Return an empty collection when Linq where returns nothing

Question

I am using the below statement with the intent of getting all of the machine objects from the MachineList collection (type IEnumerable) that have a MachineStatus of i. The MachineList collection will not always contain machines with a status of i.

At times when no machines have a MachineStatus of i I'd like to return an empty collection. My call to ActiveMachines (which is used first) works but InactiveMachines does not.

public IEnumerable<Machine> ActiveMachines
{
    get
    {
        return Customer.MachineList
            .Where(m => m.MachineStatus == "a");
    }
}

public IEnumerable<Machine> InactiveMachines
{
    get
    {
        return Customer.MachineList
            .Where(m => m.MachineStatus == "i");
    }
}

Edit

Upon further examination it appears that any enumeration of MachineList will cause subsequent enumerations of MachineList to throw an exeception: Object reference not set to an instance of an object.

Therefore, it doesn't matter if a call is made to ActiveMachines or InactiveMachines as its an issue with the MachineList collection. This is especially troubling because I can break calls to MachineList simply by enumerating it in a Watch before it is called in code. At its lowest level MachineList implements NHibernate.IQuery being returned as an IEnumerable. What's causing MachineList to lose its contents after an initial enumeration?

Answer 1

Where returns an empty sequence if there are no matches; this is a perfectly valid sequence (not null). The only way you'd get a null is if you call FirstOrDefault or SingleOrDefault.

Are you sure the bug is where you think it is?

int?[] nums = { 1, 3, 5 };
var qry = nums.Where(i => i % 2 == 0);
Console.WriteLine(qry == null); // false
Console.WriteLine(qry.Count()); // 0
var list = qry.ToList();
Console.WriteLine(list.Count); // 0
var first = qry.FirstOrDefault();
Console.WriteLine(first == null); // true

Answer 2

By default, Enumerable.Where already does return an empty IEnumerable<T>, not null. If you are seeing "Object reference not set to an instance of an object." exceptions, it's most likely that something else is the problem.

Is MachineList null, perhaps? If you hadn't created it, you'd get that exception on your call to .Where(...)