How to use WiX preprocessor pragmas? - wix

Question

I'm currently working on a test WiX project to see what I can do with it.
Recently, I've stumbled upon the fact that I can override the ProcessPragma method in my preprocessor extension to write WiX source code at compile time. Having a preprocessor function that returns an xml string without the compiler going berserk sounds neat. So I looked into it, but the response in this wix-users thread is quite brief and doesn't explain much. Google doesn't return anything interesting beyond that. So I dug through the WiX source code to learn more.
The xml documentation for the method is as follows:
/// <summary>
/// Processes a pragma defined in the extension.
/// </summary>
/// <param name="sourceLineNumbers">The location of this pragma's PI.</param>
/// <param name="prefix">The prefix of the pragma to be processed by the extension.</param>
/// <param name="pragma">The name of the pragma.</param>
/// <param name="args">The pragma's arguments.</param>
/// <param name="writer">The xml writer.</param>
/// <returns>false if the pragma is not defined.</returns>
/// <comments>Don't return false for any condition except for unrecognized pragmas.
Throw errors that are fatal to the compile. use core.OnMessage for warnings and messages.</comments>
So as a test, I had the XmlWriter produce a dummy property and then return true.
Now, to actually call it in my WiX project. In Preprocessor.cs, I found the following:
switch (reader.NodeType)
{
case XmlNodeType.ProcessingInstruction:
switch (reader.LocalName)
{
// other cases such as define, include, foreach,
// and other preprocessor directives
case "pragma":
this.PreprocessPragma(reader.Value, writer);
break;
}
break;
Which hinted that the syntax for using a pragma would be: <?pragma prefix.name?>
But this gives me the following warning: The pragma 'prefix.name' is unknown. Please ensure you have referenced the extension that defines this pragma.
I have a feeling I'm on the right track, as it gives me a warning related to pragmas, but I honestly have no clue what I'm doing here. It seems like it's uncharted territory.
Does anyone know what I'm doing wrong, or point me in the right direction?
UPDATE
Seems like my project was the problem. I used my extension in another project and it worked like a charm.
And for anyone reading this in the future, the syntax is <?pragma prefix.name args?> where the arguments are just a string. And as a side note, you don't close the XmlWriter in your override method.

Answer 1

First, ensure you are passing the -ext path\to\YourPragmaExtensionAssembly.dll to candle.exe. Candle will load your extension, look for the AssemblyDefaultWixExtension attribute that points to the class that inherits from WixExtension, then ask for your PreprocessorExtension class if you override the PreprocessorExtension property.
It's a little bit of wiring up that could be simplified in future versions (like v4.0) of the WiX toolset. But there is an example in the WiX v3.x toolset at: src\ext\PreProcExampleExtension\wixext that should show the way.