How to set PreProcessorDefinitions as a task propery for the msbuild task

This can be done without modifying the original project: the first thing done in Microsoft.Cpp.Targets, which is normally one of the last things imported in a normal C++ project, is to check if there's a property called ForceImportBeforeCppTargets and if so, import it.

So suppose you want to add ADDITIONAL to the preprocessor definitions you create a file 'override.props' like this (for full automation use the WritelinesToFile task to create the file):

<?xml version="1.0" encoding="utf-8"?> 
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">

  <ItemDefinitionGroup>
    <ClCompile>
      <PreprocessorDefinitions>%(PreprocessorDefinitions);ADDITIONAL</PreprocessorDefinitions>
    </ClCompile>
  </ItemDefinitionGroup>

</Project>

And call

<MSBuild Projects="demo.vcxproj" 
         Properties="ForceImportBeforeCppTargets=override.props"/>

or from the command line that would be

msbuild demo.vcxproj /p:ForceImportBeforeCppTargets=override.props

Note as richb points out in the comment, the above only works if override.props can be found by msbuild's lookup rules. To make sure it is always found just specify the full path.

Update

Some years later, in msbuild 15 and beyond, there are new ways to customise builds. Same principle as above, but simpler: msbuild will automatically pick up the file named Directory.Build.props in the project file's directory or even all directories above that, without needing additional commandline options (so, also works from within VS without problems). And the project files also became a bit less verbose as well:

<Project>
  <ItemDefinitionGroup>
    <ClCompile>
      <PreprocessorDefinitions>%(PreprocessorDefinitions);ADDITIONAL</PreprocessorDefinitions>
    </ClCompile>
  </ItemDefinitionGroup>
</Project>

You can't do this without modifying demo.vcxproj because you need access to the PreprocessorDefinitions of CLCompile, which is not a PropertyGroup, and thus can't be passed via the MSBuild command line.

You can modify the preprocessor definitions in the GUI via Project Properties -> Configuration Propertis -> C/C++ -> Preprocessor, or edit the XML directly:

  <ClCompile>
    ....
    <PreprocessorDefinitions>$(MyMacro);%(PreprocessorDefinitions)</PreprocessorDefinitions>
  </ClCompile>

In your MSBuild project:

  <Target Name="TestBuild" Returns="@(ManagedTargetPath)">
    <MSBuild Projects="demo.vcxproj" Targets="NormalBuild" Properties="MyMacro=THISGETSSETBUTDOESNOTHING"/>
  </Target>

This is equivalent to running MSBuild.exe as:

  MSBuild demo.vcxproj /p:MyMacro=THISGETSSETBUTDOESNOTHING

Based on @Kevin answer you need to define a user-defined macro in a PropertySheet. Then create a Preprocessor which refers to the user-defined macro. You can now use the new preprocessor value in your code. Finally, for the build, you can change the value of user-defined macro with /p flag. In here I defined a user-defined value like mymacro and a preprocessor value like VAL. Now you can simply compile the project with /p:mymacro="\"some thing new\"".

#include <iostream>


int main() {
    std::cout << VAL << std::endl;

    getchar();
}

yourproject.vcxproj:

<ClCompile>
  ...
  <PreprocessorDefinitions>VAL=$(mymacro);%(PreprocessorDefinitions)</PreprocessorDefinitions>
</ClCompile>

msbuild yourproject.vcxproj /p:mymacro="\"some thing new\""