#300 – Button is a ContentControl

Button is a ContentControl, which means that it can contain exactly one child control.  The child element can be any .NET object.

A simple string is the most common content for a Button.

        <Button Content="Click me" Click="Button_Click"/>

Examining the Button control at runtime, you can see that its stores an instance of a string in its Content property.  Also note that the base type of Content is object, meaning that the button could store any .NET object as its content.

Here’s a Button that contains a single Ellipse control as its content:

        <Button Click="Button_Click" Width="140" Height="80">
            <Ellipse Fill="PaleGreen" Stroke="Blue" StrokeThickness="5" Width="120" Height="60"/>

And here’s a Button that uses a StackPanel to host several child controls.

        <Button Click="Button_Click" Width="140" Height="120">
                <Ellipse Stroke="Blue" StrokeThickness="5" Width="120" Height="60"/>
                <Label Content="Name my ellipse"/>
                <TextBox />

#283 – A Window Can Have Only a Single Child Element

In WPF, a Window can have only a single child element.  Window inherits from ContentControl, which is a control that contains a single child element, referenced by its Content property.

Your window might have a single simple child control, like a Button.

	<Button Content="Big Daddy Button"/>

Since having a single control in your application is not very useful, it’s more common that the single child control of a Window is a container control like StackPanel, which in turn can contain multiple child elements.

		<Label Content="You can enter your name here:"/>
		<TextBox />
		<Button Content="Push to Continue"/>

#177 – A Content Presenter Is a Placeholder for a Content Control’s Content

A content control (ContentControl) in WPF is a control that displays a single piece of content.  The control will have a control template (Template property) that describes the specific visual elements that make up the control.  Somewhere in that control template will be a content presenter (ContentPresenter), which indicates the spot in the control to display the actual content.

You can think of the ContentPresenter as a little pointer that says “put your content here”.

Below is the default control template for a Label control as an example.  The control template includes a ContentPresenter inside a Border.

<ControlTemplate TargetType="{x:Type Label}">
    <Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Padding="{TemplateBinding Padding}" SnapsToDevicePixels="true">
        <ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
        <Trigger Property="IsEnabled" Value="false">
            <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>

#34 – ContentControl

Content controls are a category of controls that can hold a single nested element, which represents its content.  Since the nested element can be anything, controls that derive from ContentControl can store a variety of different types of content.

Content controls have a Content property, which points to the single element that they contain.  The element is often an UIElement, but can be any .NET object.

Here’s an example, with CheckBox using a DatePicker as its content.

 <CheckBox Height="31" HorizontalAlignment="Center" Name="checkBox1" VerticalAlignment="Center">
     <DatePicker Name="datePicker1"  />

Note that because the Window class is a content control, a Window may have only one child element.

You can effectively include a collection of controls in a content control by including as its content a single container object that in turn contains other elements.  (E.g. A Button could contain a StackPanel, which would then contain other elements).