<Port value1="" value2="" value3="">
  <Nodes>
    <Node></Node>
    <Node></Node>
  </Nodes>
</Port>

<?xml version="1.0" encoding="utf-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="Port">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="Nodes">
          <xs:complexType>
            <xs:sequence>
              <xs:element maxOccurs="unbounded" name="Node" />
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
      <xs:attribute name="value1" type="xs:string" use="required" />
      <xs:attribute name="value2" type="xs:string" use="required" />
      <xs:attribute name="value3" type="xs:string" use="required" />
    </xs:complexType>
  </xs:element>
</xs:schema>

var port = new Port { value1 = "my test value", value2 = string.Empty, value3 = string.Empty };
port.Nodes = new[] { new object(), new object(), new object() };
            
using (var writer = new StringWriter(new StringBuilder()))
{
    new XmlSerializer(typeof(Port)).Serialize(writer, port);
    string xml = writer.GetStringBuilder().ToString();
}

<?xml version="1.0" encoding="utf-16"?>
<Port xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" value1="my test value" value2="" value3="">
  <Nodes>
    <Node />
    <Node />
    <Node />
  </Nodes>
</Port>