WPF AlternateContent not working

I am trying to show different view elements in a WPF control in debug and release configurations for testing purposes. I have used this post as a guide: Does XAML have a conditional compiler directive for debug mode? (SO)

In order to test it, I have created a VS2013 solution with a single WPF app project, called TestingAlternateContent. Inside my AssemblyInfo.cs I have added the following code:

#if DEBUG
    [assembly: XmlnsDefinition("debug-mode", "TestingAlternateContent")]
#endif

In my MainWindow.xaml I have created a simple code sample to test this behaviour as follows:

<Window x:Class="TestingAlternateContent.MainWindow"
        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:debug="debug-mode"        
        mc:Ignorable="mc debug" 

        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <mc:AlternateContent>
            <mc:Choice Requires="debug">
                <TextBlock Text="Debug mode!!" />
            </mc:Choice>
            <mc:Fallback>
                <TextBlock Text="Release mode here!" />
            </mc:Fallback>
        </mc:AlternateContent>
    </Grid>
</Window>

While testing this, I always see the window with the "Release mode here!" message, regardless which configuration (Debug, Relase) I am using. I have checked that the AssemblyInfo #if DEBUG is working propperly, changing accordingly when I change between Debug/Release configurations. I have tested the same code under VS2008/VS2013 with .NET Framework 3.5/4.5 versions, and none have worked. What am I missing? Anyone knows what is wrong here or can post a working code as reference?