I have a basic UserControl
that sets its DataContext
to itself for ease of binding:
<UserControl x:Class="MyControlLib.ChildControl"
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"
DataContext="{Binding RelativeSource={RelativeSource Self}}">
</UserControl>
This is used in a parent XAML file like this:
<UserControl x:Class="MyControlLib.ParentControl"
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:ctrl="clr-namespace:MyControlLib">
<ctrl:ChildControl x:Name="ChildName"
PropertyOnChild="{Binding PropertyInParentContext}"/>
</UserControl>
For some reason, this gives a binding error that seems to indicate that the DataContext
of the parent control is getting affected by the child control setting its own DataContext
.
System.Windows.Data Error: 40 : BindingExpression path error: 'PropertyInParentContext' property not found on 'object' ''ChildControl' (Name='ChildName')'. BindingExpression:Path=PropertyInParentContext; DataItem='ChildControl' (Name='ChildName'); target element is 'ChildControl' (Name='ChildName'); target property is 'PropertyOnChild' (type 'whatever')
Why is "PropertyInParentContext" being looking for in the child control rather than in the parent's DataContext
?
If I remove the
DataContext="{Binding RelativeSource={RelativeSource Self}}
from the child control, then things operate how I would expect.
Am I missing something obvious here?
The declaration of your control and the instantiation are basically manipulating the same object, all the properties that are set in the declaration are also set on every instance. So if the properties were "visible" so to speak:
<UserControl x:Class="MyControlLib.ParentControl"
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:ctrl="clr-namespace:MyControlLib">
<ctrl:ChildControl x:Name="ChildName"
DataContext="{Binding RelativeSource={RelativeSource Self}}"
PropertyOnChild="{Binding PropertyInParentContext}"/>
</UserControl>
This is why you do not set the DataContext
of UserControls
, it will override the inherited DataContext
(and even obfuscate the fact that there is a different context). If you want to bind to properties of the UserControl
in its declaration then name the control and use ElementName
or RelativeSource
-bindings instead.
Self
means the UserControl
, so when you set the DataContext
to Self
, you are setting the DataContext
to the UserControl
object.
The correct syntax for binding to a Control's DataContext
would be {Binding RelativeSource={RelativeSource Self}, Path=DataContext}
, however since the DataContext is inherited by the Parent, this binding is totally unncessary in any situation.
Also, if you bind your DataContext
to Self.DataContext
, you would essentially be creating a loop where a value is bound to itself.
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