Question

I am trying to convert below nested for each loop into Linq. However I am still unable to do it successfully.

var objAct = new List<InformaticsBenchmarkSummary>();
foreach (var item in op)
{
    foreach (var lstTp5 in lstTopFive)
    {
        if (item.UnitOfOperations.ContainsKey(lstTp5.SystemID))
        {
            var objIbm = new InformaticsBenchmarkSummary();
            objIbm.CompanyId = item.CompanyId;
            objIbm.CompanyName = item.CompanyName;
            objIbm.LocationId = item.LocationId;
            objIbm.LocationName = item.LocationName;
            objIbm.UnitOfOperations.Add(lstTp5.SystemID, 
                                        item.UnitOfOperations[lstTp5.SystemID]);
            objAct.Add(objIbm);
        }
    }
}

Where UnitOfOperations is of type Dictionary<int,string>();
op is again List<InformaticsBenchmarkSummary>()
lstTopFive is List<int>()

I tried, something like this but was unsuccessful syntactically

var output = from item in op
from lstTp5 in lstTopFive
where item.UnitOfOperations.ContainsKey(lstTp5.SystemID)
let v = new InformaticsBenchmarkSummary()
{
     CompanyId = item.CompanyId,
     CompanyName = item.CompanyName,
     LocationId = item.LocationId,
     LocationName = item.LocationName
}
.UnitOfOperations.Add(lstTp5.SystemID, item.UnitOfOperations[lstTp5.SystemID])
select v;

Nested loop works perfectly but I think, linq on this will increase performance.
Appreciate any help.

Answer 1

It is impossible in a Linq query-syntax to use the UnitOfOperations.Add in the select. But you can do it using Method Chain and a SelectMany method:

var objAcu = (op.SelectMany(item => lstTopFive, (item, lstTp5) => new { item, lstTp5 })  // <- Bad readability
                .Where(t => t.item.UnitOfOperations.ContainsKey(t.lstTp5.SystemID))
                .Select(t =>
                        {
                            var objIbm = new InformaticsBenchmarkSummary
                            {
                                CompanyId = t.item.CompanyId,
                                CompanyName = t.item.CompanyName,
                                LocationId = t.item.LocationId,
                                LocationName = t.item.LocationName
                            };
                            objIbm.UnitOfOperations.Add(t.lstTp5.SystemID, t.item.UnitOfOperations[t.lstTp5.SystemID]);
                            return objIbm;
                        })).ToList();

If the property UnitOfOperations has a public set, in this case, you can use the query-syntax.

How do you replace 1 foreach? By a from ... in ...

Then, to replace 2 foreach, use 2 from ... in ...

var objAct = (from item in op  // First foreach loop
              from lstTp5 in lstTopFive  // Second foreach loop
              where item.UnitOfOperations.ContainsKey(lstTp5.SystemID)
              select new InformaticsBenchmarkSummary
              {
                  CompanyId = item.CompanyId,
                  CompanyName = item.CompanyName,
                  LocationId = item.LocationId,
                  LocationName = item.LocationName,
                  UnitOfOperations = { { lstTp5.SystemID, item.UnitOfOperations[lstTp5.SystemID] } }
              }).ToList();

But I doubt it will increase performance in such an operation.

Anyway, I don’t understand what you try to achieve. Because in all items of the output will have one and only one element in the dictionary UnitOfOperations. Is it really what you want to do?

UPDATE

List<int> systemIdTop5 = lstTopFive.Select(tp5 => tp5.SystemID).ToList();
var objAct = (from item in op
              select new InformaticsBenchmarkSummary
              {
                  CompanyId = item.CompanyId,
                  CompanyName = item.CompanyName,
                  LocationId = item.LocationId,
                  LocationName = item.LocationName,
                  UnitOfOperations = systemIdTop5.Intersect(item.UnitOfOperations.Keys)
                                                 .ToDictionary(systemId => systemId, systemId => item.UnitOfOperations[systemId])
              }).ToList();