title: test if an MSBuild property is defined?
body: <p>In MsBuild, is it possible to create an MSBuild condition (or another situation) that will evaluate whether a Property is 'defined' (presuming that this is previous to assigning the property a value somewhere)?</p>
  
  <p>The following seems a little too clumsy to be reliable:</p>
  
  <pre><code>&lt;PropertyGroup Label="Undefined State"&gt;
       &lt;Defined&gt;&lt;/Defined&gt;
  &lt;/PropertyGroup&gt;
  
  &lt;Choose&gt;
     &lt;When Condition="('$(Defined)' == '' OR '$(Defined)' != '')"&gt;
          &lt;Message Text="Defined is probably/likely/assuredly defined"/&gt;
      &lt;/When&gt;
      &lt;Otherwise&gt;
         &lt;Message Text="Defined is reportedly/maybe/possibly not defined"/&gt;
      &lt;/Otherwise&gt;
  &lt;Choose&gt;
  </code></pre>