it is automatically inherited by all children of the element where you assign it; therefore you don’t need to set it again on each element you want to bind. However, in some cases the DataContext is not accessible: it happens for elements that are not part of the visual or logical tree. It can be very difficult then to bind a property on those elements…
Let’s illustrate with a simple example: we want to display a list of products in a DataGrid. In the grid, we want to be able to show or hide the Price column, based on the value of a ShowPrice property exposed by the ViewModel. The obvious approach is to bind the Visibility of the column to the ShowPrice property:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding ShowPrice,
Converter={StaticResource visibilityConverter}}"/>
Unfortunately, changing the value of ShowPrice has no effect, and the column is always visible… why? If we look at the Output window in Visual Studio, we notice the following line:
System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:Path=ShowPrice; DataItem=null; target element is ‘DataGridTextColumn’ (HashCode=32685253); target property is ‘Visibility’ (type ‘Visibility’)
The message is rather cryptic, but the meaning is actually quite simple: WPF doesn’t know which FrameworkElement to use to get the DataContext, because the column doesn’t belong to the visual or logical tree of the DataGrid.
We can try to tweak the binding to get the desired result, for instance by setting the RelativeSource to the DataGrid itself:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding DataContext.ShowPrice,
Converter={StaticResource visibilityConverter},
RelativeSource={RelativeSource FindAncestor, AncestorType=DataGrid}}"/>
Or we can add a CheckBox bound to ShowPrice, and try to bind the column visibility to the IsChecked property by specifying the element name:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding IsChecked,
Converter={StaticResource visibilityConverter},
ElementName=chkShowPrice}"/>
But none of these workarounds seems to work, we always get the same result…
At this point, it seems that the only viable approach would be to change the column visibility in code-behind, which we usually prefer to avoid when using the MVVM pattern… But I’m not going to give up so soon, at least not while there are other options to consider
The solution to our problem is actually quite simple, and takes advantage of the Freezable class. The primary purpose of this class is to define objects that have a modifiable and a read-only state, but the interesting feature in our case is that Freezable objects can inherit the DataContext even when they’re not in the visual or logical tree. I don’t know the exact mechanism that enables this behavior, but we’re going to take advantage of it to make our binding work…
The idea is to create a class (I called it BindingProxy for reasons that should become obvious very soon) that inherits Freezable and declares a Data dependency property:
public class BindingProxy : Freezable
{
#region Overrides of Freezable
protected override Freezable CreateInstanceCore()
{
return new BindingProxy();
}
#endregion
public object Data
{
get { return (object)GetValue(DataProperty); }
set { SetValue(DataProperty, value); }
}
// Using a DependencyProperty as the backing store for Data. This enables animation, styling, binding, etc...
public static readonly DependencyProperty DataProperty =
DependencyProperty.Register("Data", typeof(object), typeof(BindingProxy), new UIPropertyMetadata(null));
}
We can then declare an instance of this class in the resources of the DataGrid, and bind the Data property to the current DataContext:
<DataGrid.Resources>
<local:BindingProxy x:Key="proxy" Data="{Binding}" />
</DataGrid.Resources>
The last step is to specify this BindingProxy object (easily accessible with StaticResource) as the Source for the binding:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding Data.ShowPrice,
Converter={StaticResource visibilityConverter},
Source={StaticResource proxy}}"/>
Note that the binding path has been prefixed with “Data”, since the path is now relative to the BindingProxy object.
The binding now works correctly, and the column is properly shown or hidden based on the ShowPrice property.
This is an example of the use of this pattern that worked for me on empty datagrids.
<Style.Triggers>
<DataTrigger Binding="{Binding Items.Count, RelativeSource={RelativeSource Self}}" Value="0">
<Setter Property="Background">
<Setter.Value>
<VisualBrush Stretch="None">
<VisualBrush.Visual>
<TextBlock Text="We did't find any matching records for your search..." FontSize="16" FontWeight="SemiBold" Foreground="LightCoral"/>
</VisualBrush.Visual>
</VisualBrush>
</Setter.Value>
</Setter>
</DataTrigger>
</Style.Triggers>
Binding RelativeSource={
RelativeSource Mode=FindAncestor, AncestorType={x:Type ItemType}
}
...
The default attribute of RelativeSource is the Mode property. A complete set of valid values is given here (from MSDN):
PreviousData Allows you to bind the previous data item (not that control that contains the data item) in the list of data items being displayed.
TemplatedParent Refers to the element to which the template (in which the data-bound element exists) is applied. This is similar to setting a TemplateBindingExtension and is only applicable if the Binding is within a template.
Self Refers to the element on which you are setting the binding and allows you to bind one property of that element to another property on the same element.
FindAncestor Refers to the ancestor in the parent chain of the data-bound element. You can use this to bind to an ancestor of a specific type or its subclasses. This is the mode you use if you want to specify AncestorType and/or AncestorLevel.
It's worthy of note that for those stumbling across this thinking of Silverlight:
Silverlight offers a reduced subset only, of these commands
Bechir Bejaoui exposes the use cases of the RelativeSources in WPF in his article here:
The RelativeSource is a markup extension that is used in particular binding cases when we try to bind a property of a given object to another property of the object itself, when we try to bind a property of a object to another one of its relative parents, when binding a dependency property value to a piece of XAML in case of custom control development and finally in case of using a differential of a series of a bound data. All of those situations are expressed as relative source modes. I will expose all of those cases one by one.
- Mode Self:
Imagine this case, a rectangle that we want that its height is always equal to its width, a square let's say. We can do this using the element name
<Rectangle Fill="Red" Name="rectangle" Height="100" Stroke="Black" Canvas.Top="100" Canvas.Left="100" Width="{Binding ElementName=rectangle, Path=Height}"/>But in this above case we are obliged to indicate the name of the binding object, namely the rectangle. We can reach the same purpose differently using the RelativeSource
<Rectangle Fill="Red" Height="100" Stroke="Black" Width="{Binding RelativeSource={RelativeSource Self}, Path=Height}"/>For that case we are not obliged to mention the name of the binding object and the Width will be always equal to the Height whenever the height is changed.
If you want to parameter the Width to be the half of the height then you can do this by adding a converter to the Binding markup extension. Let's imagine another case now:
<TextBlock Width="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualWidth}"/>The above case is used to tie a given property of a given element to one of its direct parent ones as this element holds a property that is called Parent. This leads us to another relative source mode which is the FindAncestor one.
- Mode FindAncestor
In this case, a property of a given element will be tied to one of its parents, Of Corse. The main difference with the above case is the fact that, it's up to you to determine the ancestor type and the ancestor rank in the hierarchy to tie the property. By the way try to play with this piece of XAML
<Canvas Name="Parent0"> <Border Name="Parent1" Width="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualWidth}" Height="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualHeight}"> <Canvas Name="Parent2"> <Border Name="Parent3" Width="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualWidth}" Height="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualHeight}"> <Canvas Name="Parent4"> <TextBlock FontSize="16" Margin="5" Text="Display the name of the ancestor"/> <TextBlock FontSize="16" Margin="50" Text="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Border}, AncestorLevel=2},Path=Name}" Width="200"/> </Canvas> </Border> </Canvas> </Border> </Canvas>The above situation is of two TextBlock elements those are embedded within a series of borders and canvas elements those represent their hierarchical parents. The second TextBlock will display the name of the given parent at the relative source level.
So try to change AncestorLevel=2 to AncestorLevel=1 and see what happens. Then try to change the type of the ancestor from AncestorType=Border to AncestorType=Canvas and see what's happens.
The displayed text will change according to the Ancestor type and level. Then what's happen if the ancestor level is not suitable to the ancestor type? This is a good question, I know that you're about to ask it. The response is no exceptions will be thrown and nothings will be displayed at the TextBlock level.
- TemplatedParent
This mode enables tie a given ControlTemplate property to a property of the control that the ControlTemplate is applied to. To well understand the issue here is an example bellow
<Window.Resources> <ControlTemplate x:Key="template"> <Canvas> <Canvas.RenderTransform> <RotateTransform Angle="20"/> </Canvas.RenderTransform> <Ellipse Height="100" Width="150" Fill="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Background}"> </Ellipse> <ContentPresenter Margin="35" Content="{Binding RelativeSource={RelativeSource TemplatedParent},Path=Content}"/> </Canvas> </ControlTemplate> </Window.Resources> <Canvas Name="Parent0"> <Button Margin="50" Template="{StaticResource template}" Height="0" Canvas.Left="0" Canvas.Top="0" Width="0"> <TextBlock FontSize="22">Click me</TextBlock> </Button> </Canvas>If I want to apply the properties of a given control to its control template then I can use the TemplatedParent mode. There is also a similar one to this markup extension which is the TemplateBinding which is a kind of short hand of the first one, but the TemplateBinding is evaluated at compile time at the contrast of the TemplatedParent which is evaluated just after the first run time. As you can remark in the bellow figure, the background and the content are applied from within the button to the control template.
Binding RelativeSource={
RelativeSource Mode=FindAncestor, AncestorType={x:Type ItemType}
}
...
The default attribute of RelativeSource is the Mode property. A complete set of valid values is given here (from MSDN):
PreviousData Allows you to bind the previous data item (not that control that contains the data item) in the list of data items being displayed.
TemplatedParent Refers to the element to which the template (in which the data-bound element exists) is applied. This is similar to setting a TemplateBindingExtension and is only applicable if the Binding is within a template.
Self Refers to the element on which you are setting the binding and allows you to bind one property of that element to another property on the same element.
FindAncestor Refers to the ancestor in the parent chain of the data-bound element. You can use this to bind to an ancestor of a specific type or its subclasses. This is the mode you use if you want to specify AncestorType and/or AncestorLevel.
Bechir Bejaoui exposes the use cases of the RelativeSources in WPF in his article here:
The RelativeSource is a markup extension that is used in particular binding cases when we try to bind a property of a given object to another property of the object itself, when we try to bind a property of a object to another one of its relative parents, when binding a dependency property value to a piece of XAML in case of custom control development and finally in case of using a differential of a series of a bound data. All of those situations are expressed as relative source modes. I will expose all of those cases one by one.
- Mode Self:
Imagine this case, a rectangle that we want that its height is always equal to its width, a square let's say. We can do this using the element name
<Rectangle Fill="Red" Name="rectangle" Height="100" Stroke="Black" Canvas.Top="100" Canvas.Left="100" Width="{Binding ElementName=rectangle, Path=Height}"/>But in this above case we are obliged to indicate the name of the binding object, namely the rectangle. We can reach the same purpose differently using the RelativeSource
<Rectangle Fill="Red" Height="100" Stroke="Black" Width="{Binding RelativeSource={RelativeSource Self}, Path=Height}"/>For that case we are not obliged to mention the name of the binding object and the Width will be always equal to the Height whenever the height is changed.
If you want to parameter the Width to be the half of the height then you can do this by adding a converter to the Binding markup extension. Let's imagine another case now:
<TextBlock Width="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualWidth}"/>The above case is used to tie a given property of a given element to one of its direct parent ones as this element holds a property that is called Parent. This leads us to another relative source mode which is the FindAncestor one.
- Mode FindAncestor
In this case, a property of a given element will be tied to one of its parents, Of Corse. The main difference with the above case is the fact that, it's up to you to determine the ancestor type and the ancestor rank in the hierarchy to tie the property. By the way try to play with this piece of XAML
<Canvas Name="Parent0"> <Border Name="Parent1" Width="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualWidth}" Height="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualHeight}"> <Canvas Name="Parent2"> <Border Name="Parent3" Width="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualWidth}" Height="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualHeight}"> <Canvas Name="Parent4"> <TextBlock FontSize="16" Margin="5" Text="Display the name of the ancestor"/> <TextBlock FontSize="16" Margin="50" Text="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Border}, AncestorLevel=2},Path=Name}" Width="200"/> </Canvas> </Border> </Canvas> </Border> </Canvas>The above situation is of two TextBlock elements those are embedded within a series of borders and canvas elements those represent their hierarchical parents. The second TextBlock will display the name of the given parent at the relative source level.
So try to change AncestorLevel=2 to AncestorLevel=1 and see what happens. Then try to change the type of the ancestor from AncestorType=Border to AncestorType=Canvas and see what's happens.
The displayed text will change according to the Ancestor type and level. Then what's happen if the ancestor level is not suitable to the ancestor type? This is a good question, I know that you're about to ask it. The response is no exceptions will be thrown and nothings will be displayed at the TextBlock level.
- TemplatedParent
This mode enables tie a given ControlTemplate property to a property of the control that the ControlTemplate is applied to. To well understand the issue here is an example bellow
<Window.Resources> <ControlTemplate x:Key="template"> <Canvas> <Canvas.RenderTransform> <RotateTransform Angle="20"/> </Canvas.RenderTransform> <Ellipse Height="100" Width="150" Fill="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Background}"> </Ellipse> <ContentPresenter Margin="35" Content="{Binding RelativeSource={RelativeSource TemplatedParent},Path=Content}"/> </Canvas> </ControlTemplate> </Window.Resources> <Canvas Name="Parent0"> <Button Margin="50" Template="{StaticResource template}" Height="0" Canvas.Left="0" Canvas.Top="0" Width="0"> <TextBlock FontSize="22">Click me</TextBlock> </Button> </Canvas>If I want to apply the properties of a given control to its control template then I can use the TemplatedParent mode. There is also a similar one to this markup extension which is the TemplateBinding which is a kind of short hand of the first one, but the TemplateBinding is evaluated at compile time at the contrast of the TemplatedParent which is evaluated just after the first run time. As you can remark in the bellow figure, the background and the content are applied from within the button to the control template.
Here's a more visual explanation in the context of a MVVM architecture:
I created a library to simplify the binding syntax of WPF including making it easier to use RelativeSource. Here are some examples. Before:
{Binding Path=PathToProperty, RelativeSource={RelativeSource Self}}
{Binding Path=PathToProperty, RelativeSource={RelativeSource AncestorType={x:Type typeOfAncestor}}}
{Binding Path=PathToProperty, RelativeSource={RelativeSource TemplatedParent}}
{Binding Path=Text, ElementName=MyTextBox}
After:
{BindTo PathToProperty}
{BindTo Ancestor.typeOfAncestor.PathToProperty}
{BindTo Template.PathToProperty}
{BindTo #MyTextBox.Text}
Here is an example of how method binding is simplified. Before:
// C# code
private ICommand _saveCommand;
public ICommand SaveCommand {
get {
if (_saveCommand == null) {
_saveCommand = new RelayCommand(x => this.SaveObject());
}
return _saveCommand;
}
}
private void SaveObject() {
// do something
}
// XAML
{Binding Path=SaveCommand}
After:
// C# code
private void SaveObject() {
// do something
}
// XAML
{BindTo SaveObject()}
You can find the library here: http://www.simplygoodcode.com/2012/08/simpler-wpf-binding.html
Note in the 'BEFORE' example that I use for method binding that code was already optimized by using RelayCommand which last I checked is not a native part of WPF. Without that the 'BEFORE' example would have been even longer.
Don't forget TemplatedParent:
<Binding RelativeSource="{RelativeSource TemplatedParent}"/>
or
{Binding RelativeSource={RelativeSource TemplatedParent}}
It's worthy of note that for those stumbling across this thinking of Silverlight:
Silverlight offers a reduced subset only, of these commands
I created a library to simplify the binding syntax of WPF including making it easier to use RelativeSource. Here are some examples. Before:
{Binding Path=PathToProperty, RelativeSource={RelativeSource Self}}
{Binding Path=PathToProperty, RelativeSource={RelativeSource AncestorType={x:Type typeOfAncestor}}}
{Binding Path=PathToProperty, RelativeSource={RelativeSource TemplatedParent}}
{Binding Path=Text, ElementName=MyTextBox}
After:
{BindTo PathToProperty}
{BindTo Ancestor.typeOfAncestor.PathToProperty}
{BindTo Template.PathToProperty}
{BindTo #MyTextBox.Text}
Here is an example of how method binding is simplified. Before:
// C# code
private ICommand _saveCommand;
public ICommand SaveCommand {
get {
if (_saveCommand == null) {
_saveCommand = new RelayCommand(x => this.SaveObject());
}
return _saveCommand;
}
}
private void SaveObject() {
// do something
}
// XAML
{Binding Path=SaveCommand}
After:
// C# code
private void SaveObject() {
// do something
}
// XAML
{BindTo SaveObject()}
You can find the library here: http://www.simplygoodcode.com/2012/08/simpler-wpf-binding.html
Note in the 'BEFORE' example that I use for method binding that code was already optimized by using RelayCommand which last I checked is not a native part of WPF. Without that the 'BEFORE' example would have been even longer.
This is an example of the use of this pattern that worked for me on empty datagrids.
<Style.Triggers>
<DataTrigger Binding="{Binding Items.Count, RelativeSource={RelativeSource Self}}" Value="0">
<Setter Property="Background">
<Setter.Value>
<VisualBrush Stretch="None">
<VisualBrush.Visual>
<TextBlock Text="We did't find any matching records for your search..." FontSize="16" FontWeight="SemiBold" Foreground="LightCoral"/>
</VisualBrush.Visual>
</VisualBrush>
</Setter.Value>
</Setter>
</DataTrigger>
</Style.Triggers>
Some useful bits and pieces:
Here's how to do it mostly in code:
Binding b = new Binding();
b.RelativeSource = new RelativeSource(RelativeSourceMode.FindAncestor, this.GetType(), 1);
b.Path = new PropertyPath("MyElementThatNeedsBinding");
MyLabel.SetBinding(ContentProperty, b);
I largely copied this from Binding Relative Source in code Behind.
Also, the MSDN page is pretty good as far as examples go: RelativeSource Class
I just posted another solution for accessing the DataContext of a parent element in Silverlight that works for me. It uses Binding ElementName.
Binding RelativeSource={
RelativeSource Mode=FindAncestor, AncestorType={x:Type ItemType}
}
...
The default attribute of RelativeSource is the Mode property. A complete set of valid values is given here (from MSDN):
PreviousData Allows you to bind the previous data item (not that control that contains the data item) in the list of data items being displayed.
TemplatedParent Refers to the element to which the template (in which the data-bound element exists) is applied. This is similar to setting a TemplateBindingExtension and is only applicable if the Binding is within a template.
Self Refers to the element on which you are setting the binding and allows you to bind one property of that element to another property on the same element.
FindAncestor Refers to the ancestor in the parent chain of the data-bound element. You can use this to bind to an ancestor of a specific type or its subclasses. This is the mode you use if you want to specify AncestorType and/or AncestorLevel.
If an element is not part of the visual tree, then RelativeSource will never work.
In this case, you need to try a different technique, pioneered by Thomas Levesque.
He has the solution on his blog under [WPF] How to bind to data when the DataContext is not inherited. And it works absolutely brilliantly!
In the unlikely event that his blog is down, Appendix A contains a mirror copy of his article.
Please do not comment here, please comment directly on his blog post.
Appendix A: Mirror of blog post
The DataContext property in WPF is extremely handy, because it is automatically inherited by all children of the element where you assign it; therefore you don’t need to set it again on each element you want to bind. However, in some cases the DataContext is not accessible: it happens for elements that are not part of the visual or logical tree. It can be very difficult then to bind a property on those elements…
Let’s illustrate with a simple example: we want to display a list of products in a DataGrid. In the grid, we want to be able to show or hide the Price column, based on the value of a ShowPrice property exposed by the ViewModel. The obvious approach is to bind the Visibility of the column to the ShowPrice property:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding ShowPrice,
Converter={StaticResource visibilityConverter}}"/>
Unfortunately, changing the value of ShowPrice has no effect, and the column is always visible… why? If we look at the Output window in Visual Studio, we notice the following line:
System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:Path=ShowPrice; DataItem=null; target element is ‘DataGridTextColumn’ (HashCode=32685253); target property is ‘Visibility’ (type ‘Visibility’)
The message is rather cryptic, but the meaning is actually quite simple: WPF doesn’t know which FrameworkElement to use to get the DataContext, because the column doesn’t belong to the visual or logical tree of the DataGrid.
We can try to tweak the binding to get the desired result, for instance by setting the RelativeSource to the DataGrid itself:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding DataContext.ShowPrice,
Converter={StaticResource visibilityConverter},
RelativeSource={RelativeSource FindAncestor, AncestorType=DataGrid}}"/>
Or we can add a CheckBox bound to ShowPrice, and try to bind the column visibility to the IsChecked property by specifying the element name:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding IsChecked,
Converter={StaticResource visibilityConverter},
ElementName=chkShowPrice}"/>
But none of these workarounds seems to work, we always get the same result…
At this point, it seems that the only viable approach would be to change the column visibility in code-behind, which we usually prefer to avoid when using the MVVM pattern… But I’m not going to give up so soon, at least not while there are other options to consider
The solution to our problem is actually quite simple, and takes advantage of the Freezable class. The primary purpose of this class is to define objects that have a modifiable and a read-only state, but the interesting feature in our case is that Freezable objects can inherit the DataContext even when they’re not in the visual or logical tree. I don’t know the exact mechanism that enables this behavior, but we’re going to take advantage of it to make our binding work…
The idea is to create a class (I called it BindingProxy for reasons that should become obvious very soon) that inherits Freezable and declares a Data dependency property:
public class BindingProxy : Freezable
{
#region Overrides of Freezable
protected override Freezable CreateInstanceCore()
{
return new BindingProxy();
}
#endregion
public object Data
{
get { return (object)GetValue(DataProperty); }
set { SetValue(DataProperty, value); }
}
// Using a DependencyProperty as the backing store for Data. This enables animation, styling, binding, etc...
public static readonly DependencyProperty DataProperty =
DependencyProperty.Register("Data", typeof(object), typeof(BindingProxy), new UIPropertyMetadata(null));
}
We can then declare an instance of this class in the resources of the DataGrid, and bind the Data property to the current DataContext:
<DataGrid.Resources>
<local:BindingProxy x:Key="proxy" Data="{Binding}" />
</DataGrid.Resources>
The last step is to specify this BindingProxy object (easily accessible with StaticResource) as the Source for the binding:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding Data.ShowPrice,
Converter={StaticResource visibilityConverter},
Source={StaticResource proxy}}"/>
Note that the binding path has been prefixed with “Data”, since the path is now relative to the BindingProxy object.
The binding now works correctly, and the column is properly shown or hidden based on the ShowPrice property.
Some useful bits and pieces:
Here's how to do it mostly in code:
Binding b = new Binding();
b.RelativeSource = new RelativeSource(RelativeSourceMode.FindAncestor, this.GetType(), 1);
b.Path = new PropertyPath("MyElementThatNeedsBinding");
MyLabel.SetBinding(ContentProperty, b);
I largely copied this from Binding Relative Source in code Behind.
Also, the MSDN page is pretty good as far as examples go: RelativeSource Class
Binding RelativeSource={
RelativeSource Mode=FindAncestor, AncestorType={x:Type ItemType}
}
...
The default attribute of RelativeSource is the Mode property. A complete set of valid values is given here (from MSDN):
PreviousData Allows you to bind the previous data item (not that control that contains the data item) in the list of data items being displayed.
TemplatedParent Refers to the element to which the template (in which the data-bound element exists) is applied. This is similar to setting a TemplateBindingExtension and is only applicable if the Binding is within a template.
Self Refers to the element on which you are setting the binding and allows you to bind one property of that element to another property on the same element.
FindAncestor Refers to the ancestor in the parent chain of the data-bound element. You can use this to bind to an ancestor of a specific type or its subclasses. This is the mode you use if you want to specify AncestorType and/or AncestorLevel.
Don't forget TemplatedParent:
<Binding RelativeSource="{RelativeSource TemplatedParent}"/>
or
{Binding RelativeSource={RelativeSource TemplatedParent}}
In WPF RelativeSource binding exposes three properties to set:
1. Mode: This is an enum that could have four values:
a. PreviousData(
value=0): It assigns the previous value of thepropertyto the bound oneb. TemplatedParent(
value=1): This is used when defining thetemplatesof any control and want to bind to a value/Property of thecontrol.For example, define
ControlTemplate:
<ControlTemplate>
<CheckBox IsChecked="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Value, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
</ControlTemplate>
c. Self(
value=2): When we want to bind from aselfor apropertyof self.For example: Send checked state of
checkboxasCommandParameterwhile setting theCommandonCheckBox
<CheckBox ...... CommandParameter="{Binding RelativeSource={RelativeSource Self},Path=IsChecked}" />
d. FindAncestor(
value=3): When want to bind from a parentcontrolinVisual Tree.For example: Bind a
checkboxinrecordsif agrid,ifheadercheckboxis checked
<CheckBox IsChecked="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type iDP:XamDataGrid}}, Path=DataContext.IsHeaderChecked, Mode=TwoWay}" />
2. AncestorType: when mode is FindAncestor then define what type of ancestor
RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type iDP:XamDataGrid}}
3. AncestorLevel: when mode is FindAncestor then what level of ancestor (if there are two same type of parent in visual tree)
RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type iDP:XamDataGrid, AncestorLevel=1}}
Above are all use-cases for
RelativeSource binding.
I created a library to simplify the binding syntax of WPF including making it easier to use RelativeSource. Here are some examples. Before:
{Binding Path=PathToProperty, RelativeSource={RelativeSource Self}}
{Binding Path=PathToProperty, RelativeSource={RelativeSource AncestorType={x:Type typeOfAncestor}}}
{Binding Path=PathToProperty, RelativeSource={RelativeSource TemplatedParent}}
{Binding Path=Text, ElementName=MyTextBox}
After:
{BindTo PathToProperty}
{BindTo Ancestor.typeOfAncestor.PathToProperty}
{BindTo Template.PathToProperty}
{BindTo #MyTextBox.Text}
Here is an example of how method binding is simplified. Before:
// C# code
private ICommand _saveCommand;
public ICommand SaveCommand {
get {
if (_saveCommand == null) {
_saveCommand = new RelayCommand(x => this.SaveObject());
}
return _saveCommand;
}
}
private void SaveObject() {
// do something
}
// XAML
{Binding Path=SaveCommand}
After:
// C# code
private void SaveObject() {
// do something
}
// XAML
{BindTo SaveObject()}
You can find the library here: http://www.simplygoodcode.com/2012/08/simpler-wpf-binding.html
Note in the 'BEFORE' example that I use for method binding that code was already optimized by using RelayCommand which last I checked is not a native part of WPF. Without that the 'BEFORE' example would have been even longer.
Don't forget TemplatedParent:
<Binding RelativeSource="{RelativeSource TemplatedParent}"/>
or
{Binding RelativeSource={RelativeSource TemplatedParent}}
I didn't read every answer, but I just want to add this information in case of relative source command binding of a button.
When you use a relative source with Mode=FindAncestor, the binding must be like:
Command="{Binding Path=DataContext.CommandProperty, RelativeSource={...}}"
If you don't add DataContext in your path, at execution time it can't retrieve the property.
If an element is not part of the visual tree, then RelativeSource will never work.
In this case, you need to try a different technique, pioneered by Thomas Levesque.
He has the solution on his blog under [WPF] How to bind to data when the DataContext is not inherited. And it works absolutely brilliantly!
In the unlikely event that his blog is down, Appendix A contains a mirror copy of his article.
Please do not comment here, please comment directly on his blog post.
Appendix A: Mirror of blog post
The DataContext property in WPF is extremely handy, because it is automatically inherited by all children of the element where you assign it; therefore you don’t need to set it again on each element you want to bind. However, in some cases the DataContext is not accessible: it happens for elements that are not part of the visual or logical tree. It can be very difficult then to bind a property on those elements…
Let’s illustrate with a simple example: we want to display a list of products in a DataGrid. In the grid, we want to be able to show or hide the Price column, based on the value of a ShowPrice property exposed by the ViewModel. The obvious approach is to bind the Visibility of the column to the ShowPrice property:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding ShowPrice,
Converter={StaticResource visibilityConverter}}"/>
Unfortunately, changing the value of ShowPrice has no effect, and the column is always visible… why? If we look at the Output window in Visual Studio, we notice the following line:
System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:Path=ShowPrice; DataItem=null; target element is ‘DataGridTextColumn’ (HashCode=32685253); target property is ‘Visibility’ (type ‘Visibility’)
The message is rather cryptic, but the meaning is actually quite simple: WPF doesn’t know which FrameworkElement to use to get the DataContext, because the column doesn’t belong to the visual or logical tree of the DataGrid.
We can try to tweak the binding to get the desired result, for instance by setting the RelativeSource to the DataGrid itself:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding DataContext.ShowPrice,
Converter={StaticResource visibilityConverter},
RelativeSource={RelativeSource FindAncestor, AncestorType=DataGrid}}"/>
Or we can add a CheckBox bound to ShowPrice, and try to bind the column visibility to the IsChecked property by specifying the element name:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding IsChecked,
Converter={StaticResource visibilityConverter},
ElementName=chkShowPrice}"/>
But none of these workarounds seems to work, we always get the same result…
At this point, it seems that the only viable approach would be to change the column visibility in code-behind, which we usually prefer to avoid when using the MVVM pattern… But I’m not going to give up so soon, at least not while there are other options to consider
The solution to our problem is actually quite simple, and takes advantage of the Freezable class. The primary purpose of this class is to define objects that have a modifiable and a read-only state, but the interesting feature in our case is that Freezable objects can inherit the DataContext even when they’re not in the visual or logical tree. I don’t know the exact mechanism that enables this behavior, but we’re going to take advantage of it to make our binding work…
The idea is to create a class (I called it BindingProxy for reasons that should become obvious very soon) that inherits Freezable and declares a Data dependency property:
public class BindingProxy : Freezable
{
#region Overrides of Freezable
protected override Freezable CreateInstanceCore()
{
return new BindingProxy();
}
#endregion
public object Data
{
get { return (object)GetValue(DataProperty); }
set { SetValue(DataProperty, value); }
}
// Using a DependencyProperty as the backing store for Data. This enables animation, styling, binding, etc...
public static readonly DependencyProperty DataProperty =
DependencyProperty.Register("Data", typeof(object), typeof(BindingProxy), new UIPropertyMetadata(null));
}
We can then declare an instance of this class in the resources of the DataGrid, and bind the Data property to the current DataContext:
<DataGrid.Resources>
<local:BindingProxy x:Key="proxy" Data="{Binding}" />
</DataGrid.Resources>
The last step is to specify this BindingProxy object (easily accessible with StaticResource) as the Source for the binding:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding Data.ShowPrice,
Converter={StaticResource visibilityConverter},
Source={StaticResource proxy}}"/>
Note that the binding path has been prefixed with “Data”, since the path is now relative to the BindingProxy object.
The binding now works correctly, and the column is properly shown or hidden based on the ShowPrice property.
It's worthy of note that for those stumbling across this thinking of Silverlight:
Silverlight offers a reduced subset only, of these commands
Bechir Bejaoui exposes the use cases of the RelativeSources in WPF in his article here:
The RelativeSource is a markup extension that is used in particular binding cases when we try to bind a property of a given object to another property of the object itself, when we try to bind a property of a object to another one of its relative parents, when binding a dependency property value to a piece of XAML in case of custom control development and finally in case of using a differential of a series of a bound data. All of those situations are expressed as relative source modes. I will expose all of those cases one by one.
- Mode Self:
Imagine this case, a rectangle that we want that its height is always equal to its width, a square let's say. We can do this using the element name
<Rectangle Fill="Red" Name="rectangle" Height="100" Stroke="Black" Canvas.Top="100" Canvas.Left="100" Width="{Binding ElementName=rectangle, Path=Height}"/>But in this above case we are obliged to indicate the name of the binding object, namely the rectangle. We can reach the same purpose differently using the RelativeSource
<Rectangle Fill="Red" Height="100" Stroke="Black" Width="{Binding RelativeSource={RelativeSource Self}, Path=Height}"/>For that case we are not obliged to mention the name of the binding object and the Width will be always equal to the Height whenever the height is changed.
If you want to parameter the Width to be the half of the height then you can do this by adding a converter to the Binding markup extension. Let's imagine another case now:
<TextBlock Width="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualWidth}"/>The above case is used to tie a given property of a given element to one of its direct parent ones as this element holds a property that is called Parent. This leads us to another relative source mode which is the FindAncestor one.
- Mode FindAncestor
In this case, a property of a given element will be tied to one of its parents, Of Corse. The main difference with the above case is the fact that, it's up to you to determine the ancestor type and the ancestor rank in the hierarchy to tie the property. By the way try to play with this piece of XAML
<Canvas Name="Parent0"> <Border Name="Parent1" Width="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualWidth}" Height="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualHeight}"> <Canvas Name="Parent2"> <Border Name="Parent3" Width="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualWidth}" Height="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualHeight}"> <Canvas Name="Parent4"> <TextBlock FontSize="16" Margin="5" Text="Display the name of the ancestor"/> <TextBlock FontSize="16" Margin="50" Text="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Border}, AncestorLevel=2},Path=Name}" Width="200"/> </Canvas> </Border> </Canvas> </Border> </Canvas>The above situation is of two TextBlock elements those are embedded within a series of borders and canvas elements those represent their hierarchical parents. The second TextBlock will display the name of the given parent at the relative source level.
So try to change AncestorLevel=2 to AncestorLevel=1 and see what happens. Then try to change the type of the ancestor from AncestorType=Border to AncestorType=Canvas and see what's happens.
The displayed text will change according to the Ancestor type and level. Then what's happen if the ancestor level is not suitable to the ancestor type? This is a good question, I know that you're about to ask it. The response is no exceptions will be thrown and nothings will be displayed at the TextBlock level.
- TemplatedParent
This mode enables tie a given ControlTemplate property to a property of the control that the ControlTemplate is applied to. To well understand the issue here is an example bellow
<Window.Resources> <ControlTemplate x:Key="template"> <Canvas> <Canvas.RenderTransform> <RotateTransform Angle="20"/> </Canvas.RenderTransform> <Ellipse Height="100" Width="150" Fill="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Background}"> </Ellipse> <ContentPresenter Margin="35" Content="{Binding RelativeSource={RelativeSource TemplatedParent},Path=Content}"/> </Canvas> </ControlTemplate> </Window.Resources> <Canvas Name="Parent0"> <Button Margin="50" Template="{StaticResource template}" Height="0" Canvas.Left="0" Canvas.Top="0" Width="0"> <TextBlock FontSize="22">Click me</TextBlock> </Button> </Canvas>If I want to apply the properties of a given control to its control template then I can use the TemplatedParent mode. There is also a similar one to this markup extension which is the TemplateBinding which is a kind of short hand of the first one, but the TemplateBinding is evaluated at compile time at the contrast of the TemplatedParent which is evaluated just after the first run time. As you can remark in the bellow figure, the background and the content are applied from within the button to the control template.
I didn't read every answer, but I just want to add this information in case of relative source command binding of a button.
When you use a relative source with Mode=FindAncestor, the binding must be like:
Command="{Binding Path=DataContext.CommandProperty, RelativeSource={...}}"
If you don't add DataContext in your path, at execution time it can't retrieve the property.
I just posted another solution for accessing the DataContext of a parent element in Silverlight that works for me. It uses Binding ElementName.
Some useful bits and pieces:
Here's how to do it mostly in code:
Binding b = new Binding();
b.RelativeSource = new RelativeSource(RelativeSourceMode.FindAncestor, this.GetType(), 1);
b.Path = new PropertyPath("MyElementThatNeedsBinding");
MyLabel.SetBinding(ContentProperty, b);
I largely copied this from Binding Relative Source in code Behind.
Also, the MSDN page is pretty good as far as examples go: RelativeSource Class
I didn't read every answer, but I just want to add this information in case of relative source command binding of a button.
When you use a relative source with Mode=FindAncestor, the binding must be like:
Command="{Binding Path=DataContext.CommandProperty, RelativeSource={...}}"
If you don't add DataContext in your path, at execution time it can't retrieve the property.
I created a library to simplify the binding syntax of WPF including making it easier to use RelativeSource. Here are some examples. Before:
{Binding Path=PathToProperty, RelativeSource={RelativeSource Self}}
{Binding Path=PathToProperty, RelativeSource={RelativeSource AncestorType={x:Type typeOfAncestor}}}
{Binding Path=PathToProperty, RelativeSource={RelativeSource TemplatedParent}}
{Binding Path=Text, ElementName=MyTextBox}
After:
{BindTo PathToProperty}
{BindTo Ancestor.typeOfAncestor.PathToProperty}
{BindTo Template.PathToProperty}
{BindTo #MyTextBox.Text}
Here is an example of how method binding is simplified. Before:
// C# code
private ICommand _saveCommand;
public ICommand SaveCommand {
get {
if (_saveCommand == null) {
_saveCommand = new RelayCommand(x => this.SaveObject());
}
return _saveCommand;
}
}
private void SaveObject() {
// do something
}
// XAML
{Binding Path=SaveCommand}
After:
// C# code
private void SaveObject() {
// do something
}
// XAML
{BindTo SaveObject()}
You can find the library here: http://www.simplygoodcode.com/2012/08/simpler-wpf-binding.html
Note in the 'BEFORE' example that I use for method binding that code was already optimized by using RelayCommand which last I checked is not a native part of WPF. Without that the 'BEFORE' example would have been even longer.
I just posted another solution for accessing the DataContext of a parent element in Silverlight that works for me. It uses Binding ElementName.
If an element is not part of the visual tree, then RelativeSource will never work.
In this case, you need to try a different technique, pioneered by Thomas Levesque.
He has the solution on his blog under [WPF] How to bind to data when the DataContext is not inherited. And it works absolutely brilliantly!
In the unlikely event that his blog is down, Appendix A contains a mirror copy of his article.
Please do not comment here, please comment directly on his blog post.
Appendix A: Mirror of blog post
The DataContext property in WPF is extremely handy, because it is automatically inherited by all children of the element where you assign it; therefore you don’t need to set it again on each element you want to bind. However, in some cases the DataContext is not accessible: it happens for elements that are not part of the visual or logical tree. It can be very difficult then to bind a property on those elements…
Let’s illustrate with a simple example: we want to display a list of products in a DataGrid. In the grid, we want to be able to show or hide the Price column, based on the value of a ShowPrice property exposed by the ViewModel. The obvious approach is to bind the Visibility of the column to the ShowPrice property:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding ShowPrice,
Converter={StaticResource visibilityConverter}}"/>
Unfortunately, changing the value of ShowPrice has no effect, and the column is always visible… why? If we look at the Output window in Visual Studio, we notice the following line:
System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:Path=ShowPrice; DataItem=null; target element is ‘DataGridTextColumn’ (HashCode=32685253); target property is ‘Visibility’ (type ‘Visibility’)
The message is rather cryptic, but the meaning is actually quite simple: WPF doesn’t know which FrameworkElement to use to get the DataContext, because the column doesn’t belong to the visual or logical tree of the DataGrid.
We can try to tweak the binding to get the desired result, for instance by setting the RelativeSource to the DataGrid itself:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding DataContext.ShowPrice,
Converter={StaticResource visibilityConverter},
RelativeSource={RelativeSource FindAncestor, AncestorType=DataGrid}}"/>
Or we can add a CheckBox bound to ShowPrice, and try to bind the column visibility to the IsChecked property by specifying the element name:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding IsChecked,
Converter={StaticResource visibilityConverter},
ElementName=chkShowPrice}"/>
But none of these workarounds seems to work, we always get the same result…
At this point, it seems that the only viable approach would be to change the column visibility in code-behind, which we usually prefer to avoid when using the MVVM pattern… But I’m not going to give up so soon, at least not while there are other options to consider
The solution to our problem is actually quite simple, and takes advantage of the Freezable class. The primary purpose of this class is to define objects that have a modifiable and a read-only state, but the interesting feature in our case is that Freezable objects can inherit the DataContext even when they’re not in the visual or logical tree. I don’t know the exact mechanism that enables this behavior, but we’re going to take advantage of it to make our binding work…
The idea is to create a class (I called it BindingProxy for reasons that should become obvious very soon) that inherits Freezable and declares a Data dependency property:
public class BindingProxy : Freezable
{
#region Overrides of Freezable
protected override Freezable CreateInstanceCore()
{
return new BindingProxy();
}
#endregion
public object Data
{
get { return (object)GetValue(DataProperty); }
set { SetValue(DataProperty, value); }
}
// Using a DependencyProperty as the backing store for Data. This enables animation, styling, binding, etc...
public static readonly DependencyProperty DataProperty =
DependencyProperty.Register("Data", typeof(object), typeof(BindingProxy), new UIPropertyMetadata(null));
}
We can then declare an instance of this class in the resources of the DataGrid, and bind the Data property to the current DataContext:
<DataGrid.Resources>
<local:BindingProxy x:Key="proxy" Data="{Binding}" />
</DataGrid.Resources>
The last step is to specify this BindingProxy object (easily accessible with StaticResource) as the Source for the binding:
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False"
Visibility="{Binding Data.ShowPrice,
Converter={StaticResource visibilityConverter},
Source={StaticResource proxy}}"/>
Note that the binding path has been prefixed with “Data”, since the path is now relative to the BindingProxy object.
The binding now works correctly, and the column is properly shown or hidden based on the ShowPrice property.
Here's a more visual explanation in the context of a MVVM architecture:
It's worthy of note that for those stumbling across this thinking of Silverlight:
Silverlight offers a reduced subset only, of these commands
In WPF RelativeSource binding exposes three properties to set:
1. Mode: This is an enum that could have four values:
a. PreviousData(
value=0): It assigns the previous value of thepropertyto the bound oneb. TemplatedParent(
value=1): This is used when defining thetemplatesof any control and want to bind to a value/Property of thecontrol.For example, define
ControlTemplate:
<ControlTemplate>
<CheckBox IsChecked="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Value, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
</ControlTemplate>
c. Self(
value=2): When we want to bind from aselfor apropertyof self.For example: Send checked state of
checkboxasCommandParameterwhile setting theCommandonCheckBox
<CheckBox ...... CommandParameter="{Binding RelativeSource={RelativeSource Self},Path=IsChecked}" />
d. FindAncestor(
value=3): When want to bind from a parentcontrolinVisual Tree.For example: Bind a
checkboxinrecordsif agrid,ifheadercheckboxis checked
<CheckBox IsChecked="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type iDP:XamDataGrid}}, Path=DataContext.IsHeaderChecked, Mode=TwoWay}" />
2. AncestorType: when mode is FindAncestor then define what type of ancestor
RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type iDP:XamDataGrid}}
3. AncestorLevel: when mode is FindAncestor then what level of ancestor (if there are two same type of parent in visual tree)
RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type iDP:XamDataGrid, AncestorLevel=1}}
Above are all use-cases for
RelativeSource binding.
Bechir Bejaoui exposes the use cases of the RelativeSources in WPF in his article here:
The RelativeSource is a markup extension that is used in particular binding cases when we try to bind a property of a given object to another property of the object itself, when we try to bind a property of a object to another one of its relative parents, when binding a dependency property value to a piece of XAML in case of custom control development and finally in case of using a differential of a series of a bound data. All of those situations are expressed as relative source modes. I will expose all of those cases one by one.
- Mode Self:
Imagine this case, a rectangle that we want that its height is always equal to its width, a square let's say. We can do this using the element name
<Rectangle Fill="Red" Name="rectangle" Height="100" Stroke="Black" Canvas.Top="100" Canvas.Left="100" Width="{Binding ElementName=rectangle, Path=Height}"/>But in this above case we are obliged to indicate the name of the binding object, namely the rectangle. We can reach the same purpose differently using the RelativeSource
<Rectangle Fill="Red" Height="100" Stroke="Black" Width="{Binding RelativeSource={RelativeSource Self}, Path=Height}"/>For that case we are not obliged to mention the name of the binding object and the Width will be always equal to the Height whenever the height is changed.
If you want to parameter the Width to be the half of the height then you can do this by adding a converter to the Binding markup extension. Let's imagine another case now:
<TextBlock Width="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualWidth}"/>The above case is used to tie a given property of a given element to one of its direct parent ones as this element holds a property that is called Parent. This leads us to another relative source mode which is the FindAncestor one.
- Mode FindAncestor
In this case, a property of a given element will be tied to one of its parents, Of Corse. The main difference with the above case is the fact that, it's up to you to determine the ancestor type and the ancestor rank in the hierarchy to tie the property. By the way try to play with this piece of XAML
<Canvas Name="Parent0"> <Border Name="Parent1" Width="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualWidth}" Height="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualHeight}"> <Canvas Name="Parent2"> <Border Name="Parent3" Width="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualWidth}" Height="{Binding RelativeSource={RelativeSource Self}, Path=Parent.ActualHeight}"> <Canvas Name="Parent4"> <TextBlock FontSize="16" Margin="5" Text="Display the name of the ancestor"/> <TextBlock FontSize="16" Margin="50" Text="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Border}, AncestorLevel=2},Path=Name}" Width="200"/> </Canvas> </Border> </Canvas> </Border> </Canvas>The above situation is of two TextBlock elements those are embedded within a series of borders and canvas elements those represent their hierarchical parents. The second TextBlock will display the name of the given parent at the relative source level.
So try to change AncestorLevel=2 to AncestorLevel=1 and see what happens. Then try to change the type of the ancestor from AncestorType=Border to AncestorType=Canvas and see what's happens.
The displayed text will change according to the Ancestor type and level. Then what's happen if the ancestor level is not suitable to the ancestor type? This is a good question, I know that you're about to ask it. The response is no exceptions will be thrown and nothings will be displayed at the TextBlock level.
- TemplatedParent
This mode enables tie a given ControlTemplate property to a property of the control that the ControlTemplate is applied to. To well understand the issue here is an example bellow
<Window.Resources> <ControlTemplate x:Key="template"> <Canvas> <Canvas.RenderTransform> <RotateTransform Angle="20"/> </Canvas.RenderTransform> <Ellipse Height="100" Width="150" Fill="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Background}"> </Ellipse> <ContentPresenter Margin="35" Content="{Binding RelativeSource={RelativeSource TemplatedParent},Path=Content}"/> </Canvas> </ControlTemplate> </Window.Resources> <Canvas Name="Parent0"> <Button Margin="50" Template="{StaticResource template}" Height="0" Canvas.Left="0" Canvas.Top="0" Width="0"> <TextBlock FontSize="22">Click me</TextBlock> </Button> </Canvas>If I want to apply the properties of a given control to its control template then I can use the TemplatedParent mode. There is also a similar one to this markup extension which is the TemplateBinding which is a kind of short hand of the first one, but the TemplateBinding is evaluated at compile time at the contrast of the TemplatedParent which is evaluated just after the first run time. As you can remark in the bellow figure, the background and the content are applied from within the button to the control template.
I didn't read every answer, but I just want to add this information in case of relative source command binding of a button.
When you use a relative source with Mode=FindAncestor, the binding must be like:
Command="{Binding Path=DataContext.CommandProperty, RelativeSource={...}}"
If you don't add DataContext in your path, at execution time it can't retrieve the property.
Source: Stackoverflow.com