Web.config transforms add tree

Question

I want to add the following to the web config on release:

<system.webServer>
    <httpProtocol>
        <customHeaders>
            <add name="Strict-Transport-Security" value="max-age=16070400; includeSubDomains" xdt:Transform="Insert" />
        </customHeaders>
    </httpProtocol>
</system.webServer>

There aren't any custom headers in the default web config so I get an error when I publish: No element in the source document matches '/configuration/system.webServer/httpProtocol/customHeaders'.

I can fix it my just adding the empty elements to the web.config like so:

  <httpProtocol>
    <customHeaders>
    </customHeaders>
  </httpProtocol>

However, it doesn't feel like the correct way.

Is there a more correct way to build the element tree on the transform?

Answer 1

Adding the empty <customHeaders> node to the web.config works because the transform you have is to insert the <add .../> node, not the <customHeaders> node. It can only insert where it matches to that point.

To insert the tree of nodes, move the xdt:Transform="Insert" up a little in the XML. If you start with a web.config of:

<?xml version="1.0">
<configuration>
  <system.webServer>
    <httpProtocol />
  </system.webServer>
</configuration>

and transform it with:

<?xml version="1.0">
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <system.webServer>
    <httpProtocol>
      <customHeaders xdt:Transform="Insert">
        <add name="Strict-Transport-Security" value="max-age=16070400; includeSubDomains" />
      </customHeaders>
    </httpProtocol>
  </system.webServer>
</configuration>

You'll end up with:

<?xml version="1.0">
<configuration>
  <system.webServer>
    <httpProtocol>
      <customHeaders>
        <add name="Strict-Transport-Security" value="max-age=16070400; includeSubDomains" />
      </customHeaders>
    </httpProtocol>
  </system.webServer>
</configuration>

Here's a helpful web.config transformation tester.