How to rename <ArrayOf> XML attribute that generated after serializing List of objects

The most reliable way is to declare an outermost DTO class:

[XmlRoot("myOuterElement")]
public class MyOuterMessage {
    [XmlElement("item")]
    public List<TestObject> Items {get;set;}
}

and serialize that (i.e. put your list into another object).

You can avoid a wrapper class, but I wouldn't:

class Program
{
    static void Main()
    {
        XmlSerializer ser = new XmlSerializer(typeof(List<Foo>),
             new XmlRootAttribute("Flibble"));
        List<Foo> foos = new List<Foo> {
            new Foo {Bar = "abc"},
            new Foo {Bar = "def"}
        };
        ser.Serialize(Console.Out, foos);
    }
}

public class Foo
{
    public string Bar { get; set; }
}

The problem with this is that when you use custom attributes you need to be very careful to store and re-use the serializer, otherwise you get lots of dynamic assemblies loaded into memory. This is avoided if you just use the XmlSerializer(Type) constructor, as it caches this internally automatically.

Change the following line from:

XmlSerializer serializer = new XmlSerializer(typeof(List<TestObject>));

To:

XmlRootAttribute root = new XmlRootAttribute("TestObjects");     

XmlSerializer serializer = new XmlSerializer(typeof(List<TestObject>), root);

It should work.

You can add an additional parameter to the XmlSerializer constructor to essentially name the root element.

XmlSerializer xsSubmit = new XmlSerializer(typeof(List<DropDownOption>), new XmlRootAttribute("DropDownOptions"));

This would result in the following structure:

<DropDownOptions xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> 
    <DropDownOption>
      <ID>1</ID>
      <Description>foo</Description>
    </DropDownOption>
    <DropDownOption>
      <ID>2</ID>
      <Description>bar</Description>
    </DropDownOption>
</DropDownOptions>