When making changes to a DataSet, it is often useful to keep a record of the changes. That way you can make a set of related changes on the client machine, then transmit just the changes back to the server. This technique saves network time, because the changes are all transmitted at once, and it saves bandwidth, because only the changes are transmitted.

You could add another line of code to Example 11-8 to see that the DataSet maintains a “before” and “after” view of the data. Add this line before the Update( ) statement:

dataSet.WriteXml(Console.Out, XmlWriteMode.DiffGram);

And you’ll see the following output when you run the program:

<diffgr:diffgram xmlns:msdata="urn:schemas-microsoft-com:xml-msdata" xmlns:diffgr=
"urn:schemas-microsoft-com:xml-diffgram-v1">
  <AngusHardware>
    <coupons diffgr:id="coupons1" msdata:rowOrder="0" diffgr:hasChanges="modified">
      <coupon_code>077GH     </coupon_code>
      <discount_amount>15</discount_amount>
      <discount_type>0</discount_type>
      <expiration_date>2002-11-09T14:17:41.6372544-05:00</expiration_date>
    </coupons>
    <coupons diffgr:id="coupons2" msdata:rowOrder="1">
      <coupon_code>665RQ     </coupon_code>
      <discount_amount>15</discount_amount>
      <discount_type>0</discount_type>
      <expiration_date>2002-11-30T00:00:00.0000000-05:00</expiration_date>
    </coupons>
    <coupons diffgr:id="coupons3" msdata:rowOrder="2">
      <coupon_code>81BIN     </coupon_code>
      <discount_amount>10</discount_amount>
      <discount_type>1</discount_type>
      <expiration_date>2003-01-31T00:00:00.0000000-05:00</expiration_date>
    </coupons>
    <coupons diffgr:id="coupons4" msdata:rowOrder="3">
      <coupon_code>99GGY     </coupon_code>
      <discount_amount>5</discount_amount>
      <discount_type>0</discount_type>
      <expiration_date>2002-12-31T00:00:00.0000000-05:00</expiration_date>
    </coupons>
  </AngusHardware>
  <diffgr:before>
    <coupons diffgr:id="coupons1" msdata:rowOrder="0">
      <coupon_code>077GH     </coupon_code>
      <discount_amount>15</discount_amount>
      <discount_type>0</discount_type>
      <expiration_date>2002-11-09T14:01:24.1830000-05:00</expiration_date>
    </coupons>
  </diffgr:before>
</diffgr:diffgram>

This is the DiffGram, and it shows the current state (“after”) of the data in the DataSet, as well as a “before” state in the diffgr:before element.

The DiffGram is an XML document that has three sections. The first, the current data instance, is represented by an XML element whose name matches the DataSet name; in this case, that’s the AngusHardware element. Under the data instance element, the current state of each row in each of the DataSet’s DataTables is serialized as a simple XML element.

The second section, diffgr:before, lists the values of any rows that have changed before the change. And the third section, diffgr:errors, shows any errors that occurred during the generation of the DiffGram.

Example 11-9 shows the general format of the DiffGram.

<diffgr:diffgram xmlns:msdata="urn:schemas-microsoft-com:xml-msdata" xmlns:diffgr=
"urn:schemas-microsoft-com:xml-diffgram-v1">
  <DataSetName>
    <DataTableName diffgr:id="DataTableName
1" msdata:rowOrder="0" diffgr:hasChanges="modified">
      <DataColumnName>DataColumnValue
</DataColumnName>
      <DataColumnName>DataColumnValue
</DataColumnName>
      ...
    </DataTableName>
    <DataTableName diffgr:id="DataTableName
2" msdata:rowOrder="1">
      <DataColumnName>DataColumnValue
</DataColumnName>
      <DataColumnName>DataColumnValue
</DataColumnName>
      ...
    </DataTableName>
  </DataSetName>
  <diffgr:before>
    <DataTableName diffgr:id="DataTableName
n" msdata:rowOrder="DataRown">
      <DataColumnName>DataColumnValue
</DataColumnName>
      <DataColumnName>DataColumnValue
</DataColumnName>
      ...
    </DataTableName>
    ...
  </diffgr:before>
  <diffgr:errors>
    <DataTableName diffgr:id="DataTableName
n" diffgr:Error="ErrorText"/>
    ...
  </diffgr:errors> 
</diffgr:diffgram>

The following details the DiffGram elements, attributes, and content:

Although the DiffGram is used internally by .NET for remoting and web services, it can also be used by any external system that needs to communicate database changes to a .NET DataSet.