Overriding project properties with a custom task - visual-c++

Question

Our C++ project uses MSBuild to build on Windows and GNU make on *nix. I'm trying to recreate the functionality of the following single line of GNU make in MSBuild:
GENN_PATH:=$(abspath $(dir $(shell which genn-buildmodel.sh))../userproject/include)
Essentially setting a variable to a path relative to an executable in the path. However, this is proving to be a battle to implement in MSBuild...
The following are the (hopefully) pertinent sections from my vcxproj. For testing purposes I am first setting the variable I want to override to something obvious:
<?xml version="1.0" encoding="utf-8"?>
<Project DefaultTargets="Build" ToolsVersion="12.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
...
<PropertyGroup Label="Configuration">
...
<GeNNUserProject>UNDEFINED</GeNNUserProject>
</PropertyGroup>
Then, in my ClCompile item definitions, I am adding the value of this property to the additional include directories
<ItemDefinitionGroup>
<ClCompile>
...
<AdditionalIncludeDirectories>include;$(GeNNUserProject)</AdditionalIncludeDirectories>
</ClCompile>
...
</ItemDefinitionGroup>
In order to find this path, I'm using the where command and redirecting it's output to a property. Then, from this, I'm finding the include directory and printing it out - this works!
<Target Name="FindUserProjects">
<Exec Command="where genn-buildmodel.bat" ConsoleToMsBuild="true">
<Output TaskParameter="ConsoleOutput" PropertyName="GeNNBuildModelPath" />
</Exec>
<PropertyGroup>
<GeNNUserProject>$([System.IO.Path]::GetFullPath($([System.IO.Path]::GetDirectoryName($(GeNNBuildModelPath)))\..\userproject\include))</GeNNUserProject>
</PropertyGroup>
<Message Text="MAGIC GENN-FINDING! $(GeNNBuildModelPath) -> $(GeNNUserProject)"/>
</Target>
I've tried a variety of ways of making this a dependency of ClCompile including setting the Target as BeforeTargets="PrepareForBuild" and the following:
<PropertyGroup>
<BeforeClCompileTargets>
FindUserProjects;
$(BeforeClCompileTargets);
</BeforeClCompileTargets>
</PropertyGroup>
</Project>
Whatever I do, my custom target runs but the property is not being overriden. Google suggests that if properties are overriden in depencies they should be visible from targets and from digging into Microsoft.CPP*.targets this is what setting BeforeClCompileTargets is doing.

Answer 1

The problem here was not that the target wasn't setting the property, it's that the AdditionalIncludeDirectories item metadata was being set from the original value. The solution is to set this directly from the target instead:
<ItemGroup>
<ClCompile>
<AdditionalIncludeDirectories>%(AdditionalIncludeDirectories);$([System.IO.Path]::GetFullPath($([System.IO.Path]::GetDirectoryName($(GeNNBuildModelPath)))\..\userproject\include))</AdditionalIncludeDirectories>
</ClCompile>
</ItemGroup>