Felix K. Felix K. - 3 months ago 31
C# Question

Adding children to UserControl

My task



Create a
UserControl
which should be able to contain any visual child which is available in WPF, the children are displayed in a container which is a child of the
UserControl
.

My Problem



I can't manage to get the children displayed correctly in my container, i tried serval ways and did not find a way which works in the designer. I also tried to use
ContentControl
but nothing gets displayed.

My approaches



First i found this link and i tried it with some variations. I managed to display the content in the right container but it does not work in the designer because the content-property is set-private and the designer want to override it. Placing everything in XAML works but this is not good when working with designers. This is may favorite way.

After this i tried to use
ContentControl
by binding it's
Content
-property to a bindable property of the
UIElementCollection
-type. This aproach is not throwing any errors in the designer, but i have to admit that i never see any control ( e.g. a
Button
) in my container. It stays empty but has the children added.

Conclusion



After serval hours of searching for a easy and quick solution i decided to ask for solutions here. I'm a little disappointed. It would be really helpful if Microsoft could get a sample into MSDN.

I'm sure there must be a easy way to archive this.

Current situation



Thanks to Andrei Gavrila and jberger i archived to create a node which displays the content ( see code below ) but there are still two issues:
- No designer support
- The border ( see xaml ) is not shown in designer and not shown when the app is running there is even no margin

public class NodeContent : ContentControl
{
static NodeContent()
{
DefaultStyleKeyProperty.OverrideMetadata(typeof(NodeContent), new FrameworkPropertyMetadata(typeof(NodeContent)));
}
}

public partial class Node : UserControl, INotifyPropertyChanged
{
UIElementCollection _Elements;

public event PropertyChangedEventHandler PropertyChanged;

public UIElementCollection NodeContent
{
get { return _Elements; }
set
{
_Elements = value;
OnPropertyChanged("NodeContent");
}
}

public Node()
{
InitializeComponent();
NodeContent = new UIElementCollection(NodeContentContainer, this);
}



protected void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
}


Node-Xaml:

<UserControl x:Class="Pipedream.Nodes.Node"
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="216" d:DesignWidth="174" Background="Transparent" Name="NodeControl" xmlns:my="clr-namespace:Pipedream.Nodes">

<Border BorderThickness="1" CornerRadius="20" BorderBrush="Black" Background="White">
<Grid>
<my:NodeContent x:Name="NodeContentContainer" Margin="20" Content="{Binding Source=NodeControl, Path=NodeContent}" />
</Grid>
</Border>
</UserControl>


Generic-Xaml:

<ResourceDictionary
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:Pipedream.Nodes">


<Style TargetType="{x:Type local:NodeContent}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type local:Node}">
<Border Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}">
<ContentPresenter />
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ResourceDictionary>

Answer

You cannot bind dependency properties of type UIElementCollection, generally. Try something like this:

MultiChildDemo.xaml

Nothing much to see here. The StackPanel will hold our child elements. You could obviously do quite a bit more.

Code:

<UserControl x:Class="Demo.MultiChildDemo"
             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:demo="clr-namespace:Demo"
             mc:Ignorable="d"
             d:DesignHeight="300" d:DesignWidth="300">
    <StackPanel x:Name="PART_Host" />
</UserControl>

MultiChildDemo.xaml.cs

Important to note:

  • The ContentPropertyAttribute attribute sets the property that will be set by any elements enclosed by the parent element of this type. Thus, any elements within <MultiChildDemo></MultiChildDemo> will be added to the Children property.
  • We are extending a UserControl. This does not necessitate a completely custom control.
  • It is good practice in WPF to make properties using DependencyProperty.Register() and variants. You will notice that there is no backing variable for the Children property: DependencyProperty takes care of storing the data for us. Were we not creating a read-only property, this would enable the use of bindings and other cool WPF features. Thus, it is important to get into the habit of using dependency properties, rather than plain properties as you often see in examples around the Internet.
  • This is a relatively simple dependency property example. All we do is copy the reference to a child's dependency property, thereby forwarding calls to UIElementCollection.Add. Much more complex examples are out there, especially on MSDN.

Code:

using System.Windows;
using System.Windows.Controls;
using System.Windows.Markup;

namespace Demo
{
    [ContentProperty(nameof(Children))]  // Prior to C# 6.0, replace nameof(Children) with "Children"
    public partial class MultiChildDemo : UserControl
    {
        public static readonly DependencyPropertyKey ChildrenProperty = DependencyProperty.RegisterReadOnly(
            nameof(Children),  // Prior to C# 6.0, replace nameof(Children) with "Children"
            typeof(UIElementCollection),
            typeof(MultiChildDemo),
            new PropertyMetadata());

        public UIElementCollection Children
        {
            get { return (UIElementCollection)GetValue(ChildrenProperty.DependencyProperty); }
            private set { SetValue(ChildrenProperty, value); }
        }

        public MultiChildDemo()
        {
            InitializeComponent();
            Children = PART_Host.Children;
        }
    }
}

MultiChildDemoWindow.xaml

Note how the labels are direct descendants of the <demo:MultiChildDemo> element. You could also enclose them in a <demo:MultiChildDemo.Children> element. The ContentPropertyAttribute attribute that we added to the MultiChild class allows us to omit this step, though.

Code:

<Window x:Class="Demo.MultiChildDemoWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:demo="clr-namespace:Demo"
        Title="MultiChildDemoWindow" Height="300" Width="300">
    <demo:MultiChildDemo>
        <Label>Test 1</Label>
        <Label>Test 2</Label>
        <Label>Test 3</Label>
    </demo:MultiChildDemo>
</Window>
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