XML validation error using nested XSD schema - Type not declared

Question

I am using a nested XSD schema to validate an XML document. The imported XSDs use their own target namespaces and I can validate the sample XML given below using Liquid XML Studio. But when I run the validation using my C# code below, it fails with the type declaration error (see below). I have spend alot of time trying to figure out, but no luck:

Main XSD Schema (DataItem.xsd):

<?xml version="1.0" encoding="utf-8" ?>
<xs:schema xmlns:DataNumeric="Doc.DataNumeric" xmlns:DataYesNo="Doc.DataYesNo"  attributeFormDefault="qualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:import schemaLocation="DataNumeric.xsd" namespace="Doc.DataNumeric" />
  <xs:import schemaLocation="DataYesNo.xsd" namespace="Doc.DataYesNo" />
  <xs:complexType name="tDataItem">
      <xs:choice>
        <xs:element name="DataNumeric" type="DataNumeric:tDataNumeric" />
        <xs:element name="DataYesNo" type="DataYesNo:tDataYesNo" />
      </xs:choice>
  </xs:complexType>
  <xs:element name="DataItem" type="tDataItem" />  
</xs:schema>

Included XSD Schema (DataNumeric.xsd):

**<?xml version="1.0" encoding="utf-8" ?>
<xs:schema xmlns:DataNumeric="Doc.DataNumeric" elementFormDefault="qualified" targetNamespace="Doc.DataNumeric" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:complexType name="tDataNumeric">
    <xs:sequence>
      <xs:element name="Answer" type="xs:double" />
    </xs:sequence>
  </xs:complexType>
  <xs:element name="DataNumeric" type="DataNumeric:tDataNumeric" />
</xs:schema>**

The XML:

<DataItem>
  <DataNumeric xmlns:DataNumeric="Doc.DataNumeric">
    <DataNumeric:Answer>37.8</DataNumeric:Answer>
  </DataNumeric>
</DataItem>

Validation Error:

XmlSchemaValidationException: Type 'Doc.DataNumeric:tDataNumeric' is not declared.

C# Validation Code:

XDocument xDoc = XDocument.Parse(xxxxxxx);
string xsdPath = ConfigUtils.GetXsdPath(XsdSchemaIdentifier.HHDataItem);
FileStream fs = new FileStream(xsdPath, FileMode.Open);
XmlReader reader = XmlReader.Create(fs);
XmlSchemaSet xss = new XmlSchemaSet();
xss.Add("", reader);
fs.Close();
fs.Dispose();

xDoc.Validate(xss, null);

Answer 1

I have found the problem after a frustrating day and a half of iterations. Changing:

FileStream fs = new FileStream(xsdPath, FileMode.Open);
XmlReader reader = XmlReader.Create(fs);
XmlSchemaSet xss = new XmlSchemaSet();
xss.Add("", reader);

to

XmlSchemaSet xss = new XmlSchemaSet();
xss.Add("", xsdPath);

solved the problem. However I am still looking for the answer to WHY? Both are valid ways of adding a schema to the schema set (i.e. 2 overloads).