foo1.cs

    private void CheckRefConditionalIssues(
      [NotNull] IConditionalTernaryExpression expression,
      [NotNull] ICSharpExpression thenResult,
      [NotNull] ICSharpExpression elseResult,
      [NotNull] IHighlightingConsumer consumer)
    {
      var thenRefExpression = thenResult as IRefExpression;
      var elseRefExpression = elseResult as IRefExpression;


      if (elseRefExpression != null && thenRefExpression == null)
      {
        consumer.AddHighlighting(new RefConditionalNeedsTwoRefsError(expression, elseRefExpression));
        return;
      }

      if (elseRefExpression == null && thenRefExpression != null)
      {
        consumer.AddHighlighting(new RefConditionalNeedsTwoRefsError(expression, thenRefExpression));
        return;
      }

      if (thenRefExpression != null && elseRefExpression != null)
      {
        // ..
      }
    }

foo2.cs

    private void CheckRefConditionalIssues(
      [NotNull] IConditionalTernaryExpression expression,
      [NotNull] ICSharpExpression thenResult,
      [NotNull] ICSharpExpression elseResult,
      [NotNull] IHighlightingConsumer consumer)
    {
      switch (thenResult, elseResult)
      {
        case (IRefExpression _, IRefExpression _):
          // ..
          break;

        case (IRefExpression thenRefExpression, _):
          consumer.AddHighlighting(new RefConditionalNeedsTwoRefsError(expression, thenRefExpression));
          return;
          
        case (_, IRefExpression elseRefExpression):
          consumer.AddHighlighting(new RefConditionalNeedsTwoRefsError(expression, elseRefExpression));
          return;
      }
    }