title: How to set optional parameter without compile-time constant
body: <p>Is there a way to write the C# method below:</p>
  
  <pre><code>public string Download(Encoding contentEncoding = null) {
      defaultEncoding = contentEncoding ?? Encoding.UTF8;
      // codes...
  }
  </code></pre>
  
  <p>with a default parameter added so it looks like this:</p>
  
  <pre><code>public string Download(Encoding contentEncoding = Encoding.UTF8) {
      // codes...
  }
  </code></pre>
  
  <p>without using a compile-time constant?</p>