foo1.cs

    [CanBeNull]
    private static ITreeNode GetHighlightingNodeFromExpressionStatement([NotNull] IExpressionStatement statement)
    {
      var expression = statement.Expression;

      while (expression != null)
      {
        var reference = expression as IReferenceExpression;
        if (reference != null)
        {
          expression = reference.QualifierExpression;
          if (expression == null)
            return reference.NameIdentifier;
        }
        else
        {
          var invocation = expression as IInvocationExpression;
          if (invocation != null)
          {
            expression = invocation.InvokedExpression;
          }
          else
          {
            var elementAccess = expression as IElementAccessExpression; // not converted
            expression = elementAccess != null ? elementAccess.Operand : null;
          }
        }
      }

      return null;
    }

foo2.cs

    [CanBeNull]
    private static ITreeNode GetHighlightingNodeFromExpressionStatement([NotNull] IExpressionStatement statement)
    {
      var expression = statement.Expression;

      while (expression != null)
      {
        expression = expression switch {
          IReferenceExpression referenceExpression => referenceExpression.QualifierExpression ?? reference.NameIdentifier,
          IInvocationExpression invocationExpression => invocationExpression.InvokedExpression,
          IElementAccessExpression elementAccessExpression => elementAccessExpression.Operand,
          _ => null
        };
      }

      return null;
    }