HOW TO: Create Key Element for an XML Schema in Visual C# .NET (318506)

SUMMARY

Use this step-by-step guide to programmatically add a key element to an existing XML schema.

In relational data representation, a primary or unique key is used to identify a single instance of data or a row. To get similar unique representation of data in hierarchical data, a key element is required. Data representation in Extensible Markup Language (XML) is an example of hierarchical data representation. In XML schema, a key element makes the data representation unique. XMLSchemaKey identifies a key constraint that represents the World Wide Web Consortium KEY element.

The XMLSchema object contains the definition of a schema. All XML schema definition language (XSD) elements are children of the schema element. The XMLSchema object represents the World Wide Web Consortium SCHEMA element.

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Requirements

The following list outlines the recommended hardware, software, network infrastructure, and service packs that you need:

• Microsoft Visual Studio .NET installed on a compatible Microsoft Windows operating system.

This article assumes that you are familiar with the following topics:

• Microsoft Visual C# .NET language.
• XML standards.
• XSD schemas.

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Create an XML Schema

In Notepad or another text editor, type or paste the following code into a text file and name it C:\Key.xsd.

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">
  <xs:element name="Parent">
    <xs:complexType>
      <xs:sequence>
        
      <xs:element name="Children">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="Child" minOccurs="0" maxOccurs="unbounded">
                <xs:complexType>
                    <xs:attribute name="name"  type="xs:string" use="required"/>
                </xs:complexType>
              </xs:element>
            </xs:sequence>
          </xs:complexType>
        </xs:element>

      </xs:sequence>
    </xs:complexType>    
  </xs:element>
</xs:schema>
				

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Create a Visual C# .NET Application

1. Create a new Visaul C# console application.
2. Replace the code in the Class1.cs file with the following:

   using System;
   using System.Xml;
   using System.Xml.Schema;
   using System.IO;
   
   namespace ConsoleApplication4
   {
   	/// <summary>
   	/// Summary description for Class1.
   	/// </summary>
   	class Class1
   	{
   		/// <summary>
   		/// The main entry point for the application.
   		/// </summary>
   		[STAThread]
   		static void Main(string[] args)
   		{
   			ValidationEventHandler eventHandler = new ValidationEventHandler(Class1.ShowCompileErrors);
   			try
   			{
   				FileStream fstream = new FileStream("C:\\key.xsd", FileMode.Open);
   				XmlSchema myschema = new XmlSchema();
   
   				myschema = XmlSchema.Read(fstream, null);
   				myschema.Compile(eventHandler);
   
   				XmlSchemaObjectTable schemaobjecttable;
   				XmlSchemaElement ParentElement;
   				XmlQualifiedName ParentElementQName = new XmlQualifiedName("Parent", "");
   
   				schemaobjecttable = myschema.Elements;
   				if (schemaobjecttable.Contains(ParentElementQName)) 
   				{	
   					// Set the parent Node.
   					ParentElement = (XmlSchemaElement) schemaobjecttable[ParentElementQName];
   					// Create a new key element.
   					XmlSchemaKey element_key = new XmlSchemaKey();
   					element_key.Name = "IDKey";
   					element_key.Selector = new XmlSchemaXPath();
   					element_key.Selector.XPath = "Children/Child";
   
   					XmlSchemaXPath field = new XmlSchemaXPath();
   					field.XPath = "@name";
   					element_key.Fields.Add(field);
   					ParentElement.Constraints.Add(element_key);
   				}
   
   				myschema.Compile(eventHandler);
   
   				// Write to an external file.
   				StreamWriter strmWriter = new StreamWriter("c:\\NewSchema.xsd");
   				myschema.Write(strmWriter);
   				strmWriter.Close();
   				// Write to Output window.
   				StringWriter strWriter = new StringWriter();
   				myschema.Write(strWriter);
   				Console.WriteLine(strWriter.ToString());
   			}
   			
   			catch (XmlException XMLExp)
   			{
   				Console.WriteLine(XMLExp.Message);
   			}
   			catch (XmlSchemaException XmlSchemaExp)
   			{
   				Console.WriteLine(XmlSchemaExp.Message);
   			}
   			catch (Exception GenExp)
   			{
   				Console.WriteLine(GenExp.Message);
   			}
   			finally
   			{
   				Console.Read();
   			}
   		}
   		public static void ShowCompileErrors(object sender, ValidationEventArgs args)
   		{
   			Console.WriteLine("Validation Error: {0}", args.Message);
   		}
   	}
   }
   
   						

   XmlSchemaObjectTable: In the Output window, notice the XSD has a new element:

   <xs:key name="IDKey">
         <xs:selector xpath="Children/Child" />
         <xs:field xpath="@name" />
       </xs:key>
   						

   A file named NewSchema.xsd is created in the root folder (C:\).

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