foo1.cs

    public void Run(ITreeNode element, ElementProblemAnalyzerData data, IHighlightingConsumer consumer)
    {
      var warningDirective = element as IWarningDirective;
      if (warningDirective != null)
      {
        var messageToken = "#warning directive";

        var tokenNode = warningDirective.Message;
        if (tokenNode != null) messageToken = tokenNode.GetText();

        consumer.AddHighlighting(new WarningPPDirectiveWarning(warningDirective, messageToken));
        return;
      }

      var errorDirective = element as IErrorDirective;
      if (errorDirective != null)
      {
        var message = "#error directive";

        var messageToken = errorDirective.Message;
        if (messageToken != null) message = messageToken.GetText();

        consumer.AddHighlighting(new ErrorPPDirectiveError(errorDirective, message));
        return;
      }
      
      var invalidDirective = element as IPreprocessorErrorDirective;
      if (invalidDirective != null)
      {
        consumer.AddHighlighting(new InvalidPPDirectiveError(invalidDirective));
        return;
      }

      var startRegion = element as IStartRegion;
      if (startRegion != null)
      {
        var daemonStageProcess = (CSharpDaemonStageProcessBase) consumer.Process;

        var fileStructure = daemonStageProcess.FileStructure.NotNull();
        if (fileStructure[startRegion] == null)
        {
          consumer.AddHighlighting(new UnmatchedRegionPPDirectiveError(startRegion));
          return;
        }
      }

      var endRegion = element as IEndRegion;
      if (endRegion != null)
      {
        var daemonStageProcess = (CSharpDaemonStageProcessBase) consumer.Process;

        var fileStructure = daemonStageProcess.FileStructure.NotNull();
        if (fileStructure[endRegion] == null)
        {
          consumer.AddHighlighting(new UnmatchedRegionPPDirectiveError(endRegion));
          return;
        }
      }
    }

foo2.cs

    public void Run(ITreeNode element, ElementProblemAnalyzerData data, IHighlightingConsumer consumer)
    {
      switch (element)
      {
        case IWarningDirective warningDirective:
        {
          var messageToken = "#warning directive";

          var tokenNode = warningDirective.Message;
          if (tokenNode != null) messageToken = tokenNode.GetText();

          consumer.AddHighlighting(new WarningPPDirectiveWarning(warningDirective, messageToken));
          return;
        }
        
        case IErrorDirective errorDirective:
        {
          var message = "#error directive";

          var messageToken = errorDirective.Message;
          if (messageToken != null) message = messageToken.GetText();

          consumer.AddHighlighting(new ErrorPPDirectiveError(errorDirective, message));
          return;
        }
        
        case IPreprocessorErrorDirective invalidDirective:
        {
          consumer.AddHighlighting(new InvalidPPDirectiveError(invalidDirective));
          return;
        }
          
        case IStartRegion startRegion:
        {
          var daemonStageProcess = (CSharpDaemonStageProcessBase) consumer.Process;

          var fileStructure = daemonStageProcess.FileStructure.NotNull();
          if (fileStructure[startRegion] == null)
            consumer.AddHighlighting(new UnmatchedRegionPPDirectiveError(startRegion));
          return;
        }
        
        case IEndRegion endRegion:
        {
          var daemonStageProcess = (CSharpDaemonStageProcessBase) consumer.Process;

          var fileStructure = daemonStageProcess.FileStructure.NotNull();
          if (fileStructure[endRegion] == null)
            consumer.AddHighlighting(new UnmatchedRegionPPDirectiveError(endRegion));
          return;
        }
      }
    }