Can not implicit convert type 'System.Collections.Generic.IEnumerable' to 'System.Collections.Generic.List'

This error happens because notificacoesAll.Union(notificacoesClient) returns a IEnumerable<TNotification> , but the signature of your method expects List<TNotification> .

So you have two alternatives:

• Change your method signature to return IEnumerable<TNotification>

• Transform the result of your union into a list:

  return notificacoesAll.Union(notificacoesClient).ToList();
  

One more detail: I do not think it's necessary to call ToList in the queries of your example, because List already returns a list (it's redundant).

Take out .ToList() from the list and let only return

Something like this:

public List<TNotification> GetNotifications(TClient client)
{
    var notificacoesClient = Session.CreateCriteria<TNotification>()
        .CreateAlias("TClientNotifications", "clientNotifications", JoinType.LeftOuterJoin)
        .SetFetchMode("clientNotifications.Client", FetchMode.Eager)
        .CreateAlias("clientNotifications.Client", "client", JoinType.LeftOuterJoin)
        .Add(Restrictions.Eq("client.Id", client.Id))
        .AddOrder(Order.Desc("Id"))
        .List<TNotification>();

    var notificacoesAll = Session.CreateCriteria<TNotification>()
        .Add(Restrictions.Eq("IsToAll", true))
        .AddOrder(Order.Desc("Id"))
        .List<TNotification>();

    return notificacoesAll.Union(notificacoesClient).ToList();
}