title: Using a Team City environment variable to override a project property
body: <p>I have a C# project property called Version defined as </p>
  
  <pre><code>&lt;Version Condition="$(Version)==''"&gt;1.2.3.4&lt;Version&gt;
  </code></pre>
  
  <p>1.2.3.4 is the default value.</p>
  
  <p>I have a Team City system property, also called Version, set up to override.  So in the custom run dialog in Team City, I can specify a value for Version and that value gets used. This works fine.</p>
  
  <p>If I leave the parameter blank in Team City, however, the default value is still overwritten with blank (null?).  If I delete the Team City parameter, the default value is used.</p>
  
  <p>Is the condition incorrect? How can I set up the Team City property to be blank, and only override if I enter some value?</p>