DefaultIfEmpty returning null

Question

I am working with C#, .NET4.5, EF6(shouldn't matter really).

I am selecting some values from db then .ToList() them and then adding DefaultIfEmpty(new ActualFee{Net = 0, Vat = 0}) if one does not exist, and I get null

public static ConveyancingSummaryVm ToConveyancingSummaryVm(this Tuple<IEnumerable<ActualFee>, ConveyancingAnswer, Customer> conveyancePricingAnswersAndCustomer)
        {
            var purchaseFees = conveyancePricingAnswersAndCustomer.Item1.Where(o => o.ConveyancingSaleType == "Purchase").ToList();

            if (purchaseFees.Any())
            {
                var discount = purchaseFees.DefaultIfEmpty(new ActualFee{Net = 0, Vat = 0}).SingleOrDefault(o => o.Title.Contains("Discount")); 

                conveyancingSummaryVm.IsPurchaseFreehold = conveyancePricingAnswersAndCustomer.Item2.PropertyBoughtIsFreehold;
...

I must be missing something obvious here.

Answer 1

There is no way for DefaultIfEmpty to return null in this case. When there is no element it returns an ActualFee instance and the Title does not contain Discount. So that's why SingleOrDefault returns null.

so you are saying that DefaultIfEmpty does not work for SingleOrDefault?

No, DefaultIfEmpty works and returns the expected value. And then SingleOrDefault runs on the return value of DefaultIfEmpty and returns null because there is no element in the sequence that satisfies your condition.

You can use null coalescing operator to get the behaviour you want:

var discount = purchaseFees.FirstOrDefault(o => o.Title.Contains("Discount")) 
              ?? new ActualFee{Net = 0, Vat = 0};

Answer 2

SingleOrDefault must've returned null. If DefaultIfEmpty had returned null, it would've returned a Object reference not set to an instance of an object. Please rewrite that statement without chaining.