foo1.cs

    private static bool CheckHasValidRefOperands(
      [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 false;
      }

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

      return thenRefExpression != null;
    }

foo2.cs

    private static bool CheckHasValidRefOperands(
      [NotNull] IConditionalTernaryExpression expression,
      [NotNull] ICSharpExpression thenResult,
      [NotNull] ICSharpExpression elseResult,
      [NotNull] IHighlightingConsumer consumer)
    {
      switch (thenResult, elseResult)
      {
        case (IRefExpression _, IRefExpression _):
          return true;
        case (IRefExpression thenRefExpression, _):
          consumer.AddHighlighting(new RefConditionalNeedsTwoRefsError(expression, thenRefExpression));
          return false;
        case (_, IRefExpression elseRefExpression):
          consumer.AddHighlighting(new RefConditionalNeedsTwoRefsError(expression, elseRefExpression));
          return false;
        default:
          return false;
      }
    }