XML

XML and the DataSet

At the heart of the .NET class library is XML, and this is true with the DataSet as well. Here are some of the XML features of the DataSet:

WriteXml
The WriteXml method writes the data, and optionally the schema, in a DataSet. There are several overloads of this method, allowing you to write the XML to any Stream, file, TextWrite, or XmlWriter, as well as overloads allowing you to write the schema as well. Here are the different overloads of the WriteXml method:

//Writes the current data for the DataSet using the specified System.IO.Stream.
public void WriteXml(Stream);
public void WriteXml(Stream, XmlWriteMode);

//Writes the current data for the DataSet to the specified file.
public void WriteXml(string);
public void WriteXml(string, XmlWriteMode);

//Writes the current data for the DataSet using the specified TextWriter.
public void WriteXml(TextWriter);
public void WriteXml(TextWriter, XmlWriteMode);

//Writes the current data for the DataSet to the specified XmlWriter.
public void WriteXml(XmlWriter);
public void WriteXml(XmlWriter, XmlWriteMode);

The values for the XmlWriteMode enumeration are:

Value Description

DiffGram Writes the DataSet as a DiffGram, including changed and original values.

IgnoreSchema Writes the XML without any schema

WriteSchema Default. Writes the XML data with an inline schema.

When you write the XML, by default it will have the following format:

<?xml version="1.0" standalone="yes"?>
<NewDataSet>
   <Table>
      <customerID>8</customerID>
      <customerName>Wily Widgets</customerName>
   </Table>
</NewDataSet>

The root node of the XML document is the name of the DataSet. This can be indicated at time of instantiation, or later with the DataSetName property, as in the following lines of code:

//Setting DataSet name at Instantiation
DataSet objDataSet = new DataSet("CustomersDataSet");

//Setting DataSet name after Instantiation
objDataSet.DataSetName = "CustomersDataSet";

The rows of data are enclosed in a tag with the name of the table, set when you use a DataAdapter to fill the DataSet, or by setting the TableName property of the Table in the DataSet. The following code snippet shows these two cases:

//Setting Table Name when Filling
SqlDataAdapter da = new SqlDataAdapter(objCmd);
da.Fill(objDataSet, "CustomersTable");

//Setting Table Name after DataSet has been Filled
objDataSet.Tables[0].TableName = "CustomersTable";

Using the WriteXml method after setting these properties results in the XML file below. Notice our DataSet name, CustomersDataSet, and our Table name, CustomersTable, in the output:

<?xml version="1.0" standalone="yes"?>
<CustomersDataSet>
   <CustomersTable>
      <customerID>8</customerID>
      <customerName>Wily Widgets</customerName>
   </CustomersTable>
</CustomersDataSet>

If you would prefer to have the columns mapped as attributes of the XML element, rather than individual elements, you have to change the ColumnMapping property of each column in your DataSet Table to MappingType.Attribute, as in the following example:

//Indicate we want columns mapped as XML attributes
foreach(DataColumn c in objDataSet.Tables[0].Columns)
{
c.ColumnMapping = MappingType.Attribute;
}

Here is the output using the same Customers data as in the previous examples:

<?xml version="1.0" standalone="yes"?>
<CustomersDataSet>
<CustomersTable customerID="8" customerName="Wily Widgets" />
</CustomersDataSet>

ReadXml
In addition to being able to extract XML from a DataSet, you can also load XML data into a DataSet using the ReadXml method.

Here are the various overloads of the ReadXml method:

//Reads XML and schema into the DataSet using the specified System.IO.Stream.
public void ReadXml(Stream);
public void ReadXml(Stream, XmlReadMode);

//Reads XML and schema into the DataSet from the specified file.
public void ReadXml(string);
public void ReadXml(string, XmlReadMode);

//Reads XML and schema into the DataSet using the specified System.IO.TextReader.
public void ReadXml(TextReader);
public void ReadXml(TextReader, XmlReadMode);

//Reads XML and schema into the DataSet using the specified System.Xml.XmlReader.
public void ReadXml(XmlReader);
public void ReadXml(XmlReader, XmlReadMode);

The values for the XmlReadMode enumeration are:

Value Description

Auto The default. Will use DiffGram, ReadSchema, or InferSchema mode as appropriate.

DiffGram Reads a diffgram and applies the changes to the dataset.

Fragment Reads xml documents, such as those generated by FOR XML sql queries.

IgnoreSchema Ignores any schema in the XML file, and loads data using the current schema in the DataSet. Used when the DataSet is already configured with a schema.

InferSchema Determines the schema based on the format of the data, and loads the data. Used when neither the DataSet nor the XML file has a schema.

ReadSchema Reads the schema in the XML file to configure the DataSet, and loads the data.