Update app.config from WIX setup? - configuration

Question

I am trying Wix 3.6 and this is how it looks right now :
<?xml version="1.0" encoding="UTF-8"?>
<Directory Id="TARGETDIR" Name="SourceDir">
<Directory Id="myappINSTALLDIR" Name="myapp5Service">
<Component Id="SampleServiceComponent" DiskId="1" Guid="6f51c0f3-776c-4aec-a200-1f199352c6c3" Win64="yes">
<File Id="myapp5.WindowsService.exe" Name="myapp5.WindowsService.exe" Source="$(var.myapp.WindowsService.TargetDir)\myapp5.WindowsService.exe" KeyPath='yes'/>
...
<ServiceInstall Id="InstallmyappService" DisplayName="myappService" Name="myapp5.WindowsService.exe" Description="myapp 5 Service - För effektivare och enklare operationsplanering" Account="LocalSystem" ErrorControl="normal" Start="auto" Type="ownProcess" Vital="yes" />
<ServiceControl Id="ControlmyappService" Name="myapp5.WindowsService.exe" Start="install" Stop="both" Remove="uninstall" Wait="yes" />
</Component>
</Directory>
</Directory>
<WixVariable Id="WixUIBannerBmp" Value="$(var.ProjectDir)\Image\myappTopBanner.bmp" />
<WixVariable Id="WixUIDialogBmp" Value="$(var.ProjectDir)\Image\myappDialogBackground.bmp" />
<Property Id="WIXUI_INSTALLDIR" Value="myappINSTALLDIR" />
<UIRef Id="WixUI_InstallDir" />
<Feature Id="ProductFeature" Title="Wix_myapp.WindowsService" Level="1">
<ComponentRef Id="SampleServiceComponent" />
</Feature>
<Media Id="1" Cabinet="SampleServiceComponent.cab" EmbedCab="yes" />
</Product>
Now I need to add a dialog to the Wix setup where one appSetting and one baseadress(WCF) is set to the app.config. This most be done before the installation becouse it will decide the name of the Windows Service that the Wix is installning.
And exampel would be great!
Edit 1:
<WixVariable Id="WixUIBannerBmp" Value="$(var.ProjectDir)\Image\myappTopBanner.bmp" />
<WixVariable Id="WixUIDialogBmp" Value="$(var.ProjectDir)\Image\myappDialogBackground.bmp" />
<Property Id="SERVICEADDRESS" Value="http://serviceaddress"/>
<Property Id="WIXUI_INSTALLDIR" Value="myappINSTALLDIR" />
<UIRef Id="WixUI_InstallDir" />
<util:XmlFile Id="UpdateBaseAddress"
Action="setValue"
File="$(var.myapp.WindowsService.TargetDir)\myapp5.WindowsService.exe.config"
SelectionLanguage="XPath"
Permanent="yes"
ElementPath="/configuration/applicationSettings/ServiceName"
Name="baseAddress" Value="[SERVICEADDRESS]" />
<Feature Id="ProductFeature" Title="Wix_myapp.WindowsService" Level="1">
<ComponentRef Id="SampleServiceComponent" />
</Feature>
<Media Id="1" Cabinet="SampleServiceComponent.cab" EmbedCab="yes" />
</Product>

Answer 1

You could add in a reference to the WixUtilExtension.dll to the installer project, then use XmlFile to update the app.config like:
<Property Id="SERVICEADDRESS" Value="http://serviceaddress"/>
<util:XmlFile Id="UpdateBaseAddress"
Action="setValue"
File="[DirApplication]$(var.app.config)"
SelectionLanguage="XPath"
Permanent="yes"
ElementPath="/configuration/applicationSettings/...."
Name="baseAddress" Value="[SERVICEADDRESS]" />
Note that you'll need to set the directory and the name of the .config file (you could just use $(var.ProjectName.TargetFileName).config which should work it out for you automatically