Applied XML Programming for Microsoft .NET.pdf - Csbdu.in

The SqlXmlAdapter ClassThe SqlXmlAdapter class is a shrink-wrapped adapter class. It does not implement theIDataAdapter interface, so technically speaking, it can't be presented as an adapterobject. Nevertheless, the class provides adapter-like methods such as Fill and Update,as shown in the following code. These are also the only public methods for the class.void Fill(DataSet ds);void Update(DataSet ds);The SqlXmlAdapter class also provides three constructors, shown in the following code,whose signatures reinforce the idea that this adapter is a mere wrapper class forSqlXmlCommand. In other words, the SqlXmlAdapter class is more a command thatmanages DataSet objects than a true data adapter object as it is described in theADO.NET specification.public SqlXmlAdapter(SqlXmlCommand cmd)public SqlXmlAdapter(string commandText,SqlXmlCommandType cmdType,string connectionString)public SqlXmlAdapter(Stream commandStream,SqlXmlCommandType cmdType,string connectionString)These constructors use the information they receive to set up an internal instance of theSqlXmlCommand class. The Fill method makes full use of all the information passedthrough the constructor. For the Update method, on the other hand, only the connectionstring information is actually needed.Filling an XML AdapterThe Fill method is rather simple. First it executes the embedded XML command usingExecuteStream. Next it uses the returned memory stream to fill the specified DataSetobject through its ReadXml method.The ReadXml method populates a DataSet object by reading XML data from a varietyof sources, including streams and text readers, and inferring the schema. We'll examinethe inference process in detail in Chapter 9. For now, suffice to say that ReadXml candetect any in-line or referenced XSD schema or determine the schema dynamically.Once the DataSet object has been filled from the XML stream generated by thecommand execution, all the changes are accepted so that the DataSet object appearsintact and with no pending changes.Updating Using an XML AdapterThe Update method takes a DataSet object and applies its pending changes to thetarget database. The parameters specified on instantiation contain the details about theconnection string. The embedded SqlXmlCommand object has command text and acommand type that are simply ignored during Update. Let's see why.When Update executes, the embedded command object is used to perform the task,but its command text and command type properties are silently and temporarilyoverwritten with DataSet-specific settings.The Update method writes the contents of the DataSet object to a newly createdmemory stream. The DataSet object is serialized as a DiffGram. Next the contents ofthe stream—that is, the DiffGram representation of the DataSet object—are copied intothe CommandText property of the underlying SqlXmlCommand object. TheCommandType property is set to Template, and ExecuteStream is called to update the321