gistfile1.cs

using (var cluster = new Cluster(new ClientConfiguration
{
  SerializationSettings =
  new JsonSerializerSettings
  {
    ContractResolver = new IgnoreFieldContractResolver("_id")
  }
}))
{
  using (var bucket = cluster.OpenBucket())
  {
    //this bucket will ignore any JSON fields with "_id"
  }
}