XML values with same name to list

If you only need to deserialize, you can use do so using XmlSerializer to the following types:

[XmlRoot(ElementName = "DUE")]
public class DueDTO
{
    [XmlAttribute(AttributeName = "idx")]
    public string Idx { get; set; }
    [XmlText]
    public string Text { get; set; }
}

[XmlRoot(ElementName = "RATE")]
public class RateDTO
{
    [XmlAttribute(AttributeName = "idx")]
    public string Idx { get; set; }
    [XmlText]
    public decimal Text { get; set; }
}

[XmlRoot(ElementName = "INVOICE")]
public partial class InvoicesDTO
{
    [XmlAttribute(AttributeName = "ID")]
    public string Id { get; set; }
    [XmlElement(ElementName = "STATUS")]
    public string Status { get; set; }
    [XmlElement(ElementName = "TOTAL")]
    public decimal Total { get; set; }

    [XmlElement(ElementName = "DUE")]
    public List<DueDTO> Due { get; set; }
    [XmlElement(ElementName = "RATE")]
    public List<RateDTO> Rate { get; set; }
}

Then, to combine the Rate and Due list into a single InvoicesDueDates collection, you can use LINQ, e.g. as follows:

public partial class InvoicesDTO
{
    public InvoicesDueDates[] InvoicesDueDates
    {
        get
        {
            // To make suure we handle cases where only a Rate or Due item of a specific index is present,
            // perform left outer joins with all indices on both Rate and Due.
            // https://learn.microsoft.com/en-us/dotnet/csharp/linq/perform-left-outer-joins
            var query = from i in Due.Select(d => d.Idx).Concat(Rate.Select(r => r.Idx)).Distinct()
                        join due in Due on i equals due.Idx into dueGroup
                        // Throw an exception if we have more than one due item for a given index
                        let due = dueGroup.SingleOrDefault()
                        join rate in Rate on i equals rate.Idx into rateGroup
                        // Throw an exception if we have more than one rate item for a given index
                        let rate = rateGroup.SingleOrDefault()
                        select new InvoicesDueDates { Id = i, DueDate = due == null ? null : due.Text, Rate = rate == null ? (decimal?)null : rate.Text };
            return query.ToArray();
        }
    }
}

public class InvoicesDueDates
{
    public string Id { get; set; }
    public string DueDate { get; set; }
    public decimal? Rate { get; set; }
}

Notes:

• This solution takes advantage of the fact that, when XmlSerializer is deserializing a List<T> property and encounters list elements interleaved with other elements, it will append each list element encountered to the growing list.

• If you re-serialize an InvoicesDTO the result will look like:

    <INVOICE ID="5">
      <STATUS>S</STATUS>
      <TOTAL>3270.00</TOTAL>
      <DUE idx="1">30.11.17</DUE>
      <DUE idx="2">07.12.17</DUE>
      <DUE idx="3">14.12.17</DUE>
      <RATE idx="1">1090.00</RATE>
      <RATE idx="2">1090.00</RATE>
      <RATE idx="3">1090.00</RATE>
    </INVOICE>
  

  Notice that all the information has been retained and re-serialized but the <RATE> and <DUE> sequences have been separated out.

• If you need to re-serialize with interleaved <RATE> and <DUE> elements, you will have to adopt a different strategy, such as the ones from serializing a list of KeyValuePair to XML or Xml Sequence deserialization with RestSharp.

• I auto-generated the DTO classes using https://xmltocsharp.azurewebsites.net/ then modified them to fit my naming contentions.

Sample working .Net fiddle.