How to return null if using SingleOrDefault() and searching a list of numbers for a number not in list?

Question

When querying a list of positive numbers using SingleOrDefault(), how can I return null, or a custom value like -1, when a number isn't found in the list, instead of the type's default value (0 in this case)?

Answer 1

You can use DefaultIfEmpty to specify a custom default value for an empty collection:

var value = numbers.Where(MyFilter)
    .DefaultIfEmpty(-1) //or any other default value you want
    .Single(); //OrDefault not needed; DefaultIfEmpty ensures there is an item

Answer 2

You could use:

var first = theIntegers
                .Cast<int?>()
                .SingleOrDefault(i => i == theValue) ?? valueIfNotFound;

This works by casting the items to a Nullable<int>, then using the null-coalescing operator to return the value you choose if null was returned (ie: not found).

Note this will throw if you have more than one matching item. If you don't want that behavior, use FirstOrDefault instead of SingleOrDefault.