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Here I have

System.Collections.Generic.KeyNotFoundException: 'The given key 'EmptyProjectionMember' was not present in the dictionary.'

var res = (from c in _context.Check
             join cp in _context.CheckProduct on c.Id equals cp.CheckId
             join p in _context.Product on cp.ProductId equals p.Id
             where c.Date.Date == date.Date
             select (cp.Quantity * Decimal.ToDouble(p.Price))).Sum();
But when I write this, the code is working:
var res = (from c in _context.Check
             join cp in _context.CheckProduct on c.Id equals cp.CheckId
             join p in _context.Product on cp.ProductId equals p.Id
             where c.Date.Date == date.Date
             select (cp.Quantity * Decimal.ToDouble(p.Price)));
double sum = 0;          
foreach(var el in res)
   sum += el;
Why Sum() is not working?
                You need to pass parameter to Sum() the attribute you want to calculate. Something like this Sum(cp.Quantity * Decimal.ToDouble(p.Price)))
– Long Luong
                Aug 23, 2020 at 9:52
                Does this answer your question? How to force LINQ Sum() to return 0 while source collection is empty
– Salik Rafiq
                Aug 23, 2020 at 14:40
Try the following:
res = (from c in _context.Check
           join cp in _context.CheckProduct on c.Id equals cp.CheckId
           join p in _context.Product on cp.ProductId equals p.Id
           where c.Date.Date == date.Date
           select (cp.Quantity * Decimal.ToDouble(p.Price)))
          .DefaultIfEmpty(0)
          .Sum();
Looks like there is nothing to select, so in that case, default to 0.
In the second case, you try to iterate an empty enumerable so it won't even go into the for each clause.
I was getting the same error when calling Any with an unsatisfied filter
public static bool IsBooking(this Address address)
    => address.ReferenceTypes.Any(referenceType 
        => referenceType == ReferenceType.Booking 
        && address.IsActive);
After first making sure that some elements exist, it is working - but it doesn't really make sense as now I'm calling Any twice
public static bool IsBooking(this Address address)
    => address.ReferenceTypes.Any()
        && address.ReferenceTypes.Any(referenceType
        => referenceType == ReferenceType.Booking
        && address.IsActive);
        
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