I have a method that is inserting into a table an Id, a set of UDT, and a date.

public class MessageQueueNext
{
    public string Id { get; set; }
    public List<UDTMessage> Messages { get; set; }
    public DateTimeOffset Updated { get; set; }
}

private static Lazy<PreparedStatement> InsertStatement = new 
    Lazy<PreparedStatement>(
        () => {
            return CassandraDB.Instance.Session.Prepare
            (
                @"INSERT INTO SomeTable
                (id, messages, updated)
                VALUES (?, ?, ?)"
            );
        });

public async Task InsertAsync(SomeObj obj)
{
    var statement = BindValues(
        InsertStatement.Value,
        new dynamic[] {
            obj.Id, obj.Messages, obj.Updated
        }
    );

    await CassandraDB.Instance.Session.ExecuteAsync(statement);
}

private BoundStatement BindValues(PreparedStatement ps, dynamic[] values)
{
    var statement = ps.Bind
    (
        values[0] ?? Unset.Value, values[1] ?? new List<UDTMessage>(), values[2] ?? Unset.Value
    );

    return statement;
}

At some point this method throws an exception.

Collection was modified; enumeration operation may not execute. at System.Collections.Generic.List`1.Enumerator.MoveNextRare() at Cassandra.Serialization.CollectionSerializer.Serialize(UInt16 protocolVersion, IEnumerable value) at Cassandra.Serialization.GenericSerializer.Serialize(ProtocolVersion version, Object value) at Cassandra.QueryProtocolOptions.Write(FrameWriter wb, Boolean isPrepared) at Cassandra.Requests.BaseRequest.WriteFrame(Int16 streamId, MemoryStream stream, ISerializer connectionSerializer) at Cassandra.OperationState.WriteFrame(Int16 streamId, MemoryStream memoryStream, ISerializer serializer, Int64 timestamp) at Cassandra.Connections.Connection.RunWriteQueueAction() --- End of stack trace from previous location where exception was thrown ---

Can anyone help me with what could possibly cause that? As I'm understanding the obj.Messages is changed somewhere down the road? Or like it's possible that it's changed in a function that calls InsertAsync(...), but right after calling InsertAsync(...) it changes the obj.Messages value?

Method that is calling InsertAsync(...)

public async Task PersistMessageQueue(UDTMessage msg)
{
    var DAO = new DAO();
    var list = await _cache.GetMessages(msg.Id);
    if (list == null)
    {
        list = new List<UDTMessage>();
    }
    list.Add(msg);
    var someObj = new SomeObj();
    someObj.ToId = msg.Id;
    someObj.Messages = list;
    someObj.Updated = DateTimeOffset.UtcNow;
    await DAO.InsertAsync(messageQueueNext);
    _cache.AddElement(msg);
}