xml.cs

using System;
using System.Xml.Serialization;

public class EnquiryResponseInfo
{
    public string EnquiryId { get; set; }
    public EnquiryViewModel Enquiry { get; set; }
}
public class EnquiryViewModel
{
    public int Whatever { get; set; }
}
static class P
{
    static void Main()
    {
        var ser = new XmlSerializer(typeof(EnquiryResponseInfo));
        ser.Serialize(Console.Out, new EnquiryResponseInfo {
            Enquiry = null, EnquiryId = "abc" });
/* output:
<?xml version="1.0" encoding="ibm850"?>
<EnquiryResponseInfo xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <EnquiryId>abc</EnquiryId>
</EnquiryResponseInfo>
*/
        Console.WriteLine();
        Console.WriteLine();


        ser.Serialize(Console.Out, new EnquiryResponseInfo {
            Enquiry = new EnquiryViewModel { Whatever = 123 }, EnquiryId = "abc" });
/* output:
<?xml version="1.0" encoding="ibm850"?>
<EnquiryResponseInfo xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <EnquiryId>abc</EnquiryId>
  <Enquiry>
    <Whatever>123</Whatever>
  </Enquiry>
</EnquiryResponseInfo>
*/
    }
}