c# Linq 链与避免 null 异常

Question

i have multi tables with one to many relation like chain

1- address has postal code id

2- postal table has area id

3- area table has city id

4- city table has county id

5- county table has country id

6- and at the last country table

from every table i need to get street name, full postal code, area name, city name, county name, country name

the query is as follow

var address = from add in _Database.Addresses
                      select add;

        address.Select(x=>new AddressClass { 
        BuildingNameOrNumber=x.BuildingNameOrNumber,
        MainStreet = x.Postcode ==null ? string.Empty: x.Postcode.StreetName,
        FullPostCode = x.Postcode == null ?  string.Empty :x.Postcode.FullPostcode,
        AreaName = x.Postcode == null ? string.Empty : x.Postcode.Area == null ? string.Empty: x.Postcode.Area.Name,
        CityName = x.Postcode == null ? string.Empty : x.Postcode.Area == null ? string.Empty : x.Postcode.Area.City == null ? string.Empty:  x.Postcode.Area.City.Name,
        CountyName = x.Postcode == null ? string.Empty : x.Postcode.Area == null ? string.Empty : x.Postcode.Area.City == null ? string.Empty : x.Postcode.Area.City.County == null ?string.Empty: x.Postcode.Area.City.County.Name,
        CountryName= x.Postcode == null ? string.Empty : x.Postcode.Area == null ? string.Empty : x.Postcode.Area.City == null ? string.Empty : x.Postcode.Area.City.County == null ? string.Empty : x.Postcode.Area.City.County.Country == null ? string.Empty:x.Postcode.Area.City.County.Country.CountryName
        })

i need to replace this multi conditions with one condition for every property

Answer 1

This is too complex for me to wrap my head around sorry:)

But what you need is the ? and ?? operators. Second to last item in your really complex query will be like:

CountyName = x.Postcode?.Area?.City?.County?.Name ?? string.Empty

x?.y returns y if x is not null and returns null otherwise. If y is not nullable then result of x?.y will become Nullable<T> where T is typeof(y)

x?? y x?? y returns x if x is not null and returns y if it is. Type of x and y should be the same here. Hope it helps

Edit

In the second look I see that you are using LINQ with IQueriable which can't use the null propagating operator. So in this case you don't have this option you can either load all the data using a ToList() run your query on them if the data size is small. Or you can load each part in separate queries (which due to the number of sub element I don't recommend). Alternatively you can use tools like this

My advice is the first option however may not be memory friendly on large datasets.

Sorry for the initial hasty answer.