FirstOrDefault returning Unexpected Value

Question

When I set a default value if the set is empty and call .FirstOrDefault() with a condition that isn't met I'm not getting my default value, but the type's default value:

int[] list = { 1, 2, 3, 4, 5 };
Console.WriteLine(list.DefaultIfEmpty(1).FirstOrDefault(i => i == 4)); // Outputs 4, as expected
Console.WriteLine(list.DefaultIfEmpty(1).FirstOrDefault(i => i > 5));  // Outputs 0, why??

This seems unintuitive since I'm setting .DefaultIfEmpty() to 1. Why doesn't this output a 1?

Answer 1

You appear to misunderstand how DefaultIfEmpty works.

list.DefaultIfEmpty(1) returns a singleton sequence containing 1 if the source collection (list) is empty. Since list is not empty, this has no effect and the source sequence is returned.

As a result, your query is effectively the same as:

int result = list.FirstOrDefault(i => i > 5);

The default of int is 0 so FirstOrDefault returns 0 if the condition is not met, which it is not since list contains no elements greater than 5.

You can get the behaviour you want using:

int result = list.Cast<int?>().FirstOrDefault(i => i > 5) ?? 1;

Answer 2

This is what you are looking for:

Console.WriteLine(list.Where(i => i > 5).DefaultIfEmpty(1).First());

By placing the Where before the DefaultIfEmpty, an empty collection will return an enumerable with one item. You can then use First to get that element.