C# code does not output the XML format i want.

That's invalid XML...

its's just input data so invalid XML is ok. it's the data format that I am focusing on.

Can you post what you're CURRENTLY getting as output for comparison?

Here's the output i get:
<?xml version="1.0" encoding="UTF-8"?>
<AddressTest xmlns="http://www.w3.org/2001/XMLSchema-instance"><AddressDetail> <Address> </Address> </AddressDetail></AddressTest>



this is the ouput i want:
<?xml version="1.0" encoding="UTF-8"?>
<AddressTest xmlns="http://www.w3.org/2001/XMLSchema-instance">
      <AddressDetail>
            <Address>101 state st</Address>
      </AddressDetail>
      <AddressDetail>
            <Address>201 exit st</Address>
      </AddressDetail>
      <AddressDetail>
            <Address>301 entrance st</Address>
      </AddressDetail>
</AddressTest>

So are you just concerned about the line breaks and indents?

Yes line breaks and indents as well as data. i am not getting any data. when i do a debug i see the data in the variable but it does not get to the output

I would just use regular XML serialization instead of doing this manually:

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Xml.Serialization;

namespace WpfApp3
{
    public class ExpertsExchange_29178928
    {

        public ExpertsExchange_29178928()
        {
            // Create test data
            var memberSpansTest = new MemberSpansTest();
            memberSpansTest.MemberSpansDetail.Add(new MemberSpansDetail() { Address = "101 state st" });
            memberSpansTest.MemberSpansDetail.Add(new MemberSpansDetail() { Address = "201 exit st" });
            memberSpansTest.MemberSpansDetail.Add(new MemberSpansDetail() { Address = "301 entrance st" });

            // Serialize data to XML
            using (var ms = new MemoryStream())
            using(var sw = new StreamWriter(ms))
            {
                var xs = new XmlSerializer(typeof(MemberSpansTest));
                xs.Serialize(ms, memberSpansTest);
                var xml = Encoding.UTF8.GetString(ms.ToArray());
            }
        }

        // -------------------------------------------------------------------------------------------------

        [Serializable]
        public class MemberSpansTest
        {
            [XmlElement(ElementName = "MemberSpansDetail")]
            public List<MemberSpansDetail> MemberSpansDetail { get; set; } = new List<MemberSpansDetail>();
        }

        [Serializable]
        public class MemberSpansDetail
        {
            public string Address { get; set; }
        }


    }
}

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Result:

<?xml version="1.0"?>
<MemberSpansTest xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <MemberSpansDetail>
    <Address>101 state st</Address>
  </MemberSpansDetail>
  <MemberSpansDetail>
    <Address>201 exit st</Address>
  </MemberSpansDetail>
  <MemberSpansDetail>
    <Address>301 entrance st</Address>
  </MemberSpansDetail>
</MemberSpansTest>

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I used the XML tags you mentioned in your original question/post (you changed to AddressDetail in another comment, but that's just a matter of changing the XmlElement name).

Did you put it in a console application in VS?

this was very good! thank you for your quick response and great solution!