Subtract two dates with 'let' variable then average()? - linq

Question

Attempting to subtract two dates from one another to figure out the number of days, then execute .Average() on the 'let' variable avgConversion.
I encounter the following error; LINQ to Entities does not recognize the method 'System.TimeSpan Subtract(System.DateTime)' method, and this method cannot be translated into a store expression.
var averageConversion =
(
from r in db.Registrations
where
(bu == "All" || r.BusinessUnit.Equals(bu)) &&
(region == "All" || r.Region.Equals(region)) &&
(startDate == null || r.StartDate >= startDate) &&
(endDate == null || r.EndDate <= endDate) &&
!r.RegistrationStatus.Equals("Cancelled") &&
!r.Status.Equals("Cancelled")
let avgConversion = r.StartDate.Value.Subtract(r.RegistrationDate.Value).Days
select avgConversion
).Average();
Thanks to Enigma, this is what ended up solving the problem.
var dates = (from r in db.Registrations
where
(bu == "All" || r.BusinessUnit.Equals(bu)) &&
(region == "All" || r.Region.Equals(region)) &&
(startDate == null || r.StartDate >= startDate) &&
(endDate == null || r.EndDate <= endDate) &&
!r.RegistrationStatus.Equals("Cancelled") &&
!r.Status.Equals("Cancelled")
select new
{
r.RegistrationDate,
r.StartDate
}).ToList();
var avgConversion = (from d in dates
let AvgConversion = d.StartDate.Value.Subtract(d.RegistrationDate.Value).Days
select AvgConversion).Average();

Answer 1

Remember that Entity Framework translates a subset of all possible LINQ statements into SQL. If you use methods or functions that can't be translated the you get the "method cannot be translated into a store expression" error.
Instead you should pull your data into memory before doing the calculation so that you can run the full LINQ expression.
Like this:
var averageConversion =
(
from r in db.Registrations
where
(bu == "All" || r.BusinessUnit.Equals(bu)) &&
(region == "All" || r.Region.Equals(region)) &&
(startDate == null || r.StartDate >= startDate) &&
(endDate == null || r.EndDate <= endDate) &&
!r.RegistrationStatus.Equals("Cancelled") &&
!r.Status.Equals("Cancelled")
select new
{
r.StartDate,
r.RegistrationDate
}
)
.ToArray()
.Select(r => r.StartDate.Value.Subtract(r.RegistrationDate.Value).Days)
.Average();