Dealing with “Circular group reference” errors in xsd.exe

I continued to research the problem of XSLT files that cannot be processed by xsd.exe. In case of xslt.xsd (contained in the Visual Studio 2010 installation under the XML directory), xsd generates the error message

>xsd "C:\Program Files\Microsoft Visual Studio 10.0\Xml\Schemas\xslt.xsd" 
      /classes

Error: Error generating classes for schema ‘C:\Program Files\Microsoft Visual Studio 10.0\Xml\Schemas\xslt.xsd’.
- Group ‘char-instructions’ from targetNamespace=’http://www.w3.org/1999/XSL/Transform’ has invalid definition: Circular group reference.

Here is how I proceeded:

Create copy1.xsd

To work around this error, I created a copy of the original xslt.xsd (named here copy1), and located the offending XSD definitions.

Replace circular references by reference to new (dummy) element

The two definitions that cause the circular reference error are the groups “char-instructions” and “instructions”. To find out where these definitions are used in the generated C# classes, the groups’ references are replaced by a reference to a new element:

  <xs:group name="char-instructions">
    <xs:choice>
<!--    
      <xs:element name="apply-templates" type="apply-templates" />
      <xs:element name="call-template" type="call-template" />
      <xs:element name="apply-imports" type="apply-imports" />
      <xs:element name="for-each" type="for-each" /> 
      <xs:element name="value-of" type="value-of" />
      <xs:element name="copy-of" type="copy-of" />
      <xs:element name="number" type="number" />
      <xs:element name="choose" type="choose" />
      <xs:element name="if" type="if" />
      <xs:element name="text" type="text" />
      <xs:element name="copy" type="copy" />
      <xs:element name="variable" type="variable" />
      <xs:element name="message" type="message" />
      <xs:element name="fallback" type="fallback" />
      -->
      <xs:any namespace="##other" processContents="lax" />
      <xs:element name="ci-dummy" type="ci-dummy" />
    </xs:choice>
  </xs:group>
  <xs:group name="instructions">
    <xs:choice>
      <xs:group ref="char-instructions" />
<!--    
      <xs:element name="processing-instruction" type="processing-instruction" />
      <xs:element name="comment" type="comment" />
      <xs:element name="element" type="element" />
-->
      <xs:element name="i-dummy" type="i-dummy" />
      <xs:element name="attribute" type="attribute" />
    </xs:choice>
  </xs:group>

Of course, the new dummy types also need to be declared in copy1.xsd:

  <xs:complexType name="ci-dummy" mixed="true">
    <xs:attribute name="dummy" type="xs:string" />
  </xs:complexType>
  <xs:complexType name="i-dummy" mixed="true">
    <xs:attribute name="dummy" type="xs:string" />
  </xs:complexType>

Running xsd.exe on copy1.xsd will now run successfully and generate copy1.cs.

Replace references to dummy classes by original classes

Search the generated classes for references to the dummy classes. You can start by commenting out the declaration of the dummy class and follow the compiler errors. In the example of xslt.xsd, replace

[System.Xml.Serialization.XmlElementAttribute("ci-dummy", typeof(cidummy))]

by

[System.Xml.Serialization.XmlElementAttribute("attribute-set", typeof(attributeset))]
[System.Xml.Serialization.XmlElementAttribute("decimal-format", typeof(decimalformat))]
[System.Xml.Serialization.XmlElementAttribute("include", typeof(include))]
[System.Xml.Serialization.XmlElementAttribute("key", typeof(key))]
[System.Xml.Serialization.XmlElementAttribute("namespace-alias", typeof(namespacealias))]
[System.Xml.Serialization.XmlElementAttribute("output", typeof(output))]
[System.Xml.Serialization.XmlElementAttribute("param", typeof(param))]
[System.Xml.Serialization.XmlElementAttribute("preserve-space", typeof(preservespace))]
[System.Xml.Serialization.XmlElementAttribute("strip-space", typeof(stripspace))]
[System.Xml.Serialization.XmlElementAttribute("template", typeof(template))]
[System.Xml.Serialization.XmlElementAttribute("variable", typeof(variable))]

However, the types that were previously referenced by other elements are not part of the generated code, as no reference to the elements exist anymore.

Add elements for previously referenced types

Create a second copy of the xsd file by copying copy1.xsd to copy2.xsd. Add the xs:element definitions that have been commented out in the first copy

    <xs:element name="apply-templates" type="apply-templates" />
    <xs:element name="apply-imports" type="apply-imports" />
    <xs:element name="call-template" type="call-template" />
    <xs:element name="for-each" type="for-each" />
    <xs:element name="value-of" type="value-of" />
    <xs:element name="copy-of" type="copy-of" />
    <xs:element name="number" type="number" />
    <xs:element name="choose" type="choose" />
    <xs:element name="if" type="if" />
    <xs:element name="text" type="text" />
    <xs:element name="copy" type="copy" />
    <xs:element name="variable" type="variable" />
    <xs:element name="message" type="message" />
    <xs:element name="fallback" type="fallback" />
    <xs:element name="processing-instruction" type="processing-instruction" />
    <xs:element name="comment" type="comment" />
    <xs:element name="element" type="element" />

Run xsd on copy2.xsd generating copy2.cs. copy2.cs need not be part of the C# project.

Copy C# classes

Next, copy all C# classes missing in copy1.cs from copy2.cs until copy1.cs compiles successfully.

Clean up attributes

Your C# classes can now be compiled and will successfully load an XML file conforming to the original XSD.

In case of the xslt.xsd, I noticed that elements and texts are handled by two different arrays, namely

// many more XmlElementAttribute declarations
[System.Xml.Serialization.XmlElementAttribute("some-element-name", typeof(someelementname))]
public object[] Items {
  get { return this.itemsField; }
  set { this.itemsField = value; }
}
[System.Xml.Serialization.XmlTextAttribute()]
public string[] Text {
  get { return this.textField; }
  set { this.textField = value; }
}

This will cause your code to lose the original order of elements and texts in the XML file. To combine both types of data into one array, add the XmlTextAttribute to the Items property as well:

[System.Xml.Serialization.XmlTextAttribute(typeof(string))]
public object[] Items

and completely remove the declaration of the string[] Text property.

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