Linq在哪里与外部联接，需要帮助才能首先选择

Question

I have a outer join with a where clause. It returns multiple responses and I need to select the first one.

from p in context.Persons
join c in context.Companies on p.PersonId equals c.CompanyId
join a1 in context.Addresses on p.AddressDeliveryId equals a1.AddressId into da from x1 in da.DefaultIfEmpty()
join a2 in context.Addresses on p.AddressInvoiceId equals a2.AddressId into ia from x2 in ia.DefaultIfEmpty()

//This line returns multiple answers
join a3 in context.Addresses on p.PersonId equals a3.PersonId into pa from x3 in pa.Where(a3 => a3.AddressLocationId == 5).DefaultIfEmpty()


orderby p.PersonId descending
where p.IsProvider.Equals(false)
&& p.Obsolete.Equals(false)
&& p.Locked.Equals(false)
&& p.IsCustomer.Equals(true)
&& p.PersonType.Equals(1)

select new
{
    p, c, x1, x2, x3

};

So I need the x3 to only return the first row. Please help

Answer 1

This worked for me.

from p in context.Persons
join c in context.Companies on p.PersonId equals c.CompanyId

join a1 in context.Addresses on p.AddressDeliveryId equals a1.AddressId into da
from x1 in da.DefaultIfEmpty()

join a2 in context.Addresses on p.AddressInvoiceId equals a2.AddressId into ia
from x2 in ia.DefaultIfEmpty()

let a3 = context.Addresses.FirstOrDefault(a3 => a3.AddressLocationId == 5 && a3.PersonId == p.PersonId && a3.Obsolete == false)
//This line worked for me

orderby p.PersonId descending
where p.IsProvider.Equals(false)
    && p.Obsolete.Equals(false)
    && p.Locked.Equals(false)
    && p.IsCustomer.Equals(true)
    && p.PersonType.Equals(1)

select new
{
    p,
    c,
    DeliveryAddress = x1.Address1,
    DeliveryAddressCity = x1.PoCity,
    DeliveryAddressZipCode = x1.PoPostalCode,
    InvoiceAddress = x2.Address1,
    InvoiceAddressCity = x2.PoCity,
    InvoiceAddressZipCode = x2.PoPostalCode,
    PhoneNumber = a3.Address1,
    PhoneNumberAddressId = (a3 != null) ? a3.AddressId : -1,

};

Answer 2

I think you can just change the .DefaultIfEmpty() to .FirstOrDefault()

join a3 in context.Addresses on p.PersonId equals a3.PersonId into pa from x3 in pa.Where(a3 => a3.AddressLocationId == 5).FirstOrDefault()

This should only return you the first x3 element