LINQ：在GROUPBY和SELECT之后WHERE子句不起作用

Question

Firstly, I am getting Debits(Non-grouped) from the database.

var groupingData = await adminService.GetDebitsGrouping();

And then, I am grouping the Debits according to types(Remittance or Non-Remittance).

//Grouping non-remittance records
        var groupedCustomerList = groupingData
            .GroupBy(u => new { u.MemberId, u.CurrencyType, u.AccountToPay, u.SortCodeOrBIC })
            .Select(grp => new DebitsGroupingViewModel
            {
                PaymentOutId = grp.FirstOrDefault().PaymentOutId,
                Sector = grp.FirstOrDefault().Sector,
                MemberId = grp.FirstOrDefault().MemberId,
                MemberName = grp.FirstOrDefault().MemberName,
                SupplierName = grp.FirstOrDefault().SupplierName,
                AccountToPay = grp.FirstOrDefault().AccountToPay,
                SortCodeOrBIC = grp.FirstOrDefault().SortCodeOrBIC,
                Transactions = grp.Sum(x => x.Transactions),
                TotalToPay = grp.Sum(x => x.TotalToPay),
                CurrencyType = grp.FirstOrDefault().CurrencyType,
                ClientCurrency = grp.FirstOrDefault().ClientCurrency,
                AccountName = grp.FirstOrDefault().AccountName,
                SupplierId = grp.FirstOrDefault().SupplierId,
                SupplierTypeId = grp.FirstOrDefault().SupplierTypeId,
                PayTypeMasterId = grp.FirstOrDefault().PayTypeMasterId,
                ClientReferenceNumber = grp.FirstOrDefault().ClientReferenceNumber
            })       
            .Where(x => x.PayTypeMasterId.GetValueOrDefault() != -1)
            .ToList();

But it seems that WHERE clause is not working properly. It includes some of the Non-Remittance records too.

I also tried searching the web before asking but did not find the correct reason/answer.

NOTE : All Non-Remittance records have id other than -1.

Answer 1

In LINQ to objects, the statement is directly turned into a series of method calls:

When you do the following:

var result = myData.GroupBy( .. ).Select( .. ).Where( .. ).ToList();

This gets translated into something like this:

var result = Enumerable.ToList(
                Enumerable.Where(
                    IGrouping.GroupBy( .. )
                )
            );

This is not what we want. We want to filter myData and then group it instead of grouping the data and then filtering it. So when you change the order:

var result = myData.Where( .. ).GroupBy( .. ).Select( .. ).ToList();

This gets translated into something like this:

var result = Enumerable.ToList(
                IGrouping.GroupBy(
                    Enumerable.Where( .. )
                )
            );

Now we are achieving what we want, filter first (innermost call) and then group our data.

Hope this helps. Additionally, you can read Query Keywords for more details on group and where clauses