title: WiX Burn - Determine what items are already installed
body: <p>I've a burn installation whereby the user can select which of three options to install - each one directly relates to one of three MsiPackages in a chain, such as :</p>
  
  <pre><code>&lt;Chain&gt;
    &lt;MsiPackage SourceFile="..\ProductA\bin\Release\ProductA.msi"  InstallCondition="chkProductA" /&gt;
    &lt;MsiPackage SourceFile="..\ProductB\bin\Release\ProductB.msi"  InstallCondition="chkProductA" /&gt;
    &lt;MsiPackage SourceFile="..\ProductC\bin\Release\ProductC.msi"  InstallCondition="chkProductC" /&gt;
  &lt;/Chain&gt;
  </code></pre>
  
  <p>All fine. However, when I run the msi next time, I only want to re-install/update the items that were selected originally - ie if only productA was selected, I don't want to install products B &amp; C.</p>
  
  <p>How do I determine what was originally selected?</p>