xml.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Xml.Serialization;

public class EnquiryResponseInfo
{
    public string EnquiryId { get; set; }
    public EnquiryViewModel Enquiry { get; set; }
}
public class EnquiryViewModel
{
    public string Name { get; set; }
    public string Address { get; set; }
}
static class P
{
    static void Main()
    {
        var list = new List<EnquiryResponseInfo>
        {
            new EnquiryResponseInfo
            {
                EnquiryId = "xxx",
                Enquiry = null,
            },
            new EnquiryResponseInfo
            {
                EnquiryId = "yyy",
                Enquiry = new EnquiryViewModel
                {
                    Name = "Test user",
                    Address = "Test Address"
                }
            },
        };
        var xml = Serialize(list);
        Console.WriteLine(xml);
/*
<ArrayOfEnquiryResponseInfo xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <EnquiryResponseInfo>
    <EnquiryId>xxx</EnquiryId>
  </EnquiryResponseInfo>
  <EnquiryResponseInfo>
    <EnquiryId>yyy</EnquiryId>
    <Enquiry>
      <Name>Test user</Name>
      <Address>Test Address</Address>
    </Enquiry>
  </EnquiryResponseInfo>
</ArrayOfEnquiryResponseInfo>
*/
    }

    public static string Serialize<T>(T toSerialize)
    {
        XmlSerializer xmlSerializer = new XmlSerializer(typeof(T));
        using (StringWriter textWriter = new StringWriter())
        {
            xmlSerializer.Serialize(textWriter, toSerialize);
            return textWriter.ToString();
        }
    }
}