title: How to update documents using another field's value using MongoDB and C#?
body: <p>I'm trying to create a new field in all my documents in a collection. That field will be an array with another field's value as (for now) the single value.</p>
  
  <p>This is my code:</p>
  
  <pre><code>var builder = new UpdateDefinitionBuilder&lt;Member&gt;();
  var update = builder.AddToSet(f =&gt; f.Sids, "$Sid");
  
  var models = new WriteModel&lt;Member&gt;[]
  {
      new UpdateManyModel&lt;Member&gt;(FilterDefinition&lt;Member&gt;.Empty, update)
  };
  
  new MongoClient().GetDatabase("mydb").GetCollection&lt;Member&gt;().BulkWrite(models);
  </code></pre>
  
  <p>It almost worked: the new field (<code>Sids</code>) was created as an array, but with the literal value <code>$Sid</code> instead of the value from the <code>Sid</code> field.</p>
  
  <p>What am I missing?</p>