When using ISerializable with DataContractSerializer, how do I stop the serializer from outputting type information?

To get more control over serialization, I have converted a class from [DataContract] to [Serializable], implementing both GetObjectData and the special deserializing constructor. When I do this, the XML emitted now has type information applied to all elements. I don't want this superfluous information, and I'm wondering how to inform the serializer to not output it.

Here's the sample code that uses [DataContract]:

[DataContract(Namespace = "")]
class Test 
{
    public Test() { }
    [DataMember]
    public Nullable<int> NullableNumber = 7;
    [DataMember]
    public int Number = 5;

    public static void Go()
    {
        var test = new Test();
        var dcs = new DataContractSerializer(typeof(Test));
        using (var s = new StreamWriter("test.xml"))
        {
            dcs.WriteObject(s.BaseStream, test);
        }
    }        
}

This outputs the following XML (notice no type info on Nullable Number and Number--this is the desired output):

<Test xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
  <NullableNumber>7</NullableNumber>
  <Number>5</Number>
</Test>

If I modify the above code as follows (adding [Serializable], : ISerializable, and the two serialization methods):

[Serializable]
class Test : ISerializable
{
    public Test() { }
    public Nullable<int> NullableNumber = 7;
    public int Number = 5;

    public static void Go()
    {
        var test = new Test();
        var dcs = new DataContractSerializer(typeof(Test));
        using (var s = new StreamWriter("test.xml"))
        {
            dcs.WriteObject(s.BaseStream, test);
        }
    }        
    public Test(SerializationInfo info, StreamingContext context)
    {
        NullableNumber = info.GetInt32("NullableNumber");
        Number = info.GetInt32("Number");
    }

    public void G