How to negate a Where clause of an IQueryable

Question

I created an extension method to encapsule some where logic like this (this is a very simplified version):

public static IQueryable<Cargo> ReadyToCarry(this IQueryable<Cargo> q)
{
    VehicleType[] dontNeedCouple = new VehicleType[] { VehicleType.Sprinter, VehicleType.Van, VehicleType.Truck };

    return q.Where(c => c.DriverId > 0 && c.VehicleId > 0)
            .Where(c => c.CoupleId > 0 || dontNeedCouple.Contains(c.Vehicle.Type));
}

So I can use it like this:

using (var ctx = new MyNiceContext())
{
    var readyCargo = ctx.Cargos.ReadyToCarry().OrderBy(c => c.Id).ToList();
    // ...more code
}

Which works nicely, this is translated to SQL and executed by Entity Framework. Now, I have another place I need cargos which are not ready to carry, which means I need exactly the opposite.

My idea was something like this:

public static IQueryable<Cargo> NotReadyToCarry(this IQueryable<Cargo> q)
{
    return !q.ReadyToCarry(); // ofc this doesn't work...
}

using (var ctx = new MyNiceContext())
{
    var readyCargo = ctx.Cargos.NotReadyToCarry().OrderBy(c => c.Id).ToList();
    // OR maybe
    var readyCargo = ctx.Cargos.ReadyToCarry(false).OrderBy(c => c.Id).ToList(); // somehow use that bool param to reverse the logic when false
}

I didn't want to recreate the reverse logic from scratch, so if I needed to change it one day, I'd change in one unique place.

I'm accepting alternatives to this approach, since it's a new project.

Answer 1

You can use Except() method:

var readyCargo = ctx.Cargos.ReadyToCarry().OrderBy(c => c.Id);
var notReadyCargo = ctx.Cargos.Except(readyCargo);

OR

you can add some parameter to ReadyToCarry():

public static IQueryable<Cargo> ReadyToCarry(this IQueryable<Cargo> q, bool ready = true)
{
    VehicleType[] dontNeedCouple = new VehicleType[] { VehicleType.Sprinter, VehicleType.Van, VehicleType.Truck };

    if (ready)
    {
        return q.Where(c => c.DriverId > 0 && c.VehicleId > 0)
                .Where(c => c.CoupleId > 0 || dontNeedCouple.Contains(c.Vehicle.Type));
    }
    else
    {
        // logic to get not ready for carrying
    }
}

OR

you can combine these two options:

public static IQueryable<Cargo> ReadyToCarry(this IQueryable<Cargo> q, bool ready = true)
{
    VehicleType[] dontNeedCouple = new VehicleType[] { VehicleType.Sprinter, VehicleType.Van, VehicleType.Truck };

    var readyToCarry = q.Where(c => c.DriverId > 0 && c.VehicleId > 0)
                        .Where(c => c.CoupleId > 0 || dontNeedCouple.Contains(c.Vehicle.Type));

    if (ready)
    {
        return readyToCarry;
    }
    else
    {
        return q.Except(readyToCarry);
    }
}

In the last case when you change logic to get ready to carry entities you don't need to change negation of that condition. You should change only one query.