Writing DataSet Contents as XML Data
In ADO.NET you can write an XML representation of a DataSet, with or without its schema. If schema information is included inline with the XML, it is written using the XML Schema definition language (XSD). The schema contains the table definitions of the DataSet as well as the relation and constraint definitions.
When a DataSet is written as XML data, the rows in the DataSet are written in their current versions. However, the DataSet can also be written as a DiffGram so that both the current and the original values of the rows will be included.
The XML representation of the DataSet can be written to a file, a stream, an XmlWriter, or a string. These choices provide great flexibility for how you transport the XML representation of the DataSet. To obtain the XML representation of the DataSet as a string, use the GetXml method as shown in the following example.
Dim xmlDS As String = custDS.GetXml()
string xmlDS = custDS.GetXml();
GetXml returns the XML representation of the DataSet without schema information. To write the schema information from the DataSet (as XML Schema) to a string, use GetXmlSchema.
To write a DataSet to a file, stream, or XmlWriter, use the WriteXml method. The first parameter you pass to WriteXml is the destination of the XML output. For example, pass a string containing a file name, a System.IO.TextWriter object, and so on. You can pass an optional second parameter of an XmlWriteMode to specify how the XML output is to be written.
The following table shows the options for XmlWriteMode.
| XmlWriteMode option | Description |
|---|---|
| IgnoreSchema | Writes the current contents of the DataSet as XML data, without an XML Schema. This is the default. |
| WriteSchema | Writes the current contents of the DataSet as XML data with the relational structure as inline XML Schema. |
| DiffGram | Writes the entire DataSet as a DiffGram, including original and current values. For more information, see DiffGrams. |
When writing an XML representation of a DataSet that contains DataRelation objects, you will most likely want the resulting XML to have the child rows of each relation nested within their related parent elements. To accomplish this, set the Nested property of the DataRelation to true when you add the DataRelation to the DataSet. For more information, see Nesting DataRelations.
The following are two examples of how to write the XML representation of a DataSet to a file. The first example passes the file name for the resulting XML as a string to WriteXml. The second example passes a System.IO.StreamWriter object.
custDS.WriteXml("Customers.xml", XmlWriteMode.WriteSchema)
custDS.WriteXml("Customers.xml", XmlWriteMode.WriteSchema);
Dim xmlSW As System.IO.StreamWriter = New System.IO.StreamWriter("Customers.xml")
custDS.WriteXml(xmlSW, XmlWriteMode.WriteSchema)
xmlSW.Close()
System.IO.StreamWriter xmlSW = new System.IO.StreamWriter("Customers.xml");
custDS.WriteXml(xmlSW, XmlWriteMode.WriteSchema);
xmlSW.Close();
Mapping Columns to XML Elements, Attributes, and Text
You can specify how a column of a table is represented in XML using the ColumnMapping property of the DataColumn object. The following table shows the different MappingType values for the ColumnMapping property of a table column, and the resulting XML.
| MappingType value | Description |
|---|---|
| Element | This is the default. The column is written as an XML element where the ColumnName is the name of the element and the contents of the column are written as the text of the element. For example:<ColumnName>Column Contents</ColumnName> |
| Attribute | The column is written as an XML attribute of the XML element for the current row where the ColumnName is the name of the attribute and the contents of the column are written as the value of the attribute. For example:<RowElement ColumnName="Column Contents" /> |
| SimpleContent | The contents of the column are written as text in the XML element for the current row. For example:<RowElement>Column Contents</RowElement>Note that SimpleContent cannot be set for a column of a table that has Element columns or nested relations. |
| Hidden | The column is not written in the XML output. |