WPF ContentControl DataTemplate not updating value - wpf

Question

I have no idea what's happening here. When I bind directly to a TextBox the value can be edited, but I want to bind in a ContentControl.
Why doesn't the ContentControl update the ViewModel?
<Window x:Class="WTFWPF.MainWindow"
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:local="clr-namespace:WTFWPF"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:sys="clr-namespace:System;assembly=mscorlib"
Title="MainWindow"
Width="525"
Height="350"
DataContext="{DynamicResource ViewModel}"
mc:Ignorable="d">
<Window.Resources>
<local:MainViewModel x:Key="ViewModel" />
<DataTemplate DataType="{x:Type sys:Int32}">
<TextBox Text="{Binding Path=., UpdateSourceTrigger=PropertyChanged}" />
</DataTemplate>
</Window.Resources>
<StackPanel>
<TextBlock Margin="5" Text="{Binding Number}" />
<TextBox Margin="5" Text="{Binding Number, UpdateSourceTrigger=PropertyChanged}" />
<ContentControl Margin="5" Content="{Binding Number}" />
</StackPanel>
</Window>

Answer 1

This seem to work fine, not sure why your version doesn't work though.
<Window.Resources>
<wpfApplication1:MainViewModel x:Key="ViewModel" />
<DataTemplate x:Key="NumberTemplate">
<TextBox Text="{Binding Path=Number, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
</DataTemplate>
</Window.Resources>
<StackPanel>
<TextBlock Margin="5" Text="{Binding Number}" />
<TextBox Margin="5" Text="{Binding Number, UpdateSourceTrigger=PropertyChanged}" />
<ContentControl Margin="5"
Content="{Binding}"
ContentTemplate="{StaticResource NumberTemplate}" />
</StackPanel>

Answer 2

Another way of making it work is to change the binding in the template:
<TextBox Text="{Binding Path=DataContext.Number,
RelativeSource={RelativeSource AncestorType=ContentControl}, UpdateSourceTrigger=PropertyChanged}" />
Unfortunately I can't explain why your version doesn't work.