SELECT DISTINCT
  o.Detail,
  o.Description,
  ai.Act,
FROM Operations o
LEFT OUTER JOIN Programs p
  ON o.Id = p.OperationId
LEFT OUTER JOIN ActImplementations ai
  ON p.ActId = ai.Id
  AND ai.Archive = false

            IEnumerable<Operation> _operations = operationRepository.GetAllOperations();
            var operations=
                (from o in db.Operations
                join p in db.Programs on o.Id equals p.OperationId into t0
                from t1 in t0.DefaultIfEmpty()
                join a in db.Acts on s1.ActId equals a.Id into t2
                from st3 in t2.DefaultIfEmpty()
                select new OperationListModel
                {
                    Id = o.Id,
                    ProductName = o.Product.Name,
                    Detail = o.Detail,
                    Description = o.Description,
                    OpNumber = o.OpNumber,
                    UnitName = o.Unit.Name,
                    EquipmentName = o.Equipment.Name,
                    Act = t3 == null ? string.Empty : t3.Archive == false ? t3.Act : string.Empty
                })
                .Distinct()
                .ToList();

IEnumerable programs = _programService.GetAllPrograms();
IEnumerable acts = _actService.GetAllActs();
var leftJoin = 
from p in programs
    join aInner in acts on p.ActId equals aInner.Id into step1
    from a in step1.DefaultIfEmpty()
    select new { Id = p.OperationId, Act = a.Number}

var _operations = _operationService.GetAllOperations();
var _programs = _programService.GetAllPrograms(null);
var _acts = _actService.GetAllActs();
var _actsList = _programs
    .GroupJoin(_acts.Where(a => a.IsArchive == false), p => p.ActId, a => a.Id, (p, a) => new { OperationId = p.OperationId, Act = a.Max(v => v.Number) });
var operations = _operations.GroupJoin(_actsList, o => o.Id, a => a.OperationId, (o, a) => new OperationListModel
{
    Id = o.Id,
    ProductName = o.Product.Name,
    Detail = o.Detail,
    Description = o.Description,
    Number = o.Number,
    EquipmentName = o.Equipment.Name,
    UnitFullName = o.Unit.FullName,
    Act = a.Max(v=>v.Act)
});