code: private String calculateProjectRelativeDirectory( VCProject vcProject ) throws MojoExecutionException
      {
          String relProjectDir = "";
          try
          {
              relProjectDir = getRelativeFile( vcProject.getBaseDirectory(), 
                      vcProject.getFile().getParentFile() ).getPath();
              if ( ".".equals( relProjectDir ) )
              {
                  relProjectDir = "";
              }
              else
              {
                  relProjectDir += "\\";
              }
          }
          catch ( IOException ioe )
          {
              throw new MojoExecutionException( ioe.getMessage(), ioe );
          }
          return relProjectDir;
      }
docs: Calculate the relative path between the project and it's base directory.
  For a project (vcxproj) file this will be an empty string.
  For a solution (sln) file this will be the path from the soltuion to the project terminated with a \
  @param vcProject the parsed project
  @return a String containing the relative path
  @throws MojoExecutionException if there is an IOException checking the file system