There are string properties that are constrained to several valid values such as:

    <StringProperty
      Name="CompilerFlavor"
      HelpUrl="http://doc.qt.io/qt-5/moc.html"
      DisplayName="Compiler Flavor"
      Description=
"Set the compiler &lt;flavor&gt;: either &quot;msvc&quot; or &quot;unix&quot;.
(--compiler-flavor &lt;flavor&gt;)"
      Switch="--compiler-flavor [value]"/>

which is either msvc or unix in this case.

The user is expected to know the valid values of the property (or pay attention to the description text), and enter one of the valid values correctly to a text box. This is an error-prone process that can be avoided by using EnumProperty instead of StringProperty.

How to proceed: Scan "<StringProperty" within the solution, for each property, see if it is constrained by reading its description (which should be identical to the -h documentation), convert it to an EnumProperty if it is so.