public Envelope Location {get; set;}

{
  // Note : Needs QueuedTask to run
  {
    //Get the bookmark's definition
    var bookmarkDef = oldBookmark.GetDefinition();

    //Modify the bookmark's location
    bookmarkDef.Location = envelope;

    //Clear the camera as it is no longer valid.
    bookmarkDef.Camera = null;

    //Set the bookmark definition
    oldBookmark.SetDefinition(bookmarkDef);
  }
}