More and more programs use XML-based configuration files these days instead of the older .ini style. WiX contains prefabricated custom actions to modify such files during installation and uninstallation. Let's suppose that we install a Settings.xml file together with our application. Originally, the file only contains the outermost tags:


We want the installer to add new nodes, one of those with attribute values:

  <add key="a_key" value="a_value">key_item

To achieve this, we can use the XmlFile tag. Apart from its usual attributes like Id and File, its Action, Name and Value attributes will determine what to do in the XML file while the ElementPath specifies where to do it. This last attribute uses the standard XPath specification language to describe the XML node the actual operation will be applied to.

  • Action=createElement a new element named Name will be created as a child of the one specified in ElementPath
  • Action=setValue If Name is omitted, the inner text of the element specified in ElementPath will be set to Value. If both Name and Value are specified, the node specified in ElementPath will receive an attribute named Name with a value of Value.
  • Action=deleteValue If both Name and Value are omitted, the inner text of the element specified in ElementPath will be deleted. If Name is specified, the attribute named Name will be deleted from the node specified in ElementPath.

So, to carry out the modifications we planned, we need the following entries (note that we have to provide our own sequencing, this is important to ensure that the changes will be removed in the proper reverse order upon uninstallation):

<Component Id='Settings' Guid='YOURGUID-574D-4A9A-A266-5B5EC2C022A4'>
  <File Id='XmlSettings' Name='settings.xml' DiskId='1' Source='settings.xml' Vital='yes' />
  <util:XmlFile Id='XmlSettings1' File='[INSTALLDIR]settings.xml'
    Action='createElement' Name='add' ElementPath='//settings' Sequence='1' />
  <util:XmlFile Id='XmlSettings2' File='[INSTALLDIR]settings.xml'
    Action='setValue' Name='key' Value='a_key' ElementPath='//settings/add' Sequence='2' />
  <util:XmlFile Id='XmlSettings3' File='[INSTALLDIR]settings.xml'
    Action='setValue' Name='value' Value='a_value' ElementPath='//settings/add' Sequence='3' />
  <util:XmlFile Id='XmlSettings4' File='[INSTALLDIR]settings.xml'
    Action='setValue' Value='key_item' ElementPath='//settings/add' Sequence='4' />
  <util:XmlFile Id='XmlSettings5' File='[INSTALLDIR]settings.xml'
    Action='createElement' Name='inside' ElementPath='//settings/add' Sequence='5' />
  <util:XmlFile Id='XmlSettings6' File='[INSTALLDIR]settings.xml'
    Action='setValue' Value='inside_item' ElementPath='//settings/add/inside' Sequence='6' />

If there are several nodes of the same name in the XML file, we can use the usual XPath format of 'node[@attr=&quot;value&quot;]' to specify which one we refer to. As ElementPath expects a formatted string, we have to escape the brackets with backslashes ([\[] and [\]]) to keep them from being evaluated as properties.

As this functionality resides in the standard utilities module, we have to link against that:

light.exe -ext WixUtilExtension -out Sample.msi Sample.wixobj