Conditional compilation and framework targets

An alternative that is working for me so far is to add the following to the project file:

 <PropertyGroup>
    <DefineConstants Condition=" !$(DefineConstants.Contains(';NET')) ">$(DefineConstants);$(TargetFrameworkVersion.Replace("v", "NET").Replace(".", ""))</DefineConstants>
    <DefineConstants Condition=" $(DefineConstants.Contains(';NET')) ">$(DefineConstants.Remove($(DefineConstants.LastIndexOf(";NET"))));$(TargetFrameworkVersion.Replace("v", "NET").Replace(".", ""))</DefineConstants>
  </PropertyGroup>

This takes the value of TargetFrameworkVersion property, which is like "v3.5", replaces the "v" and "." to get "NET35" (using the new Property Functions feature). It then removes any existing "NETxx" value and adds it to the end of the DefinedConstants. It may be possible to streamline this, but I haven't got the time to fiddle.

Looking on the Build tab of the project properties in VS you will see the resulting value in the conditional compilation symbols section. Changing the target framework version on the Application tab then changes the symbol automatically. You can then use #if NETxx preprocessor directives in the usual way. Changing the project in VS does not seem to lose the custom PropertyGroup.

Note that this doesn't give seem to give you anything different for the Client Profile target options, but that's not an issue for me.

One of the best ways to accomplish this is to create different build configurations in your project:

<PropertyGroup Condition="  '$(Framework)' == 'NET20' ">
  <DefineConstants>NET20</DefineConstants>
  <OutputPath>bin\$(Configuration)\$(Framework)</OutputPath>
</PropertyGroup>


<PropertyGroup Condition="  '$(Framework)' == 'NET35' ">
  <DefineConstants>NET35</DefineConstants>
  <OutputPath>bin\$(Configuration)\$(Framework)</OutputPath>
</PropertyGroup>

And in one of your default configurations:

<Framework Condition=" '$(Framework)' == '' ">NET35</Framework>

Which would set the default if it wasn't defined anywhere else. In the above case the OutputPath will give you a separate assembly each time you build each version.

Then create a AfterBuild target to compile your different versions:

<Target Name="AfterBuild">
  <MSBuild Condition=" '$(Framework)' != 'NET20'"
    Projects="$(MSBuildProjectFile)"
    Properties="Framework=NET20"
    RunEachTargetSeparately="true"  />
</Target>

This example will recompile the entire project with the Framework variable set to NET20 after the first build (compiling both and assuming that the first build was the default NET35 from above). Each compile will have the conditional define values set correctly.

In this manner you can even exclude certain files in the project file if you want w/o having to #ifdef the files:

<Compile Include="SomeNet20SpecificClass.cs" Condition=" '$(Framework)' == 'NET20' " />

or even references

<Reference Include="Some.Assembly" Condition="" '$(Framework)' == 'NET20' " >
  <HintPath>..\Lib\$(Framework)\Some.Assembly.dll</HintPath>
</Reference>

I had problems with these solutions, possibly because my initial constants were pre-built by these properties.

<DefineConstants />
<DefineDebug>true</DefineDebug>
<DefineTrace>true</DefineTrace>
<DebugSymbols>true</DebugSymbols>

Visual Studio 2010 also threw up an error because of the semi-colons, claiming they are illegal characters. The error message gave me a hint as I could see the pre-built constants seperated by commas, eventually followed by my "illegal" semi-colon. After some reformatting and massaging I was able to come up with a solution that works for me.

<PropertyGroup>
  <!-- Adding a custom constant will auto-magically append a comma and space to the pre-built constants.    -->
  <!-- Move the comma delimiter to the end of each constant and remove the trailing comma when we're done.  -->
  <DefineConstants Condition=" !$(DefineConstants.Contains(', NET')) ">$(DefineConstants)$(TargetFrameworkVersion.Replace("v", "NET").Replace(".", "")), </DefineConstants>
  <DefineConstants Condition=" $(DefineConstants.Contains(', NET')) ">$(DefineConstants.Remove($(DefineConstants.LastIndexOf(", NET"))))$(TargetFrameworkVersion.Replace("v", "NET").Replace(".", "")), </DefineConstants>
  <DefineConstants Condition=" $(TargetFrameworkVersion.Replace('v', '')) >= 2.0 ">$(DefineConstants)NET_20_OR_GREATER, </DefineConstants>
  <DefineConstants Condition=" $(TargetFrameworkVersion.Replace('v', '')) >= 3.5 ">$(DefineConstants)NET_35_OR_GREATER</DefineConstants>
  <DefineConstants Condition=" $(DefineConstants.EndsWith(', ')) ">$(DefineConstants.Remove($(DefineConstants.LastIndexOf(", "))))</DefineConstants>
</PropertyGroup>

I would post a screenshot of the Advanced Compiler Settings dialog (opened by clicking the "Advanced Compile Options..." button on the Compile tab of your project). But as a new user, I lack the rep to do so. If you could see the screenshot, you would see the custom constants auto-filled by the property group and then you'd be saying, "I gotta get me some of that."

EDIT: Got that rep surprisingly fast.. Thanks guys! Here's that screenshot: