[XmlInclude(typeof(BookCatalogItem))]
    [XmlInclude(typeof(NewspaperCatalogItem))]
    [XmlInclude(typeof(PattentCatalogItem))]
    [XmlRoot("Catalog", Namespace="https://Kashin/CatalogSchema.xsd")]
    [XmlInclude(typeof(CatalogItem))] 
    public class CatalogList: List<CatalogItem>
    {
    }

     CatalogList cl = new CatalogList();
            cl.Add(new BookCatalogItem("AAAA1", "BBBB", "CCC", "DDDD", DateTime.Now, 10, "dddd", "789-1-12-123456-1"));
            cl.Add(new PattentCatalogItem("dasd", "asdq", "Russia", 29, DateTime.Now.Date, DateTime.Now, 20, "dsada"));
            cl.Add(new BookCatalogItem("AAAA", "BBBB", "CCC", "DDDD", DateTime.Now.Date, 10, "dddd", "789-1-12-123456-3"));
 
            XmlSerializer xs = new XmlSerializer(typeof(CatalogList));
            using (StreamWriter s = new StreamWriter("catalog.xml"))
                xs.Serialize(s, cl);

<?xml version="1.0" encoding="utf-8"?>
<Catalog xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="https://Kashin/CatalogSchema.xsd">
  <CatalogItem xsi:type="Book">
    <Title>AAAA1</Title>
    <Page>10</Page>
    <Author>BBBB</Author>
    <City>CCC</City>
    <Issue>DDDD</Issue>
    <DateIssue>2014-03-21T09:24:51.4555401+04:00</DateIssue>
    <PS>dddd</PS>
    <ISBN>789-1-12-123456-1</ISBN>
  </CatalogItem>
  <CatalogItem xsi:type="Pattent">
    <Title>dasd</Title>
    <Page>20</Page>
    <Inventor>asdq</Inventor>
    <Country>Russia</Country>
    <Number>29</Number>
    <Date>2014-03-21T00:00:00+04:00</Date>
    <DateIssue>2014-03-21T09:24:51.4565416+04:00</DateIssue>
    <PS>dsada</PS>
  </CatalogItem>
</Catalog>

<CatalogItem xsi:type="Pattent">

<Pattent>

<?xml version="1.0" encoding="utf-8"?>
<xs:schema id="CatalogSchema"
    targetNamespace="https://Kashin/CatalogSchema.xsd"
    elementFormDefault="qualified"
    xmlns="https://Kashin/CatalogSchema.xsd"
    xmlns:mstns="https://Kashin/CatalogSchema.xsd"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
>
  
  <xs:element name="Catalog">    
    <xs:complexType>
      <xs:choice maxOccurs="unbounded">
        <xs:element name="CatalogItem">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="Title" type="xs:string"></xs:element>
              <xs:element name="Author" type="xs:string"></xs:element>
              <xs:element name="City" type="xs:string"></xs:element>
              <xs:element name="Issue" type="xs:string"></xs:element>
              <xs:element name="DateIssue" type="xs:date"></xs:element>
              <xs:element name="Page" type="xs:int"></xs:element>
              <xs:element name="PS" type="xs:string"></xs:element>
              <xs:element name="ISBN">
                <xs:simpleType>
                  <xs:restriction base="xs:string">
                    <xs:pattern value="[7-9]{3}-[0-9]-[0-9]{2}-[0-9]{6}-[0-9]"/>
                  </xs:restriction>
                </xs:simpleType>
              </xs:element>
            </xs:sequence>           
          </xs:complexType>
        </xs:element>
....

<xs:element name="CatalogItem">

<xs:element name="CatalogItem" xsi:type="Book">