处理queryover投影中的空值

Question

我有一个对应的，它有一个地址，可能分配了一个国家。

我该如何处理：

InvoiceAddress invoiceAddres = null;
Country InvoiceAddressCountry = null;
Counterpart counterpart = null;
CounterpartTabDTO result = null;

// projections for DTO-mapping
var projections = new[]
                      {
                          Projections.Property(() => counterpart.CounterpartId).WithAlias(() => result.InternalID),
                          Projections.Property(() => counterpart.GlobalCounterpartId).WithAlias(() => result.BasicInfo_GlobalCounterpartyID),
                          Projections.Property(() => counterpart.Name).WithAlias(() => result.BasicInfo_Name),
                          Projections.Property(() => counterpart.ShortName).WithAlias(() => result.BasicInfo_ShortName),
                          Projections.Property(() => counterpart.PhoneNumber).WithAlias(() => result.BasicInfo_Telephone),
                          Projections.Property(() => counterpart.Webpage).WithAlias(() => result.BasicInfo_WWW),
                          Projections.Property(() => counterpart.Language).WithAlias(() => result.BasicInfo_Language),
                          Projections.Property(() => counterpart.VAT).WithAlias(() => result.BasicInfo_VAT),
                          Projections.Property(() => counterpart.CompanyRegistrationNumber).WithAlias(() => result.BasicInfo_CompanyRegistationno),
                          Projections.Property(() => invoiceAddres.Name).WithAlias(() => result.BasicInfo_InvoiceAddressContactPerson),
                          Projections.Property(() => invoiceAddres.Street).WithAlias(() => result.BasicInfo_InvoiceAddressAddress),
                          Projections.Property(() => invoiceAddres.PostalCode).WithAlias(() => result.BasicInfo_InvoiceAddressPostalCode),
                          Projections.Property(() => invoiceAddres.City).WithAlias(() => result.BasicInfo_InvoiceAddressCity),
                          Projections.Property(() => invoiceAddres.Area).WithAlias(() => result.BasicInfo_InvoiceAddressArea),
                          Projections.Property(() => InvoiceAddressCountry.PrintableName).WithAlias(() => result.BasicInfo_InvoiceAddressCountry),
                          Projections.Property(() => invoiceAddres.Department).WithAlias(() => result.BasicInfo_InvoiceAddressDepartment),
                          Projections.Property(() => invoiceAddres.Fax).WithAlias(() => result.BasicInfo_InvoiceAddressFax),
                          Projections.Property(() => invoiceAddres.MainEmail).WithAlias(() => result.BasicInfo_InvoiceAddressEmail),
                      };
var query = Session.QueryOver(() => counterpart)
    .JoinQueryOver<InvoiceAddress>(x => x.InvoiceAddresses, () => invoiceAddres)
    .Where(x => x.IsDefault)
    .JoinQueryOver<Country>(() => invoiceAddres.Country, () => InvoiceAddressCountry)
    .Select(projections);

问题出在InvoiceAddressCountry，它可能为空。如果发生这种情况，我希望result.BasicInfo_InvoiceAddressCountry属性保持为空。需要说明的是，上面的代码不起作用。它不能处理null。

Answer 1

从您的话中，我可以假设您需要使用left join：

.Left.JoinQueryOver(() => invoiceAddres.Country, () => InvoiceAddressCountry)