title: How can I have a WiX Property default to null?
body: <p>I am working on a set of WiX installers that are going to share a common form. Every application needs the values set, but they are going to be different for each application.</p>
  
  <p>I am trying to allow for the properties (which are linked to controls) to have a default value (or not) and to allow for the property values to be set via command line. </p>
  
  <p>in my "SharedDialog.wxs" I have:</p>
  
  <pre><code>&lt;Fragment&gt;
     &lt;PropertyRef Id="PROP1"/&gt;
     &lt;PropertyRef Id="PROP2"/&gt;
     &lt;UI&gt;
        &lt;Dialog Id="SharedDialog" Width="370" Height="270" Title="[ProductName]"&gt;
           &lt;Control Type="Edit" Id="1" Property="PROP1" Wid... Indirect="no" /&gt;
           &lt;Control Type="CheckBox" Id="2" Property="PROP2" Wid...  
             CheckBoxValue="1" Indirect="no"/&gt;
        &lt;/Dialog&gt;
  &lt;/Fragment&gt;
  </code></pre>
  
  <p>In a file for the application specific project:</p>
  
  <pre><code>&lt;Fragment&gt;
     &lt;Property Id="PROP1" Value="Test"/&gt;
     &lt;Property Id="PROP2" Value="1"/&gt;
  &lt;/Fragment&gt;
  </code></pre>
  
  <p>This all works for what I am trying to do, but the problem is that when I want to clear the values as so: (so they don't have a default)</p>
  
  <pre><code>&lt;Fragment&gt;
     &lt;Property Id="PROP1"/&gt;
     &lt;Property Id="PROP2"/&gt;
  &lt;/Fragment&gt;
  </code></pre>
  
  <p>I get this error:</p>
  
  <pre><code>Unresolved reference to symbol 'Property:PROP1' in section 'Fragment:'. 
  Unresolved reference to symbol 'Property:PROP2' in section 'Fragment:'.
  </code></pre>
  
  <p>WiX also will not let you set value to "". The problem is that as far I can tell the checkbox will always be checked if the property has a value.   How can I set the property "PROP2" to "null"?</p>