<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="root">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="item">
          <xs:complexType>
            <xs:simpleContent>
              <xs:extension base="xs:int">
                <xs:attribute name="name" type="xs:string" use="required" />
                <xs:attribute name="type" type="xs:string" use="required" />
              </xs:extension>
            </xs:simpleContent>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

[System.Serializable]
[XmlType(AnonymousType = true)]
[XmlRoot(ElementName = "root", Namespace = "", IsNullable = false)]
public class Root
{
  [XmlElement("item")]
  public RootItem Item { get; set; }
}

[System.Serializable]
[XmlType(AnonymousType = true)]
public class RootItem
{
  [XmlAttribute("name")]
  public string Name { get; set; }
  [XmlAttribute("type")]
  public string Type { get; set; }
  [XmlText]
  public int Value { get; set; }
}

try
{
  using (var sr = new StringReader("<root><item name='foo' type='bar'>123</item></root>"))
  {
    if (!(new XmlSerializer(typeof(Root)).Deserialize(sr) is Root r))
      throw new InvalidOperationException("Not instace of Root");
    Console.WriteLine(r);
  }
}
catch(Exception e)
{
  Console.WriteLine(e);
}

public class RootItemBase
{
  [XmlAttribute("name")]
  public string Name { get; set; }
  [XmlAttribute("type")]
  public string Type { get; set; }
}

[System.Serializable]
[XmlType(AnonymousType = true)]
public class RootItem : RootItemBase
{
  [XmlText]
  public int Value { get; set; }
}

System.InvalidOperationException: There was an error reflecting type 'test2.Root'.
  ---> System.InvalidOperationException: There was an error reflecting property 'Item'.
  ---> System.InvalidOperationException: There was an error reflecting type 'test2.RootItem'.
  ---> System.InvalidOperationException: Cannot serialize object of type 'test2.RootItem'.
 Consider changing type of XmlText member 'test2.RootItem.Value' from System.Int32 to string or string array. 

public class RootItemBase
{
}

[System.Serializable]
[XmlType(AnonymousType = true)]
public class RootItem : RootItemBase
{
  [XmlAttribute("name")]
  public string Name { get; set; }
  [XmlAttribute("type")]
  public string Type { get; set; }
  [XmlText]
  public int Value { get; set; }
}