Alan Dean

CTO, Developer, Agile Practitioner

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Sunday, February 20, 2011

How to conditionally skip compilation

As I mentioned in my last post, I was having problems getting an MVC web application to play nice with my framework targeting builds. Specifically, my CI server would fall over if I tried to build using Framework 3.5 when the web application was using 4.0 (due to 4.0-specific web.config settings that 3.5 does not recognise). I managed to spend a large portion of the afternoon searching for an answer until I had one of those “eureka” moments. My objective was to simply not build the web application unless the framework was 4.0 (the same would apply to any project that is framework version-specific). I tried putting a conditional on the project element. No Joy. I tried a whole bunch of other things and it was starting to look like I would have to manually configure each project to be built – very nasty, not at all what I want.

My “eureka” moment happened as I was gazing forlornly at a project file. My eye alighted upon an attribute of DefaultTargets="Build" on the project element. Build? I thought… I’ve not seen a target called that. A quick check confirmed my suspicion. The “Build” target is effectively inherited. In a flash I thought I wonder if I can intercept this? I changed the attribute to DefaultTargets="Conditional" and then added the following target and all is well again in Narnia:

<Target Name="Conditional">
<CallTarget Targets="Build" Condition=" '$(TargetFrameworkVersion)' == 'v4.0' " />

Framework version targeting with MSBUILD

I faced a question this weekend: how best should I support conditional compilation by framework? The proximate reason for asking this was a need to use BigInteger in the Cavity project in order to be able to support 128-bit base-36 notation with a minimum of fuss. This is a new value type in Framework 4.0 and up until now Framework 3.5 have been the target for Cavity assemblies. As 3.5 is going to be around for a good while yet (heck, there is still plenty of 2.0 in production today) I didn’t want to orphan support for that version. I also didn’t want to add complexity to my build or maintenance activities. This ruled out brute force options such as creating a branch for 3.5 or creating a new solution and project files for one or other of the framework versions.

A search yielded a varied collection of advice. Some was plainly potty but a couple of StackOverflow answers [1] [2] looked promising. Taking the answers and combining a bit of common sense, I put together a quick spike to verify that I had the gist of it. Here is what I learnt:

  1. The best way to control targeting is to use property parameters with a build file. Here is a .bat file to build release versions targeting each major framework version:
    MSBUILD build.xml /p:Configuration=Release /p:TargetFrameworkVersion=v2.0
    MSBUILD build.xml /p:Configuration=Release /p:TargetFrameworkVersion=v3.5
    MSBUILD build.xml /p:Configuration=Release /p:TargetFrameworkVersion=v4.0

  2. You don’t need to do anything in the build.xml or the .sln file for this to work. You do, however, need to do a little work in the .csproj files:
    • First, you need to decide which framework version you want to work with in Visual Studio (typically, this will be the latest version) and configure TargetFrameworkVersion accordingly:

      <TargetFrameworkVersion Condition=" '$(TargetFrameworkVersion)' == '' ">v4.0</TargetFrameworkVersion>

    • To make the build output obvious, set the OutputPath to use properties:

      <OutputPath>bin\$(Configuration) $(TargetFrameworkVersion)\</OutputPath>

    • Next, you should set up some conditional property groups:
      <PropertyGroup Condition=" '$(TargetFrameworkVersion)' == 'v2.0' ">
      <PropertyGroup Condition=" '$(TargetFrameworkVersion)' == 'v3.5' ">
      <PropertyGroup Condition=" '$(TargetFrameworkVersion)' == 'v4.0' ">

    • This allows you to configure framework-specific assembly references, such as System.Core and Microsoft.CSharp:

      <Reference Include="System.Core" Condition=" '$(TargetFrameworkVersionNumber)' >= '3.5' " />
      <Reference Include="Microsoft.CSharp" Condition=" '$(TargetFrameworkVersionNumber)' >= '4.0' " />

    • You should now be able to target framework versions in a straightforward manner by running the batch file above.

  3. If you want to conditionally include classes, simply apply a framework condition:
    <Compile Include="Class20.cs" Condition=" '$(TargetFrameworkVersion)' == 'v2.0' " />

  4. I also defined constants above to allow conditional compilation at code level:
    namespace Example
        public sealed class Class1
    #if NET20
            public void Net20()
    #if NET35
            public void Net35()
    #if NET40
            public void Net40()

The only pain I have really encountered so far is getting MVC web applications to play nicely.