How can multiple elements be added to an XML config file with wix?

Question

I am trying to edit an XML file with Wix. I am using the WixUtilExtension bundled with Wix 3.7. The xml file is a settings file created in Visual Studio 2010 for a C# application. In this file, I am using an element which is used to store multiple string values in an array. This is the content of the unaltered settings file:

<configuration>
    <applicationSettings>
        <AppName.Properties.Settings>
            <setting name="StringArray" serializeAs="Xml">
                <value>
                    <ArrayOfString xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                        xmlns:xsd="http://www.w3.org/2001/XMLSchema">
                    </ArrayOfString>
                </value>
            </setting>
        </AppName.Properties.Settings>
    </applicationSettings>
</configuration>

I want to add <string> elements to the <ArrayOfString> element in this file. One way to do this is by using an <XmlConfig> element from the wix/UtilExtension namespace. I have added this element to the component which holds the config file like this:

<Component Id="ProductComponent" Guid="$(var.ConfigGuid)">
    <File Source="SettingsFile.exe.config" KeyPath="yes" Id="FILE_config" />
    <util:XmlConfig
      Name="string"
      Value="My value"
      File="[INSTALLFOLDER]SettingsFile.exe.config"
      Id="String1"
      On="install"
      Action="create"
      Node="element"
      ElementPath="/configuration/applicationSettings/AppName.Properties.Settings/setting[\[]@name='StringArray'[\]]/value/ArrayOfString"
      Sequence="100"
      />
</Component>

This results in the addition of one <string> element to the <ArrayOfString> element. To add another <string> element to the settings file, another XmlConfig element has to be added to the <Component> element of the setup project with a different Id attribute and a higher value for the Sequence attribute like this:

<util:XmlConfig
    Name="string"
    Value="My second value"
    File="[INSTALLFOLDER]SettingsFile.exe.config"
    Id="String2"
    On="install"
    Action="create"
    Node="element"
    ElementPath="/configuration/applicationSettings/AppName.Properties.Settings/setting[\[]@name='StringArray'[\]]/value/ArrayOfString"
    Sequence="101"
/>

After installation of the msi, the <ArrayOfString> element in the settings file looks like this:

<ArrayOfString xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                            xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<string>My value</string><string>My second value</string></ArrayOfString>

I have found out that it is possible to set the Value attribute of an <XmlConfig> attribute to the value of a property like this:

<Property Id="STRING1VALUE" Value="My value" />
<util:XmlConfig Value="[STRING1VALUE]" ... />

This is good. I would like the user to be able to add multiple values in the installation process dynamically so that a variable amount of <string> elements can be added to the settings file. My first approach was to use a <?foreach?> statement like this:

<?define values="My value;My second value"?>
<?foreach value in $(var.values)?>
    <util:XmlConfig
        Name="string"
        Value="$(var.value)"
        File="[INSTALLFOLDER]SettingsFile.exe.config"
        Id="String$(var.value)"
        On="install"
        Action="create"
        Node="element"
        ElementPath="/configuration/applicationSettings/AppName.Properties.Settings/setting[\[]@name='StringArray'[\]]/value/ArrayOfString"
        Sequence="101"
    />
<?endforeach?>

There are a few problems with this approach:

1. The foreach statement uses a preprocessor variable which cannot be set to the value of a property.
2. The value of the Sequence attribute stays the same.

I would like the user to store the values for the string elements in a Property which separates the values by semicolons and then parse them in a foreach statement like this:

<Property Id="VALUES" Value="My value;My second value" />
<?foreach value in [VALUES]?>
    <util:XmlConfig
        Name="string"
        Value="$(var.value)"
        File="[INSTALLFOLDER]SettingsFile.exe.config"
        Id="String$(var.value)"
        On="install"
        Action="create"
        Node="element"
        ElementPath="/configuration/applicationSettings/AppName.Properties.Settings/setting[\[]@name='StringArray'[\]]/value/ArrayOfString"
        Sequence="101"
    />
<?endforeach?>

This throws the following error:

The util:XmlConfig/@Id attribute's value, 'String[VALUES]', is not a legal identifier. 
Identifiers may contain ASCII characters A-Z, a-z, digits, underscores (_), or periods (.). 
Every identifier must begin with either a letter or an underscore.

Is there any way I can create a variable amount of elements with the XmlFile or the XmlConfig element? Is the only solution to this problem a CustomAction?

Answer 1

Based on Rob's answer, here is my new approach to adding multiple elements to an XML config file with Wix. I did not want to write C++ code, that is why I used DTF in my CustomAction.

I am going to describe how to turn a string containing multiple elements using a delimiter into multiple XML elements.

First there needs to be a property in the setup file containing the delimited string.

<Property Id="STRINGARRAY" Value="string1;string2;string3" />

This property could be populated by the user in a dialog, of course.

Next, a CustomAction has to be written. To make use of the DTF, a reference to the Microsoft.Deployment.WindowsInstaller.dll has to be added to the C# CustomAction project. The namespace Microsoft.Deployment.WindowsInstaller should be included with a using directive in that project. My CustomAction looks like this:

[CustomAction]
public static ActionResult Insert(Session session)
{
    string strings = session["STRINGARRAY"];
    string[] stringArray = strings.Split(';');
    Database db = session.Database;
    View view = db.OpenView("select * from `XmlConfig`");
    string xpath = "/configuration/applicationSettings/AppName.Properties.Settings/setting[\\[]@name='StringArray'[\\]]/value/ArrayOfString";
    for (int i = 0; i < stringArray.Length; i++)
    {
        string id = String.Format("String{0}", i);
        int sequence = 100 + i;
        string value = stringArray[i].Trim();
        Record rec = new Record(
            id,
            "[INSTALLFOLDER]SettingsFile.exe.config",
            xpath,
            null,
            "string",
            value,
            273,
            "ProductComponent",
            sequence);
        view.InsertTemporary(rec);
    }
    db.Close();
    return ActionResult.Success;
}

Here, at first the Property StringArray is read into a local variable which is converted to a string array. The following line establishes a connection to the current database used by the installer. A handle on the table XmlConfig is created, which is the table where the XML elements are added to. To insert the right values into that table, it is best to create an installer file which contains such a table and then take a look at that table in an editor like orca or InstEd.

In the xpath, backslashes have to be escaped by using double backslashes. The id variable holds the name of the temporary record, using a simple string and a number works flawlessly. The sequence has to be incremented for each element. I could not find any documentation on the values of the flags column, but I have found out that its value is set to 273 for elements that are created and 289 for elements that get deleted.

Once the record is filled with the correct values, it gets added to the XmlConfig table by using the InsertTemporary method of the view object. This is done for each element found in the delimited string.

A problem I have come across is that this CustomAction fails, if the XmlConfig table does not exist. To counter this problem I have added the following code to the setup file, which adds an element to the XML file and immediately deletes that element. I guess there could be a cleaner solution, but this was the easiest one for me.

<util:XmlConfig
    Name="string"
    Value="Dummy"
    File="[INSTALLFOLDER]SettingsFile.exe.config"
    Id="DummyEntry"
    On="install"
    Action="create"
    Node="element"
    ElementPath="/configuration/applicationSettings/AppName.Properties.Settings/setting[\[]@name='StringArray'[\]]/value/ArrayOfString"
    Sequence="1" />
<util:XmlConfig
    On="install"
    Action="delete"
    Id="DeleteDummyEntry"
    Node="element"
    File="[INSTALLFOLDER]SettingsFile.exe.config"
    VerifyPath="/configuration/applicationSettings/AppName.Properties.Settings/setting[\[]@name='StringArray'[\]]/value/ArrayOfString/string"
    ElementPath="/configuration/applicationSettings/AppName.Properties.Settings/setting[\[]@name='StringArray'[\]]/value/ArrayOfString"
    Sequence="2" />

Finally, the CustomAction has to be added to the setup project. By adding a reference to the CustomAction project in the setup project, the location of the binary can be specified like this:

<Binary Id="XmlCustomActionDLL" SourceFile="$(var.XmlCustomAction.TargetDir)XmlCustomAction.CA.dll" />

The CustomAction has to be executed immediately, otherwise it won't be able to access the session variable:

<CustomAction Id="CA_XmlCustomAction" BinaryKey="XmlCustomActionDLL" DllEntry="Insert" Execute="immediate" Return="check" />
<InstallExecuteSequence>
  <Custom Action="CA_XmlCustomAction" Before="RemoveRegistryValues" />
</InstallExecuteSequence>

To determine the right position for the CustomAction in the installation sequence, I relied on this article by Bob Arnson.