Develop Reference

Cannot bind ContentPresenter child element to outer control by name

I have a user control with ContentObject property and I use it to specify custom content. On my main window I created a nested label bound to a dependency property. It works fine if I bind it with RelativeSource, but for some reason it doesn't work if I reference element by name:
MainWindow.xaml:
<Window x:Class="TestContentPresenter.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:TestContentPresenter"
Title="MainWindow" Height="350" Width="525" Name="MyWindow">
<Grid>
<local:MyUserControl>
<local:MyUserControl.ContentObject>
<!--<TextBlock Text="{Binding MyText, ElementName=MyWindow}"/>-->
<TextBlock Text="{Binding MyText, RelativeSource={RelativeSource AncestorType=Window}}"/>
</local:MyUserControl.ContentObject>
</local:MyUserControl>
</Grid>
</Window>
UserControl.xaml:
<UserControl x:Class="TestContentPresenter.MyUserControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:TestContentPresenter"
Height="300" Width="300" Name="MainControl">
<StackPanel>
<ContentPresenter Content="{Binding ContentObject, ElementName=MainControl}"/>
</StackPanel>
</UserControl>
Main window has MyText dependency property and the commented line is what doesn't work. I suspect that this has something to do with name scopes, but is there anything I'm doing wrong?

No, you are not really doing anything wrong, it's just that the by the time the binding to the TextBlock's Text property gets resolved there is no element named "MyWindow" in scope. You set the ContentObject property of the UserControl to a TextBlock but when the UserControl eventually renders it doesn't know anything about any "MyWindow" name.
You could solve this by simply replacing the ElementName with a RelativeSource - as you have already discovered - since the TextBlock will always have a parent window. ElementName won't work.

WPF Binding inner control with parent data context

I made a user control
<UserControl x:Class="MyApp.MyControl"
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"
mc:Ignorable="d" x:Name="uc">
<Grid Width="Auto" Height="Auto">
<TextBlock Text="{Binding Path=DataContext.TextContent, ElementName=uc}"/>
<TextBlock Text="{Binding Path=DataContext.TextContent2, ElementName=uc}"/>
</Grid>
I want the sub-controls in the defined control(uc) will bind to the properties of uc.DataContext. I used the defined control as follows:
<Window x:Class="Tms.TMSClient.Views.MainWindow" Name="window"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:control="clr-namespace:MyApp"
xmlns:ribbon="clr-namespace:Microsoft.Windows.Controls.Ribbon;assembly=RibbonControlsLibrary">
<control:MyControl DataContext="{Binding Path=MyControlVM}"/>
The DataContext which is assigned to the window has this structure: WindowVM.MyControlVM.TextContent.
The given code does not work because the textbox's DataContext is bound to WindowVM instead. I think the problem may be because the inner textbox is bound before the defined control (uc) is, thus the bounded DataContext for uc does not take effect yet.
What I want is: the custom control (MyControl) will be bound to its corresponding viewmodel (MyControlVM), and the inner elements of MyControl will be bound to the properties of MyControlVM.
Do you have any solutions for this problem?

If I understand you correctly, you want to data bind a property from your MyControl view model to a TextBox.Text property inside the MyControl UserControl. If that is correct, then you can use a RelativeSource Binding, or the ElementName syntax that you are already using.
First, make sure that your view model is set as the DataContext for the UserControl:
public MyControl()
{
DataContext = new YourControlViewModel();
}
As child controls automatically inherit their parent's DataContext objects, you can now reference this view model from the TextBox through the MyControl.DataContext property from the UserControl's XAML:
<TextBlock Text="{Binding DataContext.TextContent,
RelativeSource={RelativeSource AncestorType={x:Type Window}}}" />
That's all you need.

<TextBlock Text="{Binding Path=TextContent}"/>
works for me in my test-application.
MainWindow.xaml
<Window x:Class="DataContextTest.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:my="clr-namespace:DataContextTest"
Title="MainWindow" Height="350" Width="525">
<Window.DataContext>
<my:MyOuterDataContext />
</Window.DataContext>
<Grid>
<my:MyControl DataContext="{Binding Path=MyInnerDataContext}" />
</Grid>
MyControl.xaml
<UserControl x:Class="DataContextTest.MyControl"
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"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300">
<Grid>
<TextBlock Text="{Binding Path=TextContent}" />
</Grid>
DataContexts:
public class MyOuterDataContext
{
public MyInnerDataContext MyInnerDataContext { get; set; }
public MyOuterDataContext()
{
MyInnerDataContext = new MyInnerDataContext();
}
}
public class MyInnerDataContext
{
public string TextContent { get { return "foo"; } }
}

By default every control inherits its DataContext from its parent control. Thus there is no need to explicitly bind to it.
Indeed, when you want to bind a control's DataContext to a nested property then you have to specifiy this:
<control:MyControl DataContext="{Binding Path=TextContent}"/>

Showing a control based on type of databound viewmodel?

I'm learning WPF at the moment. I'm finding xaml quite tough to use. I have MainWindow.xaml defined like this:
<Window x:Class="Compliance.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<ResourceDictionary Source="MainWindow.Resources.xaml"></ResourceDictionary>
</Window.Resources>
<Grid>
</Grid>
</Window>
And MainWindow.Resources.xaml like this:
<ResourceDictionary
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:vm="clr-namespace:Compliance.ViewModel"
xmlns:vw="clr-namespace:Compliance.View">
<DataTemplate DataType="{x:Type vm:Entities.AbstractEntityViewModel}">
<vw:AbstractEntityView></vw:AbstractEntityView>
</DataTemplate>
</ResourceDictionary>
AbstractEntityView is like this:
<UserControl x:Class="Compliance.View.AbstractEntityView"
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"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300">
<StackPanel>
<Label Content="ID:"></Label>
<TextBlock Text="{Binding Path=EntityId}"></TextBlock>
</StackPanel>
</UserControl>
Then in App.xaml.cs I am overriding OnStartup like this:
MainWindow window = new MainWindow();
//Model class
Individual ind = new Individual(1,"Name");
//subclass of AbstractEntityViewModel
var vm = new Entities.IndividualEntityViewModel(ind);
window.DataContext = vm;
window.Show();
However, nothing appears in the window.
I used the answer from this question to get my control to render. However, this requires you to refer to elements in the view from the code, which I don't want to do.
Is it possible to get a window to pick a View to render based on the ViewModel set as its datacontext? Or do I have the wrong idea about how MVVM is supposed to work?

You have the right idea, but you're not actually telling WPF to display your ViewModel anywhere
I usually host a ViewModel in a ContentControl object if I am binding to a single ViewModel
<Window x:Class="Compliance.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<ResourceDictionary Source="MainWindow.Resources.xaml"></ResourceDictionary>
</Window.Resources>
<Grid>
<ContentControl Content="{Binding }" />
</Grid>
</Window>
The ContentControl is usually not needed for lists of Models or ViewModels, since the object is automatically inserted as the Content property of the ContentPresenter of each item. For example, no ContentControl is needed when binding a ListBox to a collection of ViewModels
<ListBox ItemsSource="{Binding MyCollectionOfViewModel}" />

TwoWay Binding with UserControl

I am trying to set twoway binding on a UserControl that I have created.
When I use the control in Xaml is set the DataContext like so...
<uc:MyUserControl DataContext="{Binding Path=MyObject, Mode=TwoWay}" />
My user control is defined as the following....
<UserControl x:Class="SilverlightApplication1.XText"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="400">
<Grid x:Name="LayoutRoot" Background="White">
<TextBox x:Name="Text" Text="{Binding}"/>
</Grid>
</UserControl>
The data is displayed correctly, however if I make I change I wanted it to update with TwoWay binding.
I have tried this below, but it errors at runtime since no Path is defined.
<Grid x:Name="LayoutRoot" Background="White">
<TextBox x:Name="Text" Text="{Binding Mode=TwoWay}"/>
</Grid>
</UserControl>
Any ideas on how to get the control inside the usercontrol to twoway bind to the DataContext?

While your above (self-answered) answer seems to fix the problem, I can't help but think this is a problem domain issue. I have a hard time thinking why you'd want to bind directly like that in the first place, especially since it gives you less control over what happens with the data.
Take the following:
<UserControl
x:Class="SilverlightApplication1.XText"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
x:Name="UserControl"
d:DesignHeight="300"
d:DesignWidth="400">
<Grid x:Name="LayoutRoot" Background="White">
<TextBox x:Name="Text" Text="{Binding Path=Value, ElementName=UserControl, Mode=TwoWay}"/>
</Grid>
</UserControl>
Then in the codebehind:
public partial class XText
{
public static DependencyProperty ValueProperty =
DependencyProperty.Register(
"Value",
typeof(string),
typeof(XText),
new FrameworkPropertyMetadata(null)
);
public string Value
{
get { return ((string)(base.GetValue(XText.ValueProperty))); }
set { base.SetValue(XText.ValueProperty, value); }
}
...
}
Then, when you're ready to use it:
<uc:XText Value="{Binding Path=MyObject, Mode=TwoWay}" />
Yes, it's more code, but it gives you much more control over what happens with Value inside of your UserControl, and makes working with this code much much simpler in the future.
Thoughts?
-Doug
EDIT: fixed a couple typos.

I have found a solution that doesn't require you to give a name to the base control.
When I defined a name for the base UserControl it was creating issues for me when I was adding multiple instances to my Grid, since they were defined as the same name.
This is a combination of my first answer and Doug's answer.
Note the UserControl lacks the name property and the TextBox has no Binding declared in the XAML
XAML
<UserControl
x:Class="SilverlightApplication1.XText"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
d:DesignHeight="300"
d:DesignWidth="400">
<Grid x:Name="LayoutRoot" Background="White">
<TextBox x:Name="MyText"/>
</Grid>
</UserControl>
CodeBehind
public partial class XText
{
public XText()
{
InitializeComponent();
MyText.SetBinding(TextBox.TextProperty, new Binding()
{
Source = this,
Path = new PropertyPath("Value"),
Mode = BindingMode.TwoWay
});
}
public static DependencyProperty ValueProperty =
DependencyProperty.Register(
"Value",
typeof(string),
typeof(XText),
new PropertyMetadata(null)
);
public string Value
{
get { return ((string)(GetValue(ValueProperty))); }
set { SetValue(ValueProperty, value); }
}
...
}
When you are ready to use it do the following
<uc:XText Value="{Binding Path=MyObject, Mode=TwoWay}" />

Ok I think I have come up with a way to get this to work....
First I set a public property in my UserControl's code behind...
public Binding BindingValue
{
set { this.MyTextBox.SetBinding(TextBox.TextProperty, value); }
}
Then in XAML
<uc:MyUserControl BindingValue="{Binding Path=MyObject, Mode=TwoWay}" />

Escape from DataContext

I have a window that get its data from another class that is passed as DataContext. But I now also want to do data binding within the window. The window looks as follows:
<Window x:Class="WpfApplication1.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window1"
Height="300"
Width="300">
<StackPanel>
<TextBlock Text="{Binding UserName}" />
<TextBlock x:Name="TestTextBlock"
Text="Hello World" />
<TextBlock x:Name="TestTextBlock2"
Text="{Binding ElementName=TestTextBlock,Path=Text}" />
</StackPanel>
</Window>
The binding between the text blocks TestTextBlock and TestTextBlock2 works fine, but only until I change the DataContext-property of the window. How can I bind between those two textblocks so that changing the DataContext will not break the data binding?
Thanks in advance,
Stefan

Try setting the Binding.Mode to OneTime explicitly.
That way, TestTextBlock2.Text will only be set once (if that's what you want).