Why XSD.exe does not add XmlArrayItemAttribute for the derived type in the list item?

I have this XSD schema. I used XSD.exe to create the cs file, but only
XmlArrayItemAttribute for the base type is specified for the list item
(Not the derived type). Why is the derived type not added? Can anyone
tell me how to solve this problem?

My XSD Schema:

<?xml version = "1.0" encoding = "UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
targetNamespace="http://example.org/" xmlns="http://
example.org/" elementFormDefault="qualified">
<xsd:element name="baseInstance" type="MyBaseType" abstract="true" / <xsd:complexType name="MyBaseType">
<xsd:sequence>
<xsd:element name="Field1" type="xsd:string"/>
<xsd:element name="Field2" type="xsd:string"/>
</xsd:sequence>
</xsd:complexType>


<xsd:complexType name="BaseListType">
<xsd:sequence>
<xsd:element maxOccurs="unbounded" minOccurs="0"
ref="baseInstance" />
</xsd:sequence>
</xsd:complexType>
<xsd:element name="BaseList" type="BaseListType" />

<xsd:element name="derivedAInstance" type="DerivedTypeA"
substitutionGroup="baseInstance" />
<xsd:complexType name="DerivedTypeA">
<xsd:complexContent>
<xsd:extension base="MyBaseType">
<xsd:attribute name="ExtraInfoForA" type="xsd:string"/>
</xsd:extension>
</xsd:complexContent>
</xsd:complexType>

<xsd:element name="derivedBInstance" type="DerivedTypeB"
substitutionGroup="baseInstance" />
<xsd:complexType name="DerivedTypeB">
<xsd:complexContent>
<xsd:extension base="MyBaseType">
<xsd:attribute name="ExtraInfoForB" type="xsd:string"/>
</xsd:extension>
</xsd:complexContent>
</xsd:complexType>

<xsd:element name="myThings" type="MyThingsType" />
<xsd:complexType name="MyThingsType">
<xsd:sequence>
<xsd:element ref="BaseList" />
</xsd:sequence>
</xsd:complexType>

</xsd:schema>


The following code for the MyThingsType class was generated. Why
derivedAInstance and derivedBInstance are not added?


/// <remarks/>
[System.CodeDom.Compiler.GeneratedCodeAttribute("xsd",
"2.0.50727.312")]
[System.SerializableAttribute()]
[System.Diagnostics.DebuggerStepThroughAttribute()]
[System.ComponentModel.DesignerCategoryAttribute("code")]
[System.Xml.Serialization.XmlTypeAttribute(Namespace="http://
example.org/")]
[System.Xml.Serialization.XmlRootAttribute("myThings",
Namespace="http://example.org/", IsNullable=false)]
public partial class MyThingsType {

private MyBaseType[] baseListField;

/// <remarks/>
[System.Xml.Serialization.XmlArrayItemAttribute("baseInstance",
IsNullable=false)] <-- Why derivedAInstance and derivedBInstance
are not added here by xsd.exe?
public MyBaseType[] BaseList {
get {
return this.baseListField;
}
set {
this.baseListField = value;
}
}
}

Regards,
Peter