You need to call the XmlSchemaSet.Reprocess(XmlSchema) method if you have changed a schema (or one of its includes/imports) after adding it to the System.Xml.Schema.XmlSchemaSet. The XmlSchemaSet.Reprocess(XmlSchema) method will check the schema for structural validity according to the rules of W3C XML Schema. However, it will not perform a full validation check. It will also resolve references to internal and external schema components. Any imported or included schemas that are successfully retrieved are also added to the System.Xml.Schema.XmlSchemaSet. Imported schemas are added as separate System.Xml.Schema.XmlSchema objects while included schemas are made part of the including System.Xml.Schema.XmlSchema. If the call to reprocess is successful, the XmlSchemaSet.IsCompiled property is set to false.