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MSBuild - What It Does and What You Can Expect in the Future

The standard customizable build platform for the .NET Framework

In Visual Studio 2003 and earlier, the build process for Visual Basic and C# projects was hard-coded, and built into Visual Studio itself. The only build scripting tool that Microsoft offered was nmake, and a companion tool called build.exe that provided some support for concurrent builds. Visual Studio users whose build systems were based on makefiles had to maintain project files in parallel. For Visual Studio 2005, we thought it would be great if it was possible to completely customize the build process, and to build Visual Studio projects on machines that didn't even have Visual Studio installed, exactly the same as they built inside Visual Studio. We also wanted to be able to plug in reuseable build loggers and build steps.

MSBuild shipped in .NET Framework 2.0, and Visual Studio 2005 was based on it. To date we've only released build processes for Visual Basic and C#, but our vision is to make this truly the common build platform for Microsoft technologies. It's fair to say that internal Microsoft needs have motivated MSBuild almost as much as the needs of our customers. We're making exciting progress towards this goal, and we'll talk about our future plans later in this article.

MSBuild File Format
If you use Visual Studio 2005, you don't necessarily have to know anything about MSBuild syntax. Visual Studio's Visual Basic and C# project files are MSBuild format, and you can go right ahead and build them on machines without Visual Studio installed.

However if you want to customize the build process or create your own from scratch, you'll need to know something about the MSBuild syntax. We tried to make the file format as simple as possible, so it's easy to learn. It's XML, so any XML tool can work with it, and there are only a few different tags to know about. Let's do a quick tour.

MSBuild properties represent key/value pairs used during the build process. A property can only take one value at a time. Properties are always inside a PropertyGroup tag. If you open a Visual Basic or C# project file you'll see it uses properties to store whether the compiler should build with optimizations (the "Optimize" property), the output path (the "OutputPath" property), and what warning level it should use (the WarningLevelproperty). Think of properties as scalars. You can get their values with an expression like "$(PropertyName)".

MSBuild Item lists are just ordered bags, like arrays. An item list contains multiple MSBuild items of the same type. Each has a value and optional metadata. The syntax to access their values is different than properties: "@(ItemType)." The items in an item list are often file paths, but don't have to be - MSBuild doesn't care. Take a look at a Visual Basic or C# project again and you'll see items used to store all the source files (the "Compile" items) and references (the "Reference" items) in your project. They don't have to be hard-coded though. You can have MSBuild generate a list of items using wildcards such as "*.txt".

Like properties, items have an ItemGroup tag they always sit inside. Unlike properties, items can have arbitrary "metadata" attached to each one, which you can define with sub-elements below the item's element. Here's an example of metadata used to represent a Culture value on a resource item:

   <Resource Include="Form1.en-CA.resx ">

Metadata is accessed with yet another kind of expression that looks like this "%(MetadataName)" or "%(ItemType.MetadataName)". There's also a variety of built-in metadata such as "Filename" and "Full-Path", which are useful if the item represents a file.

One item list can be converted to another by a transform operation. For example, if you write "@(Compile -> ‘%(Filename).obj'") you'll get the collection of "Compile" items, but the extension of each item will be changed to "obj". That can be useful for specifying the outputs of a target or constructing a command line string.

Targets are the unit of build: it's not possible to build only part of a target. This time open up your Microsoft.CSharp.targets file, which you'll find in the .NET Framework install folder, e.g., Windows\ Microsoft.NET\Framework\v2.0.50727. (By convention, targets are stored in files that have the ‘.targets' extension.) Now look for the ‘Target' tag. A target groups a collection of tasks together in a particular order to form a unit of build. For example, the "CoreCompile" target contains the "CSC" task that will invoke the C# compiler. Like every other MSBuild tag, Target tags can have conditions on them, but you can also add Input and Output attributes, which MSBuild will compare to see if the target is up-to-date already: if the target is up-todate, MSBuild will skip the tasks within it in part or entirely. A "DependsOnTargets" attribute lets you define what targets need to run beforehand so you can organize the order in which targets run. Targets can override previous targets with the same name. Targets have some other powers: partial incremental build and target batching; you can find out more about these on MSDN.

Tasks are reuseable build actions that define the steps in a target. Tasks are always inside a target. Each task is implemented as a .NET class, and the parameters to a task - where you pass in the items and properties it needs - are just the .NET properties on that task. MSBuild comes with a set of built-in tasks like Delete, Message, GenerateResource (which is much like resgen.exe), plus tasks for the Visual Basic and C# compilers. There's also a special MSBuild task that runs MSBuild on another project. If those aren't enough, you can write your own - start by deriving from Task or ToolTask. Use a "UsingTask" tag to help MSBuild to find the assembly that implements your task. Tasks can also emit items and properties back into the build using Output sub-elements for subsequent tasks and targets to consume.

Every element in MSBuild syntax can have a Condition attribute. These have simple syntax something like conditions in XSL: logical operators "and", "or". parentheses, and so on. You can also use Choose, When, and Otherwise elements to make decisions. Here are some examples, so you can see what they look like:

<Compile Include="MyFile.cs" Condition="'$(Confi guration)'=='debug'"/>

In this example, the item "MyFile.cs" will be created only when the project is building in the debug configuration.

<Target Name="CreateOutputDirectory" Condition="!Exists(‘$(OutputDir)')" />

In this example, the target CreateOutputDirectory and the targets it depends on will run only if the output directory doesn't exist.

More Stories By Xin Yan

Xin Yan has been a software design engineer at Microsoft for over 7 years. He works on Visual Studio developer tools platform team.

More Stories By Dan Moseley

Dan Moseley is a software developer on the MSBuild and Visual Studio Project team.

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Most Recent Comments
Paul 11/28/07 04:17:42 PM EST

I enjoyed the article on MSBuild, well written, concise, and interesting. But I was most interested in reading about Batching. In that section, you refer to an example using the Culture metadata, but I don't see the example you're referring to. Am I missing something?