title: Error when an agg pipeline assigns a value to the $$Maincode builtin variable
body: <pre><code>   var lookup = 
                          new BsonDocument(&quot;$lookup&quot;,
                          new BsonDocument
                          {
                              {&quot;from&quot;,&quot;DepoStatus&quot; },
                              {&quot;let&quot;,new BsonDocument{ {Maincode,&quot;$Maincode&quot;},{ColorCode , &quot;$Renkcode&quot; } } },
                              {&quot;pipeline&quot;,
                                  new BsonArray{
                                      new BsonDocument(&quot;$match&quot;,
                                      new BsonDocument(&quot;$expr&quot;,
                                      new BsonDocument(&quot;$and&quot;,
                                        new BsonArray{new BsonDocument(&quot;$eq&quot;,
                                        new BsonArray { &quot;$Maincode&quot;, &quot;$$Maincode&quot; }),new BsonDocument(&quot;$eq&quot;, new BsonArray { &quot;$Renkcode&quot;, &quot;$$Renkcode&quot; })}))),
                                  }
                              },
                              {&quot;as&quot;,&quot;statuses&quot;}
                          });
                     
                      var pipeline1 = new[] { lookup };
                      var depostatus = czProductcollection.Aggregate&lt;Product&gt;(pipeline1).ToList();
  </code></pre>
  <p>MongoDB.Driver.MongoCommandException: 'Command aggregate failed: '20.296' starts with an invalid character for a user variable name.'
  I get an error. What can I do ?</p>