Question

Why the GetOrders() does not return any element I get an InvalidOperationException that there are no elements in the sequence.

   int maxNumber = GetOrders().Max(o => o.Number);

How can I fix this so I get the integer default value which is 0 when the sequence is empty else I want the max value is the sequence is not empty.

Answer 1

You can use the overload of Enumerable.DefaultIfEmpty:

int maxNumber = GetOrders().DefaultIfEmpty(new Order { Number = 0 }).Max(o => o.Number);

If you just want to select the int value anyway, this approach is even simpler:

int maxNumber = GetOrders().Select(o => o.Number).DefaultIfEmpty(0).Max(o => o);