Your question has many different components:
In order to make the property OsobaNiefizyczna go in the correct namespace, decorate it with XmlElement and set the Namespace property correctly.
In order to add additional namespaces to the root document object, attach the [XmlNamespaceDeclarations] attribute to a public XmlSerializerNamespaces xmlsn synthetic property returning the namespaces. Also set the XmlRootAttribute.Namespace properly on the root object.
To make the root object have a schemaLocation attribute, you must add a synthetic property along the lines of the answer in How to add xsi schemalocation to root c # object XmlSerializer -- but use a property instead of a field. If you use a field you will actually add to the footprint of your class in memory.
To omit the standard xsd namespace (though I recommend you do not), use the XmlSerializer.Serialize(XmlWriter, Object, XmlSerializerNamespaces) method and pass in only the namespaces you want to see.
To make an empty Zalaczniki element show up, you need to set the property value to string.Empty.
Putting these together we get:
public static class NameSpaces
{
static readonly XmlSerializerNamespaces namespaces;
static NameSpaces()
{
namespaces = new XmlSerializerNamespaces();
namespaces.Add("", NameSpaces.Default);
namespaces.Add("xsi", NameSpaces.Xsi);
namespaces.Add("etd", NameSpaces.Etd);
namespaces.Add("zzu", NameSpaces.Zzu);
}
public static XmlSerializerNamespaces XmlSerializerNamespaces
{
get
{
return namespaces;
}
}
public const string Default = "http://crd.gov.pl/wzor/2014/12/08/1887/";
public const string Etd = "http://crd.gov.pl/xml/schematy/dziedzinowe/mf/2011/06/21/eD/DefinicjeTypy/";
public const string Xsi = "http://www.w3.org/2001/XMLSchema-instance";
public const string Zzu = "http://crd.gov.pl/xml/schematy/dziedzinowe/mf/2011/10/07/eD/ORDZU/";
public const string SchemaLocation = "http://crd.gov.pl/wzor/2014/12/08/1887/ http://crd.gov.pl/wzor/2014/12/08/1887/schemat.xsd";
}
[XmlRoot("Deklaracja", Namespace = NameSpaces.Default)]
public class EPIT11V21
{
public EPIT11V21() {
this.Zalaczniki = string.Empty;
}
public EPIT11V21(int XPDeclarationID) : this()
{
this.Pouczenie = "Za uchybienie obowiązkom płatnika grozi odpowiedzialność przewidziana w Kodeksie karnym skarbowym.";
}
[XmlAttribute("schemaLocation", Namespace = "http://www.w3.org/2001/XMLSchema-instance")]
public string XSDSchemaLocation
{
get
{
return NameSpaces.SchemaLocation;
}
set {
// Do nothing - fake property.
}
}
public Podmiot1PIT11V21 Podmiot1 = new Podmiot1PIT11V21();
public String Pouczenie { get; set; }
public String Zalaczniki { get; set; }
[XmlNamespaceDeclarations]
public XmlSerializerNamespaces xmlsn
{
get
{
return NameSpaces.XmlSerializerNamespaces;
}
set {
// Do nothing - fake property.
}
}
}
public class Podmiot1PIT11V21
{
public Podmiot1PIT11V21() { }
[XmlAttribute("rola")]
public string rola = "Płatnik";
[XmlElement("OsobaNieFizyczna", Namespace = NameSpaces.Etd)]
public OsobaNiefizycznaPIT11V21 OsobaNieFizyczna = new OsobaNiefizycznaPIT11V21();
}
public class OsobaNiefizycznaPIT11V21
{
public OsobaNiefizycznaPIT11V21() { }
public string NIP = "0000000000";
public string PelnaNazwa = "XXXXXXXX";
}
And, to test:
public static class TestClass
{
public static void Test()
{
var curpit11 = new EPIT11V21(10101);
var xml = XmlSerializationHelper.GetXml(curpit11, NameSpaces.XmlSerializerNamespaces);
Debug.WriteLine(xml);
}
}
public static class XmlSerializationHelper
{
public static string GetXml<T>(this T obj)
{
return GetXml(obj, false);
}
public static string GetXml<T>(this T obj, bool omitNamespace)
{
return GetXml(obj, new XmlSerializer(obj.GetType()), omitNamespace);
}
public static string GetXml<T>(this T obj, XmlSerializer serializer)
{
return GetXml(obj, serializer, false);
}
public static string GetXml<T>(T obj, XmlSerializer serializer, bool omitStandardNamespaces)
{
XmlSerializerNamespaces ns = null;
if (omitStandardNamespaces)
{
ns = new XmlSerializerNamespaces();
ns.Add("", ""); // Disable the xmlns:xsi and xmlns:xsd lines.
}
return GetXml(obj, serializer, ns);
}
public static string GetXml<T>(T obj, XmlSerializerNamespaces ns)
{
return GetXml(obj, new XmlSerializer(obj.GetType()), ns);
}
public static string GetXml<T>(T obj, XmlSerializer serializer, XmlSerializerNamespaces ns)
{
using (var textWriter = new StringWriter())
{
XmlWriterSettings settings = new XmlWriterSettings();
settings.Indent = true; // For cosmetic purposes.
settings.IndentChars = " "; // For cosmetic purposes.
using (var xmlWriter = XmlWriter.Create(textWriter, settings))
{
if (ns != null)
serializer.Serialize(xmlWriter, obj, ns);
else
serializer.Serialize(xmlWriter, obj);
}
return textWriter.ToString();
}
}
}
Produces
<Deklaracja xmlns:zzu="http://crd.gov.pl/xml/schematy/dziedzinowe/mf/2011/10/07/eD/ORDZU/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:etd="http://crd.gov.pl/xml/schematy/dziedzinowe/mf/2011/06/21/eD/DefinicjeTypy/" xsi:schemaLocation="http://crd.gov.pl/wzor/2014/12/08/1887/ http://crd.gov.pl/wzor/2014/12/08/1887/schemat.xsd" xmlns="http://crd.gov.pl/wzor/2014/12/08/1887/">
<Podmiot1 rola="Płatnik">
<etd:OsobaNieFizyczna>
<etd:NIP>0000000000</etd:NIP>
<etd:PelnaNazwa>XXXXXXXX</etd:PelnaNazwa>
</etd:OsobaNieFizyczna>
</Podmiot1>
<Pouczenie>Za uchybienie obowiązkom płatnika grozi odpowiedzialność przewidziana w Kodeksie karnym skarbowym.</Pouczenie>
<Zalaczniki />
</Deklaracja>
