空のクラスをXMLシリアライズした時にドキュメントも空になる

class SampleClass
{
    string value1;
    string value2;
}

SampleClass obj = new SampleClass();
StringWriter wr = new StringWriter();
            
XmlSerializer serializer = new XmlSerializer(obj.GetType());
serializer.Serialize(wr, obj);

Console.WriteLine(writer.ToString());

"<?xml version="1.0" encoding="utf-16"?>
<SampleClass xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="">
</SampleClass>

"<?xml version="1.0" encoding="utf-16"?>
<SampleClass xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="">
<value1></value1>
<value2></value2>
</SampleClass >"

class SampleClass
{
    public SampleClass()
    {
        value1 = "";
        value2 = "";
    }

    public string value1;
    public string value2;
}

SampleClass obj = new SampleClass();
obj.value1 = "";
obj.value2 = "";