Tài liệu Writing and Reading XML Using a DataSet Object ppt

Writing and Reading XML Using a DataSet Object
XML is a convenient format for moving information around. You can write out the
contents of the DataTable objects contained in a DataSet to an XML file using the
WriteXml() method. The XML file written by this method contains the DataTable
column names and values.
You can write out the schema of a DataSet object to an XML file using the
WriteXmlSchema() method. The XML file written by this method contains the structure
of the DataTable objects contained in the DataSet. You can also get the XML in a
DataSet using the GetXml() method, which returns the XML in a string.
You can read the contents of the DataTable objects in an XML file into a DataSet using
the ReadXml() method. You can also read the schema contained in an XML file using the
ReadXmlSchema() method.

Note SQL Server also contains extensive built-in XML functionality, which you'll learn
about in Chapter 16
, "Using SQL Server's XML Support."
Using the WriteXml() Method
Let's say you have a DataSet object named myDataSet. Assume that myDataSet has a
DataTable that contains the CustomerID, CompanyName, ContactName, and Address
columns for the top two rows from the Customers table. The following code shows this:
SqlCommand mySqlCommand = mySqlConnection.CreateCommand();
mySqlCommand.CommandText =
"SELECT TOP 2 CustomerID, CompanyName, ContactName, Address " +
"FROM Customers " +
"ORDER BY CustomerID";
SqlDataAdapter mySqlDataAdapter = new SqlDataAdapter();
mySqlDataAdapter.SelectCommand = mySqlCommand;
DataSet myDataSet = new DataSet();
mySqlConnection.Open();
Console.WriteLine("Retrieving rows from the Customers table");

mySqlDataAdapter.Fill(myDataSet, "Customers");
mySqlConnection.Close();
You can write out the contents of myDataSet to an XML file using the WriteXml()
method. For example:
myDataSet.WriteXml("myXmlFile.xml");
This writes an XML file named myXmlFile.xml, as shown in Listing 10.9.
Listing 10.9: MYXMLFILE.XML

<?xml version="1.0" standalone="yes"?>
<NewDataSet>
<Customers>
<CustomerID>ALFKI</CustomerID>
<CompanyName>Alfreds Futterkiste</CompanyName>
<ContactName>Maria Anders</ContactName>
<Address>Obere Str. 57</Address>
</Customers>
<Customers>
<CustomerID>ANATR</CustomerID>
<CompanyName>Ana Trujillo Emparedados y helados</CompanyName>
<ContactName>Ana Trujillo</ContactName>
<Address>Avda. de la Constitución 2222</Address>
</Customers>
</NewDataSet>


As you can see, this file contains the columns for the rows retrieved from the Customers
table.
The WriteXml() method is overloaded as follows:
void WriteXml(Stream myStream);
void WriteXml(string fileName);

void WriteXml(TextWriter myTextWriter);
void WriteXml(XmlWriter myXmlWriter);
void WriteXml(stream myStream, XmlWriteMode myXmlWriteMode);
void WriteXml(string fileName, XmlWriteMode myXmlWriteMode);
void WriteXml(TextWriter myTextWriter, XmlWriteMode myXmlWriteMode);
void WriteXml(XmlWriter myXmlWriter, XmlWriteMode myXmlWriteMode);
where myXmlWriteMode is a constant from the System.Data.XmlWriteMode
enumeration that specifies how to write XML data and the schema. Table 10.8
shows the
constants defined in the XmlWriteMode enumeration.
Table 10.8: XmlWriteMode ENUMERATION MEMBERS
CONSTANT DESCRIPTION
DiffGram Writes out the DataSet as a DiffGram, which contains the original values
Table 10.8: XmlWriteMode ENUMERATION MEMBERS
CONSTANT DESCRIPTION
and the changes to those values to make them current. You can generate a
DiffGram that contains only the changes by calling the GetChanges()
method of your DataSet, and then call WriteXml().
IgnoreSchema Writes out only the data in the DataSet, without writing the schema.
IgnoreSchema is the default.
WriteSchema Writes out the schema in the DataSet.
The following example shows the use of the XmlWriteMode.WriteSchema constant:
myDataSet.WriteXml("myXmlFile2.xml", XmlWriteMode.WriteSchema);
This writes an XML file named myXmlFile2.xml, as shown in Listing 10.10
.
Listing 10.10: MYXMLFILE2.XML

<?xml version="1.0" standalone="yes"?>
<NewDataSet>
<xsd:schema id="NewDataSet" targetNamespace="" xmlns=""

xmlns:xsd=" xmlns:msdata="urn:schemas-
microsoft-
com:xml-msdata">
<xsd:element name="NewDataSet" msdata:IsDataSet="true">
<xsd:complexType>
<xsd:choice maxOccurs="unbounded">
<xsd:element name="Customers">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="CustomerID" type="xsd:string" minOccurs="0" />
<xsd:element name="CompanyName" type="xsd:string" minOccurs="0" />
<xsd:element name="ContactName" type="xsd:string" minOccurs="0" />
<xsd:element name="Address" type="xsd:string" minOccurs="0" />
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:choice>
</xsd:complexType>
</xsd:element>
</xsd:schema>
<Customers>
<CustomerID>ALFKI</CustomerID>
<CompanyName>Alfreds Futterkiste</CompanyName>
<ContactName>Maria Anders</ContactName>
<Address>Obere Str. 57</Address>
</Customers>
<Customers>
<CustomerID>ANATR</CustomerID>
<CompanyName>Ana Trujillo3 Emparedados y helados</CompanyName>
<ContactName>Ana Trujillo</ContactName>

<Address>Avda. de la Constitución 2222</Address>
</Customers>
</NewDataSet>

As you can see, this file contains the schema definition for the columns used in the
original SELECT statement, as well as the column values for the rows retrieved.
Using the WriteXmlSchema() Method
You can write out the schema of myDataSet to an XML file using the WriteXmlSchema()
method. For example:
myDataSet.WriteXmlSchema("myXmlSchemaFile.xml");
This writes an XML file named myXmlSchemaFile.xml, as shown in Listing 10.11
.
Listing 10.11: MYXMLSCHEMAFILE.XML

<?xml version="1.0" standalone="yes"?>
<xsd:schema id="NewDataSet" targetNamespace="" xmlns=""
xmlns:xsd="
xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
<xsd:element name="NewDataSet" msdata:IsDataSet="true">
<xsd:complexType>
<xsd:choice maxOccurs="unbounded">
<xsd:element name="Customers">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="CustomerID" type="xsd:string" minOccurs="0" />
<xsd:element name="CompanyName" type="xsd:string" minOccurs="0" />
<xsd:element name="ContactName" type="xsd:string" minOccurs="0" />
<xsd:element name="Address" type="xsd:string" minOccurs="0" />
</xsd:sequence>
</xsd:complexType>

</xsd:element>
</xsd:choice>
</xsd:complexType>
</xsd:element>
</xsd:schema>

As you can see, this file contains the schema definition for the columns retrieved from the
Customers table by the original SELECT statement.
Using the ReadXml() Method
You can read the contents of an XML file into a DataSet object using the ReadXml()
method. This method reads the rows and columns from the XML file into DataTable
objects of the DataSet. For example, the following statement uses the ReadXml() method
to read the XML file myXmlFile.xml previously written by the WriteXml() method:
myDataSet.ReadXml("myXmlFile.xml");
The ReadXml() method is overloaded as follows:
void ReadXml(Stream myStream);
void ReadXml(string fileName);
void ReadXml(TextReader myTextReader);
void ReadXml(XmlReader myXmlReader);
void ReadXml(stream myStream, XmlReadMode myXmlReadMode);
void ReadXml(string fileName, XmlReadMode myXmlReadMode);
void ReadXml(TextReader myTextReader, XmlReadMode myXmlReadMode);
void ReadXml(XmlReader myXmlReader, XmlReadMode myXmlReadMode);
where myXmlReadMode is a constant from the System.Data.XmlReadMode enumeration
that specifies how to read XML data and the schema. Table 10.9
shows the constants
defined in the XmlReadMode enumeration.
Table 10.9: XmlReadMode ENUMERATION MEMBERS
CONSTANT DESCRIPTION
Auto Reads the XML file in an appropriate manner:

•
If the XML file contains a DiffGram, then XmlReadMode is set to
DiffGram.
•
If the DataSet already contains a schema or the XML file contains
a schema, then XmlReadMode is set to ReadSchema.
•
If the DataSet doesn't contain a schema and the XML file doesn't