SingleOrDefault 返回 null 时的 LINQ 新实例

Question

I have this simple method:

    #region Fields

    private Collection<Address> _addresses;

    #endregion

    #region Public methods

    public Address DeliveryAddress()
    {
        if (_addresses == null)
            if (this.Id > 0)
                _addresses = Core.Data.Addresses.GetClient(this.Id);

        return _addresses.SingleOrDefault(x => x.TypeId == AddressType.Delivery);
    }

    public Address InvoiceAddress()
    {
        if (_addresses == null)
            if (this.Id > 0)
                _addresses = Core.Data.Addresses.GetClient(this.Id);

        return _addresses.SingleOrDefault(x => x.TypeId == AddressType.Invoice);
    }

    #endregion

As you can see I trying to return one result for a DeliveryAddress and one result for an InvoiceAddress . My problem is that I would like the link expression to create a new instance of Address() if SingleOrDefault returns null . I am really new to linq, so I am not sure whether SingleOrDefault is the correct expression I should be using.

Answer 1

You could use DefaultIfEmpty and use that instance as default value:

return _addresses.Where(x => x.TypeId == AddressType.Delivery)
                 .DefaultIfEmpty(new Adress())
                 .Single();

Answer 2

Use the null-coalescing operator :

return _addresses
    .SingleOrDefault(x => x.TypeId == AddressType.Delivery) ?? new Address();

---

The expression

x ?? y

yields x if x is not null , otherwise y . You can chain the operator

x ?? y ?? z ?? t

This returns the first non-null value or null if all of them are null .

---

UPDATE

Note that SingleOrDefault throws an exception if the sequence has more than one element. If you need the first element of a sequence possibly having no or more than one element, use FirstOrDefault instead.

Answer 3

You could create your own extension method, like this:

public static T NewIfNull<T>(this T obj) where T: class, new()
{
   return obj ?? new T();
}

... then tack a usage onto the end of SingleOrDefault:

var singleResult = myCollection.SingleOrDefault().NewIfNull();

... or because the logic is so simple, just inline it as other answers have said.

Answer 4

Instead of

return _addresses.SingleOrDefault(x => x.TypeId == AddressType.Delivery);

Do something like this:

var address = _addresses.SingleOrDefault(x => x.TypeId == AddressType.Delivery);

if(address == null)
    address = new Address();

return address;

Answer 5

I'd be inclined to write both of these as extension methods on IEnumerable<Address> . You can use the null-coalesing operator to return a new instance if the SingleOrDefault() call returns null .

public static class AddressExtensions
{
    public static Address DeliveryAddress(this IEnumerable<Address> addresses)
    {
        return addresses.SingleOrDefault(x => x.TypeId == AddressType.Delivery) 
               ?? new Address();
    }

    public static Address InvoiceAddress(this IEnumerable<Address> addresses)
    {
        return addresses.SingleOrDefault(x => x.TypeId == AddressType.Invoice) 
               ?? new Address();
    }
}

Answer 6

Apart from alternatives in other answers, you can also create your own SingleOrNew Extension method.

public static TSource SingleOrNew<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate ) where T:new()
{
     return source.SingleOrDefault(predicate) ?? new T();
}

It can be used as

return _addresses.SingleOrNew(x => x.TypeId == AddressType.Delivery);