Does XAML have a conditional compiler directive for debug mode?

This is not possible in WPF/Silverlight/WP7.

On an interesting note, the standards document, ISO/IEC 29500 (Office Open XML File Formats), covers how this should be handled in an XML document, and XAML does support one of the items from that spec mc:Ignorable which allows us to do things like this:

<Page xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
      xmlns:c="Comments"
      mc:Ignorable="c">
    <Button Content="Some Text"
            c:Content="Some other text" />
</Page>

to comment out attributes.

The XAML parser team (SL4, WP7.1, WPF) chose to use that spec to solve their needs for ignoring attributes, rather than just making something up. That is why some of the default XAML pages have the 'mc' namespace defined. I do think it would be cool if XAML one day supported the rest of the spec that allows the loading of alternate content.

The mc:Ignorable attribute is used by Blend to support design time functionality.

You could use a template selector. The DataTemplateSelector class is something you code. With the template selection method that you override, you could put your preprocessor directives.

http://msdn.microsoft.com/en-us/library/system.windows.controls.datatemplateselector.aspx

I recently had to do this and was suprised at how simple it was when I couldn't easily find any clear examples. What I did was add the following to AssemblyInfo.cs:

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

Then, use the markup-compatability namespace's AlternateContent tag to choose your content based on the presense of that namespace definition:

<Window x:Class="Namespace.Class"
        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="debug-mode"

        Width="400" Height="400">

        ...

        <mc:AlternateContent>
            <mc:Choice Requires="d">
                <Style TargetType="{x:Type ToolTip}">
                    <Setter Property="FontFamily" Value="Arial"/>
                    <Setter Property="FlowDirection" Value="LeftToRight"/>
                </Style>
            </mc:Choice>
            <mc:Fallback>
                <Style TargetType="{x:Type ToolTip}">
                    <Setter Property="FontFamily" Value="Tahoma"/>
                    <Setter Property="FlowDirection" Value="RightToLeft"/>
                </Style>
            </mc:Fallback>
        </mc:AlternateContent>

        ...
</Window>

Now, when DEBUG is defined, "debug-mode" will also be defined, and the "d" namespace will be present. This makes the AlternateContent tag choose the first block of code. If DEBUG is not defined, the Fallback block of code will be used.

This sample code was not tested, but it's basically the same thing that I'm using in my current project to conditionally show some debug buttons.

I did see a blog post with some example code that relied on the "Ignorable" tag, but that seemed a lot less clear and easy to use as this method.