Peter Marcu's Blog

WiX: Patching something you didnt build with WiX using WiX

Many times people want to take advantage of the Wix v3 patch building features but didnt build their original product using Wix. for example, they may have used Install Shield, WiX v2 or one of the other tools out there. I ran into this when I needed to create a patch for a product we shipped from Visual Studio before the Visual Studio build moved to WiX v3.

 WiX v3 has a feature I added a while back to support this scenario. It allows you to point at an admin image of your msi based install package. This will be very familiar to some of you who have used patchwiz and pcp's in the past. The difference here is that your patch authoring can be done using the WiX v3 Patch element and it follows the same workflow as patching a WiX v3 based msi after the first stage of creating the diffs.

Something I found really easy, and I could see this being used by smaller products who dont have a lot of infrastructure, is the ability to hand edit your admin image to product the patch. For example, I wanted to update a single file in my product:

1. I created an admin image
2. I made a copy of the admin image
3. I updated the loose file in my copy of the admin image

After running it through the series of command lines, I got a patch that updated the file. Pretty simple...

The command lines you need to run are all the same as the ones in my sample: Building a Patch using the new Patch Building System - Part 3. The only difference is that you need to pass the -ax command with a path to extract embedded binaries to instead of the -xi switch. This tells torch that the input is admin image instead of xml. Also, specify the path to the msi in the admin images instead of the wixpdb as the target and upgrade. Everything after that should follow the same process.

Let me know how it works! :)

Bonus feature: One of the most useful things in the WiX v3 patching system is the ability to filter whats in your patch using patch families to only ship your customers the updates they need. This allows you to grab changes per Fragment from your wix source. When patching from admin images, there is no source so you would think that the filtering isn't suppoted. Not so! This was one of the more fun parts of this implementation. We "auto-fragment" the msi into what we consider to be the optimal chunks to give you the ability to filter if you choose to do so.

WiX: Introducing the WixPdb

I should have made this post about 6 months ago as I was implementing the WixPdb feature in WiX but better late than never :).  The wixpdb was conceived as a solution for a limitation in the original patch building system. Originally, WiX v3 patching only supported creating transforms between two wixout files. This was flawed in that many things modify the output after the wixout is generated by light which weren't being reflected in patches. I knew I needed the patch build system to be able to create transforms from something closer to the final output. Thats what the WixPdb is. It is all the data in the output right before creating the final msi. With that data, we can now create more accurate transforms to what people expect when diffing two products. I updated my blog with samples on how to build patches to show that I now recommend using the wixpdb instead of the wixout.

Down the road, I look forward to seeing what other features we can get out of the wixpdb. It provides a ton of information an allows you to potentially connect install time errors all the way back to the wix source and line number. Static analysis is another benefit. I updated smoke to be able to take the wixpdb so it will give you source and line number information for ICE errors. Statis analysis on the wixpdb itself may also be interesting because it contains more data than in the msi. It is the one place where the wix source data and the msi data connect. Let me know if you have any ideas of how to use the wixpdb in some cool ways...

Design your Application to Fit Your Shipping Container

Let me start by saying I'd love to get people's feedback on the content of this post...

When Toyota designs a car they would never ignore the fact that no matter how great a car they build, they have to be able to get it to their customers. Additionally, they have to be able to get it to them at a reasonable cost. Imagine that Toyota decided that it would be cool to create a car that is 4 seats wide. Assume they ignored that the world's infrastructure is built to handle cars two seats wide. Can you imagine what would happen if they did that? Cars wouldn't fit on boats, on trucks, in garages, etc. In short, the cars would sit at the factory. Assuming the world wants the car people would scramble to create the infrastructure to get the cars to the users. In the end all of this would drive up the cost of shipping the cars and cause customers to pay more money to get it.

Software is no different

Unfortunately, software is often designed without considering its ship vehicle. What usually matters to developers is that it runs on their machine. It is designed and built until it works and is tested. Only then is deployment thought about. At that point, the application usually does tons of things that make software deployment a nightmare and costs way more that it would have if it were considered up front.

There's a way out

The standard for deployment on Windows is the Windows Installer (MSI). Luckily, for the applications that think about deployment last, Windows Installer is very extensible. If you want to run a bunch of custom code during install, you can. Having to do this is usually a sign that your application was not designed to make your deployment simple. You have to build a bunch of infrastructure as a band-aid for something that could have likely been avoided. Your custom code inevitably wont behave properly in some scenarios causing your entire deployment to become fragile. Whenever you see this, consider an upstream design change. This is more likely feasible if you aren't just about to ship your product and you aren't scrambling to build an installer package.

Make it simple

Setup can be really simple. Everything on a machine boils down to very simple things: Files, registry keys, and shortcuts. Windows Installer happens to handle Files, registry keys, and shortcuts very well. It's actually really easy to use if all you need to do to deploy your application is copy files, write registry keys, and add shortcuts to the system.

Its often not your application's fault

IIS did not make it easy to copy files to a location to install a web site. .NET didn't make it easy to GAC an assembly or NGEN it by copying it to a location. Both of these are things I like to call custom stores. Applications that expect other applications to extend them or use them as a platform need to be even more conscious of the deployment problems they cause. When platforms don't consider what it will take to deploy on top of them they make every one of their customers building solutions has to modify their shipping container to handle their custom store. Although we cant get rid of the custom stores that have already shipped, we can hopefully prevent more and more custom stores from being produced. I think people building on top of platforms should push back more when their platforms create more and more custom stores.

Start development with deployment

When building your applications, think about deployment as you design your app. Think, "Can I put this on my customers machine by only copying files"? If it starts to get hard, don't just write custom code to solve it. Think, "Could my application make itself easier to deploy?" Ideally, you could build your application to a folder and just run it without any configuration steps. You should create an empty installer package at the beginning of your project and build it up as you ad functionality to your product.

Application virtualization is on the horizon

If you make your application deployment completely declarative where no custom code has to run on your customers machine, your application will fit into the virtualized application world much better. It will also allow scenarios like drag and drop deployment, running from USB key.

Disclaimer

Many of the things I've said as far as guidance are optimistic. The world isn't as clean as I wish it were. But in the end, I dont think we'll ever be in a better situation unless people start thinking about the problems and stop making things worse for the Windows eco system by creating custom stores in platforms.

WiX: BinderExtensions and the BinderFileManager

As of a few weeks ago, the BinderExtension class in the wix extensibility model has taken on a new meaning. If you currently have a BinderExtension, you will need to rename it to BinderFileManager. Over the past year or so, numerous requests came in to give WixExtensions access to data and bind time to pull disperate data together or get access to data only available at bind time.

For example, if I have a custom table where I need to know the exports of all my native dll's and I want this table to be populated dynamically by wix, I would need to have access to populate my table after files are resolved. With the new BinderExtension class, this is now possible.

Key Changes:

What used to be called BinderExtension was renamed to BinderFileManager. The BinderFileManager handles things specific to a files, mainly file resolution and comparison. Only one instance of a BinderFileManager can be present in the WixExtensions passed to the wix tools and is commonly used in an extension specific to a build environment.

It is now possible to have a BinderExtension defined in each WixExtension passed to the command line tools. This allows you to:

1. Link disperate data associated with compile time language extensions. It essentially enables extensions to make additional decisions that the WiX linker cant do for you because it doesnt natively support your language extension. Once the linker has pulled all the data together, it may be neccessary for you to read that data and populate additional tables.

2. Populate tables with bind time data such as information from files which are resolved during bind.

 In my series abut writing WixExtensions, I plan to show an example of this functionality. I plan to pick that series back up soon.

WiX: More powerful foreach

I've been meaning to post about this for a while but just never got around to it. A few months ago I extended the implementation in WiX to allow foreach loops using any variable over any given set of values. Previously it was limited to only those variables that were defined in your preprocessor extension.

While this example wont do much for you, it demonstrates what a foreach loop will do. <?foreach?> loops can simplify authoring in many ways, especially for complex products.

Example:

<?foreach number in 1;2;3 ?>
    <Property Id="Property$(var.number)" Value="value$(var.number) />
<?endforeach ?>

Will result in the following being passed to the compiler:

    <Property Id="Property1" Value="value1 />
    <Property Id="Property2" Value="value2 />
    <Property Id="Property3" Value="value3 />

I want to note as a caveat, It can be very easy to break component rules by changing the contents of a component by simply changing the values you iterate over. I recommend not using foreach loops anywhere near components unless you have really thought it through.  Also, while using the preprocessor can be very handy for minimizing the amount of wix code you need to maintain, it can really confuse some of the tools that are out there that help you generate or maintain your wix source. In general, think about other options before you use the preprocessor as a solution but if you think its your best option, enjoy :)

WiX: "Rosario" WiX toolset vs. the community WiX toolset

This post is a follow-up to Rob Mensching's "Visual Studio ships the WiX toolset" blog entry.

One of the key tenets WiX development is operating under is to make all changes available to both community WiX as well as "Rosario" WiX. (They actually get fixed for the community first :))

Each time we make a major "Rosario" WiX release, I plan to list the major differences between community WiX and "Rosario" WiX.

Differences:

Signing - The WiX binaries shipped with Visual Studio will be signed with a Microsoft signature rather than the community WiX signature.
Contents of the installer - In order to limit our test surface and ship the mainline tools in the highest possible quality, there are a few tools that will not be shipping in the VS package. Another option we are exploring is to supply some sort of SDK for the tools we don't plan to provide mainline support for.
Supported VS versions - The "Rosario" WiX toolset will only install on VS 2008 while the community version will continue to install on both VS 2005 and VS 2008.
Nant - No support for Nant will ship in the "Rosario" box.

WiX: Binary Delta patch support added to WiX 3.0

Last night, everything came together and the changes to add binary delta patch support to WiX 3.0 went in. The feature is minimally documented but the schema and docs have descriptions of the new elements and attributes. I hope to get time to provide more details soon but I wanted to tell everyone that its in there so you can start playing with it if you'd like.

WiX: Writing your own WiX Extension Part 2: Preprocessor

The preprocessor in WiX allows extensibilty at a few levels. In this article I will describe how to add a PreprocessorExtension to your WixExtension and have it handle variables and functions you define in your own namespace.

[This sample assumes you have already gone through Part 1]

1. Add a new class to your project called SamplePreprocessorExtension.
2. If you added a new file for this class, make sure you add: using Microsoft.Tools.WindowsInstallerXml to you file.
3. Make your SamplePreprocessorExtension class implement PreprocessorExtension.

public class SamplePreprocessorExtension : PreprocessorExtension

4. Add your SamplePreprocessorExtension to your SampleWixExtension class and override the PreprocessorExtension property from the base class. This will make it so when WiX asks your extension for your preprocessor extension, you extension will know what to do.

private SamplePreprocessorExtension preprocessorExtension;

public override PreprocessorExtension PreprocessorExtension
{
    get 
    {
        if (this.preprocessorExtension == null)
        {
            this.preprocessorExtension = new SamplePreprocessorExtension(); 
        }
        return this.preprocessorExtension;

    }
}

5. Now, back in your SamplePreprocessorExtension class, you need to specify what prefixes (or namespaces) your extension will handle. For example, if you want to be able to define a variable $(sample.ReplaceMe) then you need to specify that your extension will handle the "sample" prefix.

private static string[] prefixes = { "sample" };
public override string[] Prefixes { get { return prefixes; } }

6. Now that you have specified your prefixes, you now need to handle variables and functions that are passed to you from WiX. You do this by overriding the GetVariable and EvaluateFunction methods from the PreprocessorExtension base class.

public override string GetVariableValue(string prefix, string name)
{
     string result = null;
    // Based on the namespace and name, define the resulting string.
    switch (prefix)
    {
        case "sample":
            switch (name)
            {
                case "ReplaceMe":
                   // This could be looked up from any where you can access from your code.
                   result = "replaced";
                   break;
            }
            break;
    } 
    return result;
} 
  
public override string EvaluateFunction(string prefix, string function, string[] args)
{
    string result = null;
    switch (prefix)
    {
        case "sample":
            switch (function) 
            {
                case "ToUpper":
                    if (0 < args.Length) 
                    {
                        result = args[0].ToUpper();
                    }
                    else
                    {
                        result = String.Empty; 
                    }
                    break; 
            } 
            break; 
    } 
    return result;
}

7. Build
8. With this you can now pass your extension on the command line to candle and expect variables and functions in your namespace to be passed to your extension and be evaluated. To prove this, try adding the following properties your WiX source.

    <Property Id="VARIABLETEST" Value="$(sample.ReplaceMe)" />
    <Property Id="FUNCTIONTEST" Value="$(sample.ToUpper(lowercase))" />

You resulting msi should have entries in the Property table with the values "replaced" and "LOWERCASE" in the property table.

WiX: Writing Your Own WiX Extension Part 1

Wix extensions are used to extend and customize what WiX builds and how it builds it. I plan on talking about the 3 most common types of extensions in this article series; ProprocessorExtension, CompilerExtension, and BinderExtension.

The first step in creating any set of extensions is to create a class that implements WixExtension. This class will be the container for all the extensions you plan on implementing.

Creating a bare-bones WixExtension

1. Create a new C# library (.dll) project named SampleWixExtension
2. Add a reference to wix.dll to your project
3. Add a using statement: using Microsoft.Tools.WindowsInstallerXml;
4. Make your SampleWixExtension class inherit from WixExtension.

 public class SampleWixExtension : WixExtension {}

5. Add the AssemblyDefaultWixExtensionAttribute to your AssemblyInfo.cs.

 [assembly: AssemblyDefaultWixExtension(typeof(SampleWixExtension.SampleWixExtension))]

6. Build

7. Although it wont do anything yet, you can now pass the your SampleWixExtension.dll on the command line to the candle and light with the -ext flag like this:  candle Product.wxs -ext SampleWixExtension.dll

The next article in this series will explain how to add a Preprocessor extension to your WixExtension.

WiX: Building a Patch using the new Patch Building System - Part 3

This example is aimed at demonstrating the workflow and command lines required to build a patch using the Patch element and Pyro.exe. I would appreciate any feedback anyone has.

Setting up the Sample:

1. Create a Directory that you plan on running the sample from. I will call this directory the sample root

2. Under the sample root directory create two subdirectories called "1.0" and "1.1"

3. Create a text file in the 1.0 directory called Sample.txt and put some text in it telling you that it is the 1.0 version of the file.

4. Create a text file in the 1.1 directory called Sample.txt and put some text in it telling you that it is the 1.1 version of the file.

You should now have 2 sub-directories under your sample root called 1.0 and 1.1 each containing a Sample.txt file whose contents state which version it is.

5. Create your product authoring in the sample root folder called Product.wxs with the following contents:

<?xml version="1.0" encoding="UTF-8"?>
<Wix xmlns="http://schemas.microsoft.com/wix/2006/wi">
    <Product Id="48C49ACE-90CF-4161-9C6E-9162115A54DD" Name="WiX Patch Example Product" Language="1033" Version="1.0.0" Manufacturer="Dynamo Corporation" UpgradeCode="48C49ACE-90CF-4161-9C6E-9162115A54DD">
        <Package Description="Installs a file that will be patched." Comments="This Product does not install any executables" InstallerVersion="200" Compressed="yes" />

        <Media Id="1" Cabinet="product.cab" EmbedCab="yes" />
        <FeatureRef Id="SampleProductFeature"/>
    </Product>

    <Fragment>
        <Feature Id="SampleProductFeature" Title="Sample Product Feature" Level="1">
            <ComponentRef Id="SampleComponent" />
        </Feature>
    </Fragment>

    <Fragment>
        <DirectoryRef Id="SampleProductFolder">
            <Component Id="SampleComponent" Guid="{C28843DA-EF08-41CC-BA75-D2B99D8A1983}" DiskId="1">
                <File Id="SampleFile" Name="Sample.txt" Source=".\$(var.Version)\Sample.txt" />
            </Component>
        </DirectoryRef>
    </Fragment>

    <Fragment>
        <Directory Id="TARGETDIR" Name="SourceDir">
            <Directory Id="ProgramFilesFolder" Name="PFiles">
                <Directory Id="SampleProductFolder" Name="Patch Sample Directory">
                </Directory>
            </Directory>
        </Directory>
    </Fragment>
</Wix>

6. Create your patch authoring in the sample root called Patch.wxs with the following contents:

<?xml version="1.0" encoding="UTF-8"?>
<Wix xmlns="http://schemas.microsoft.com/wix/2006/wi">
    <Patch
        AllowRemoval="yes"
        Manufacturer="Dynamo Corp"
        MoreInfoURL="http://www.dynamocorp.com/"
        DisplayName="Sample Patch"
        Description="Small Update Patch"
        Classification="Update"
        >
       
        <Media Id="5000" Cabinet="RTM.cab">
            <PatchBaseline Id="RTM"/>
        </Media>

        <PatchFamilyRef Id="SamplePatchFamily"/>
    </Patch>

    <Fragment>   
        <PatchFamily Id='SamplePatchFamily' Version='1.0.0' Supersede='yes'>
            <ComponentRef Id="SampleComponent"/>
        </PatchFamily>
    </Fragment>
</Wix>

You should now have a Product.wxs and Patch.wxs file in the sample root.

Instructions for building a Patch using the sample:

Open a command prompt where you normally run the WiX tools from.

Required WiX executables:

Candle.exe
Light.exe
Torch.exe
Pyro.exe

Your WiX toolset version should be at least 3.0.3001.0

1. Build Target Layout:

> candle.exe -dVersion=1.0 product.wxs
> light.exe -sval Product.wixobj -out 1.0\Product.msi

2. Build the Upgrade Layout:

> candle.exe -dVersion=1.1 product.wxs
> light.exe -sval -xo Product.wixobj -out 1.1\Product.msi

3. Create the transform between your products:

> torch.exe -p -xi 1.0\Product.wixpdb 1.1\Product.wixpdb -out Patch\Diff.Wixmst

4. Build the Patch:

> candle.exe Patch.wxs
> light.exe Patch.wixobj -out Patch\Patch.WixMsp
> pyro.exe Patch\Patch.WixMsp -out Patch\Patch.msp -t RTM Patch\Diff.wixmst

I am currently working on reducing the number of command line switches required for building a simple patch. Sometime soon I hope to have the commands somewhat simplified by making the defaults match up with the patch building system.

Verifying what you have done here works: 

1. Install the 1.0 Product by running 1.0\Product.msi

2. Go to "Program Files\Patch Sample Directory" and open up Sample.txt. You should see that it is the 1.0 version.

3. Close Sample.txt.

3. Install the Patch from the sample root Patch\Patch.msp.

4. Repeat step 2 and notice that Sample.txt now contains the new 1.1 content.

5. Go to Add/Remove Programs and make sure "Show Updates" is checked.

6. Uninstall the Patch and repeat step 2 to see that the file was rolled back to its original version.

7. Uninstall the Product itself to clean off your system.

WiX: Building a Patch using the new Patch Building System - Part 2

I dont have much time today to write but I wanted to take a chance to answer a few questions that I've been asked a lot recently.

What is Pyro? 

Pyro is the new tool in the WiX toolset that will pull together your patch authoring and your product transforms in order to build you an msp.

What is the new system and what is the old? 

It is important to distinguish the new patch building system from the previous one provided by WiX. There has been some confusion and I want to try and clear it up.  

The Previous Way:
The PatchCreation element has been supported in WiX for a while to help people build patches. It essentially helps users build a pcp file which is then used as input to the PatchWiz tools. The PatchCreation element is not used in the new patch building system.

The New Way:
The Patch Element is a newly added element that lets you describe the contents of your patch. It contains information about where the files your patch contains should be stored as well as summary information that describes your patch to your end user. It also supplies a mechanism to filter out specific changes to be included in your patch. When this element is passed to candle to be compiled and the resulting wixobj's are passed to light to link, the result is a WixMsp. This is an xml representation of the patch database (msp).

What Next?

The next thing I'd like to do is to provide a simple example of how to build a patch using Pyro.exe. I plan on getting this started in the next few days. Keep bugging me if I dont to remind me to do it and feel free to ask me more questions as this new functionality isn't all that well documented yet.

WiX: What is a WiX Extension?

A WiX extension is a dll that allows users to add their own custom elements, preprocessor functionality, and build customizations. These extensions take advantage of the various extension points in WiX. The most commonly used ones are Preprocessor, Compiler, and Binder extentions.

Mosts WiX extensions shipped with the WiX toolset are used to encapsulate common funtionality and make it easy for users to author things into their setups. They are targeted at solving setup problems in a standard way so everyone can benefit from and build on the WiX toolset.

Example: WixNetFxExtension.dll

Many people who write applications for the .NET Framework have various needs. I will discuss 2 here in an effort to describe 2 uses of extensions.

AppSearch (ComponentSearch, RegistrySearch, etc..):

Sometimes people want to find information about the system as part of their install. One way to get current system info is using an AppSearch.

Detecting if the .NET Framework is installed:

In reality, in order to detect if the .NET framework is installed, you need to search for a specific registry key and set a property if that key exists. Using the WixNetFxExtension you can just add the following to detect if the 2.0 Framework is installed.

<PropertyRef Id="NETFRAMEWORK20"/>

 The logic defining this property and all the logic behind it is stored in a WixLib which embedded inside the WixNetFxExtension.dll

Custom Actions:

Sometimes people want to do things that are not supported by Windows Installer. In this case they write custom actions. WiX provides a good mechanism for sharing custom actions.

Ngen’ing an Assembly:

To add a custom action to a product there is a lot of authoring to do. You need to define it and its binary, schedule it, and condition it. This doesn’t even consider that most custom actions come in sets, all sequenced at different times. This can all be done using a CustomActionRef in a similar way as the previous example. After that, you need to populate any custom tables your custom action may rely on for its data. This is a lot complexity for users to author using standard WiX constructs. One use of a compiler extension can be to add your own element to the authoring and have that translate into a custom table in your msi that your customaction can read. The ngen custom actions have such an element.

<netfx:NativeImage Platform=”all” … />

The compiler will take this line and its context and populate a NativeImage table in the resulting MSI that will contain all the data needed by the ngen custom actions.

The custom action also requires a dll binary in the binary table. In WiX v2 you had to ship your binary along with your extension. WiX v3 has support for binary WixLib’s. The key thing here is that the binary will get added into the WixLib for you and you don’t have to worry about shipping 2 separate files.

The resulting extension file:

The result of this is a .NET dll with a WixLib embedded in it as a resource. That WixLib then in turn has a cabinet at the beginning of it that contains the custom action binary. All of this nesting is handled for you by the linker and the binder so all you have to do to use stuff you put in your WixLib is add the extension to your candle and light command lines and reference the things defined in the WixLib from your product authoring.

WiX: Preprocessor error and warning

This is simple but could be handy. You can now add preprocessor instructions to you WiX authoring that will allow you to display a warning or error message when the preprocessor encounters that line.

Often errors and warnings would be used inside other prepropcessor constructs.

Example:

<?warning Must define the variable variableThatMustBeDefined ?>
<?ifndef variableThatMustBeDefined ?> 
    <?error You did not define the variable variableThatMustBeDefined ?>
<?endif ?>

WiX: PatchFamily patch filtering

Using the Patch system in WiX 3.0 allows you to select which differences between 2 builds you want to ship. By default, all the changes will go into a patch, but sometimes people want to ship a targeted update to fix a security issue or major customer problem. This can be done using the PatchFamily concept built into WiX.

A patch family has 2 meanings in this context, it specifies which changes you would like in any given patch, and also provides a mechanism for supercedence and dependency tracking. I'm not going to get into supercedence here other than to say that for each patch family in a patch will get an entry in the MsiPatchSequence table.

The PatchFamily element is a child of the Patch element. You can have as many PatchFamily elements in a Patch as you want. Under a PatchFamily element, there is a set of Reference elements that can be used to pull various items into your patch. Just as in your product authoring, referencing anything in a Fragment will result in that entire fragment being pulled in. WiX will handle file and media sequencing as well as the Sequence tables for you.

Example:

This PatchFamily would select the Fragment that contains the Component "MyComponent" and add it to the patch.

<PatchFamily Id="MyPatchFamily" supercede="yes">
    <ComponentRef "MyComponent" />
</PatchFamily>

Sidenote:

One thing to note is that there are rules about patch families. Once you ship a patch family, you must keep its references the same or add to it (grow it) but never remove items from it. All items in your product must be a part of only one patch family. When you change the build that you are targeting (re-baseline), patch families start over so you can change the contents and asociations at this time.

WiX: Functional Preprocessing

The preprocessor in WiX has a new feature called Functional Preprocessing. It is a way for WiX users to define functions that they can use in their authoring to dynamically fill in content.

 Most people who use WiX are familiar with preprocessor variables which look like:

 "$(var.VariableName)"

 The preprocessor now has support for functions in addition to variables which look like:

"$(func.FunctionName(arg1,arg2,arg3))"

While there have been no additions as of yet to the standard WiX preprocessor function library many functions have been implemented and are being used in people's preprocessor extensions. In an extension you tell the preprocessor that you would like to have all functions with a specific qualifier be handled by your extension.

Example:

Say you want to keep track of all of your component GUID's in a single file. The file has 4 pieces of data for each GUID. Id, Language, Architecture, and of course the GUID.

MyComponentId, English, Intel, 365453-34235-32525-23325345223

If you implement a preprocessor extension to handle "guid" functions and define a function called "GetGuid" that takes 3 arguments you could represent the guid in your authoring as:

"$(guid.GetGuid(MyComponentId,English,Intel))"

This function would be replaced in your authoring with "365453-34235-32525-23325345223".

 There are a lot of possible uses of this type of functionality. I'd love to hear how other people use it.