List<object>.RemoveAll - How to create an appropriate Predicate

Question

This is a bit of noob question - I'm still fairly new to C# and generics and completely new to predicates, delegates and lamda expressions...

I have a class 'Enquiries' which contains a generic list of another class called 'Vehicles'. I'm building up the code to add/edit/delete Vehicles from the parent Enquiry. And at the moment, I'm specifically looking at deletions.

From what I've read so far, it appears that I can use Vehicles.RemoveAll() to delete an item with a particular VehicleID or all items with a particular EnquiryID. My problem is understanding how to feed .RemoveAll the right predicate - the examples I have seen are too simplistic (or perhaps I am too simplistic given my lack of knowledge of predicates, delegates and lambda expressions).

So if I had a List<Of Vehicle> Vehicles where each Vehicle had an EnquiryID, how would I use Vehicles.RemoveAll() to remove all vehicles for a given EnquiryID?

I understand there are several approaches to this so I'd be keen to hear the differences between approaches - as much as I need to get something working, this is also a learning exercise.

As an supplementary question, is a Generic list the best repository for these objects? My first inclination was towards a Collection, but it appears I am out of date. Certainly Generics seem to be preferred, but I'm curious as to other alternatives.

Thanks

Answer 1

This should work (where enquiryId is the id you need to match against):

vehicles.RemoveAll(vehicle => vehicle.EnquiryID == enquiryId);

What this does is passes each vehicle in the list into the lambda predicate, evaluating the predicate. If the predicate returns true (ie. vehicle.EnquiryID == enquiryId), then the current vehicle will be removed from the list.

If you know the types of the objects in your collections, then using the generic collections is a better approach. It avoids casting when retrieving objects from the collections, but can also avoid boxing if the items in the collection are value types (which can cause performance issues).

Answer 2

A predicate in T is a delegate that takes in a T and returns a bool. List<T>.RemoveAll will remove all elements in a list where calling the predicate returns true. The easiest way to supply a simple predicate is usually a lambda expression, but you can also use anonymous methods or actual methods.

{
    List<Vehicle> vehicles;
    // Using a lambda
    vehicles.RemoveAll(vehicle => vehicle.EnquiryID == 123);
    // Using an equivalent anonymous method
    vehicles.RemoveAll(delegate(Vehicle vehicle)
    {
        return vehicle.EnquiryID == 123;
    });
    // Using an equivalent actual method
    vehicles.RemoveAll(VehiclePredicate);
}

private static bool VehiclePredicate(Vehicle vehicle)
{
    return vehicle.EnquiryID == 123;
}

Answer 3

The RemoveAll() methods accept a Predicate delegate (until here nothing new). A predicate point to a method that simply return true or false. Of course, the RomveAll will remove from the collection all the T instance that return True with the predicate applied. C# 3.0 make the developer able to use several methods to pass a predicate to the RemoveAll method (and not oly this on…). You can use:

lamba expression

vehicles.RemoveAll(vehicle => vehicle.EnquiryID == 123);

Anonymous methods

vehicles.RemoveAll(delegate(Vehicle v) { return vehicle.EnquiryID == 1; });

Normal Methods

vehicles.RemoveAll(VehicleCustomPredicate); private static bool VehicleCustomPredicate (Vehicle v) { return vehicle.EnquiryID == 1; }