title: Optional parameters "must be a compile-time constant"
body: <p>I have a class divided in two partial files, like this:</p>
  
  <pre><code>public partial class PersonRepository : BaseRepository&lt;Person&gt;
  {
      public static readonly string ColumnID = "ID";
      ...
  </code></pre>
  
  <p>and</p>
  
  <pre><code>public partial class PersonRepository : BaseRepository&lt;Person&gt;
  {
      public List&lt;Person&gt; GetByCompany(int companyID, string sortExpression = ColumnID)
      {
      ...
  </code></pre>
  
  <p>But the compiler keeps saying that <code>sortExpression</code> "must be a compile-time constant". To me it seems a perfect compile-time constant, so I don't understand where the problem is.</p>