UserControl's DataContext

Question

I'm creating a UserControl I want to use something like this:

<controls:ColorWithText Color="Red" Text="Red color" />

So far, I've implemented similar controls like this:

<UserControl x:Class="Namespace.ColorWithText" Name="ThisControl">
    <StackPanel Orientation="Horizontal" >
        <Border Width="15" Height="15" Background="{Binding Color, ElementName=ThisControl}" />
        <TextBlock Text="{Binding Text, ElementName=ThisControl}" />
    </StackPanel>
</UserControl>

where Color and Text are dependency properties of the control defined in code. This works, but specifying ElementName every time seems unnecessary.

Another option that works is using

<UserControl x:Class=… DataContext="{Binding ElementName=ThisControl}" Name="ThisControl">

and not specifying ElementNames, but that doesn't seem like a clean solution to me either.

I have two questions:

1. Why doesn't <UserControl DataContext="{RelativeSource Self}"> work?
2. What is the best way to do something like this?

Involution

Answer 1

For first one, try :

<UserControl DataContext="{Binding RelativeSource={RelativeSource Self}}">

And for second question, I think using ElementName or AncestorBinding is best way to bind to UserControl's properties.

Cornelison

Question. If you set RelativeSource like this, how does it know what is the VM of this control? How do you set it up? – Buke

I know this is an old post but for anyone else coming her...You don't set up a VM for an individual control. You set the properties on your control and those properties should be enough to make it "work". If the control is depending on some VM or is tightly coupled / depends on being placed into a specific context to work then it isn't a "control". You've violated the separation of concerns principle. – Abisia

As far as I can tell, this breaks any UserControl with a DependencyProperty. See nikolalukovic.com/programming/… and/or the answer by @jdawiz. – Schiff

A UserControl should never explicitly set its own DataContext. Doing so will break the standard binding mechanism that uses a view model instance in the inherited DataContext as the source object of Bindings of the controls properties. This has already cause a huge amount of trouble for people that are new to WPF. – Arroyo

Answer 2

Why can't you use <UserControl DataContext="{RelativeSource Self}">?

This is how you would use the control

<Grid DataContext="{StaticResource ViewModel}">
    <!-- Here we'd expect this control to be bound to -->
    <!-- ColorToUse on our ViewModel resource          -->
    <controls:ColorWithText Color="{Binding ColorToUse}" />
</Grid>

Now because we've hardcoded our data-context in the control it will instead attempt to lookup ColorToUse property on the ColorWithText object not your ViewModel, which will obviously fail.

This is why you can't set the DataContext on the user control. Thanks to Brandur for making me understand that.

What is the best way to do something like this?

Instead you should set the DataContext in the first child UI element in your control.

In your case you want

<StackPanel 
  DataContext="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}}"
  Orientation="Horizontal" >

Now you have a DataContext which refers to your control so you can access any properties of that control using relative bindings.

Doubletongued

This is definitely the best solution! It preserves the control bindings and doesn't require any specific element naming. – Nikaniki

Answer 3

I know this has been answered but none of the explanations give an Understanding of DataContext and how it works. This link does a great job for that.

EVERYTHING YOU WANTED TO KNOW ABOUT DATABINDING IN WPF, SILVERLIGHT AND WP7 (PART TWO)

In answer to your question #1

Why doesn't <UserControl DataContext="{RelativeSource Self}"> work?

This is a summary of the above link. DataContext should not be set to Self at UserControl Element level. This is because it breaks the Inheritance of the DataContext. If you do set it to self and you place this control on a Window or another control, it will not inherit the Windows DataContext.

DataContext is inherited to all lower Elements of the XAML and to all the XAML of UserControls unless it is overwritten somewhere. By setting the UserControl DataContext to itself, this overwrites the DataContext and breaks Inheritance. Instead, nest it one Element deep in the XAML, in your case, the StackPanel. Put the DataContext binding here and bind it to the UserControl. This preserves the Inheritance.

See also this link below for a detailed explanation of this.

A SIMPLE PATTERN FOR CREATING RE-USEABLE USERCONTROLS IN WPF / SILVERLIGHT

In answer to your question #2
What is the best way to do something like this?

See code example below.

<UserControl x:Class="Namespace.ColorWithText" Name="ThisControl">
    <StackPanel Orientation="Horizontal" DataContext="{Binding ElementName=ThisControl}">
        <Border Width="15" Height="15" Background="{Binding Color" />
        <TextBlock Text="{Binding Text}" />
    </StackPanel>
</UserControl>

Note that once you do this, you will not need the ElementName on each binding.

Ahlers

The only elegant solution that preserves UserControl external bindings. Thanks. – Milkweed

Good explanation! – Anabantid

This was by far the most helpful answer here since it does not break the datacontext Inheritance – Succursal

Answer 4

You should be using

{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}, Path=Color}

for Databinding Related doubts always refer this sheet.
http://www.nbdtech.com/Blog/archive/2009/02/02/wpf-xaml-data-binding-cheat-sheet.aspx

Futurism

I should write this every time? I don't want to bind to anything else in this control and I think repeating code is bad. – Involution

Answer 5

You can set the datacontext to self at the constructor itself.

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

Now you can simply say

<UserControl x:Class="Namespace.ColorWithText" Name="ThisControl">
    <StackPanel Orientation="Horizontal" >
        <Border Width="15" Height="15" Background="{Binding Color}" />
        <TextBlock Text="{Binding Text}" />
    </StackPanel>
</UserControl>

Subjective

or even in the loaded event this.Loaded += (sender, e) => { this.DataContext = this; }; – Bouncy

That is very simple and elegant. I like it. – Belabor

Answer 6

For the desperate souls, who are trying to make pdross's answer work and can't:

It's missing an essential detail - Path=DataContext. The lower code segment starts working when you add it there with this being the result:

    <StackPanel DataContext="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}, Path=DataContext}">

Octarchy