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I am a new C# developer and I have been programming in 2 month now. I have now come to a part where I want to create an own collection class, but I have a few problems.
What do I want to do?
- I want to create an own generic collection class (list) for storing data. My class should have methods for adding item to the end of list or removing item from list.
My Class should have properties for the number of items in the list and capacity of the iist.
I must store the list in an array inside my class and the aray must have a predefined length.
so first I want to create an design and write down how my class should look like,where i only write down all the things that I need like methods, properties, and this is what ive done so far:
public void Propertie () // depending on how many items I have, I create an propertie for each item
{
}
public void addItems() // method for adding items
{
}
public void removeItems() // method for removing items
{
}
int[] storeList; // array for storing the list
}
So before I start the real coding, I just want to know if i have missed or done something wrong?
Edit: I am Not allowed to use any of the c# collection or generic classes in this assignment.
I just want to know if i have missed or done something wrong?
The design you're showing here is missing a few things. First, you said you want to create a generic list. How are you going to store generic data in an integer array?
Also, the signature you give for addItems is
public void addItems()
and the signature you give for removeItems is
public void removeItems()
How does your list object know what to add, if it isn't given any data? Similarly, how does it know what to remove, if the client code doesn't tell it (you could always remove the first item, or always remove the last item, but then you're in stack or queue territory, not the standard object-oriented list API).
I'd recommend looking at the API for System.Collections.Generic.List, which is the built-in class you should be trying to emulate here. You don't need to implement all the properties and methods that List implements, but you should implement parts of that API. I would recommend
public sealed class MyList<T>
{
public int Count { get; }
public T this[int index] { get; set; }
public MyList();
public void Add(T item);
public void RemoveAt(int index);
}
as the bare minimum API for a list class. You'd implement it by declaring a field of type T[] as the backing store, plus an internal size field. Don't forget that, if client code adds more items than the length of the backing store, you have to allocate a new backing store array (probably double the size of the old one), copy the elements to the new array, and start treating the new array as the backing store.
You should also look into implementing System.Collections.Generic.IEnumerable, which isn't strictly necessary, but makes a collection class much more useful.
Assuming that you've assured yourself that you really want to do this, you should consider:
What types you are going to support (you say generic, but have an int array in your example
How are you going to support access and iteration (e.g. will you support foreach, array style [])
how will the programmer specify the length (you say fixed length, does the programming specify that up front, is there a default)
Will you support find methods, sort methods, etc...
Can the programmer copying one collection into another larger collection? A smaller one?
Are you going to support FIFO, LIFO, bidirectional iteration, etc...
Creating a good generic collection will be difficult, so I recommend that you look at everything that you would want a collection to support and go from there.
So, yes your initial thoughts are still incomplete. At a minimum it needs a method to access the data and to know how much data is there.
you can implement the IList<T> interface, it contains all the required signatures to build own generic list class. MSDN documentation for IList
I have a class called GestorePersonale which holds a list of instances of another class:
public List<Dipendente> Dipendenti
{
get;
private set;
}
I want to keep this list modifiable only from the methods the class exposes, and not directly. I noticed that with the code above, one could just do var gp = new GestorePersonale();
gp.Dipendenti.Add( new Dipendente( ... ) );
and be able to perform any other kind of action on the List<Dipendente> itself.
I considered converting the first code snippet to
private List dipendenti;
but I could find a few downsides to that:
This would break the personal rule of mine to try to always use the public fields over the private ones from inside the class's methods whenever possible (even though I'm not sure if it is good practice to do so, so any clarification would be welcome);This would impair any external entities' ability to access the contents of the list for reading purposes only, like, say, to execute a LINQ query over the contents of the list.
What would be the best way to solve this situation?
You can wrap the list in a ReadOnlyCollection<T> and expose that:
private List<Dipendente> dipendenti;
private ReadOnlyCollection<Dipendente> readOnlyDipendenti;
public GestorePersonale()
{
dipendenti = new List<Dipendente>();
readOnlyDipendenti = new ReadOnlyCollection<Dipendente>(dipendenti);
}
public ReadOnlyCollection<Dipendente> Dipendenti
{
get { return readOnlyDipendenti; }
}
Internally, you have access to dipendenti and can add/remove items. External entities have access only to the ReadOnlyCollection<T> that wraps the list, so they can only read, but not add/remove items.
I would agree with dtb that ReadOnlyCollections is the way to go. However, you can return it from the property getter (using AsReadOnly) and drop the method.
private List<Dipendente> dipendenti = new List<Dipendente>();
public ReadOnlyCollection<Dipendente> ReadOnlyDipendenti
{
get
{
return dipendenti.AsReadOnly();
}
}
there are a couple of things you can do:
you use ReadOnlyCollection
you can return an IEnumerable<_type>
you can wrap the list in another class
you can roll your own collection class, implementing the appropriate interface
the method you use depends on the functionality you need and what you want/need to expose to the user of your class
What you have is a public property with a private accessor. It is very useful. It allows an instance to expose a value that is controlled (set) by the instance itself, e.g. a state.
For example, take a collection with a Count property. It makes no sense for it have a public accessor. An implementation could be to update the property (internally) when the collection is changed (to avoid having to count it each time).
Do a setter method or wrap the field in another class. This is a classic collection set and collection.add problem.
I know there has been a lot of posts on this but it still confuses me why should you pass in an interface like IList and return an interface like IList back instead of the concrete list.
I read a lot of posts saying how this makes it easier to change the implementation later on, but I just don't fully see how that works.
Say if I have this method
public class SomeClass
{
public bool IsChecked { get; set; }
}
public void LogAllChecked(IList<SomeClass> someClasses)
{
foreach (var s in someClasses)
{
if (s.IsChecked)
{
// log
}
}
}
I am not sure how using IList will help me out in the future.
How about if I am already in the method? Should I still be using IList?
public void LogAllChecked(IList<SomeClass> someClasses)
{
//why not List<string> myStrings = new List<string>()
IList<string> myStrings = new List<string>();
foreach (var s in someClasses)
{
if (s.IsChecked)
{
myStrings.Add(s.IsChecked.ToString());
}
}
}
What do I get for using IList now?
public IList<int> onlySomeInts(IList<int> myInts)
{
IList<int> store = new List<int>();
foreach (var i in myInts)
{
if (i % 2 == 0)
{
store.Add(i);
}
}
return store;
}
How about now? Is there some new implementation of a list of int's that I will need to change out?
Basically, I need to see some actual code examples of how using IList would have solved some problem over just taking List into everything.
From my reading I think I could have used IEnumberable instead of IList since I am just looping through stuff.
Edit
So I have been playing around with some of my methods on how to do this. I am still not sure about the return type(if I should make it more concrete or an interface).
public class CardFrmVm
{
public IList<TravelFeaturesVm> TravelFeaturesVm { get; set; }
public IList<WarrantyFeaturesVm> WarrantyFeaturesVm { get; set; }
public CardFrmVm()
{
WarrantyFeaturesVm = new List<WarrantyFeaturesVm>();
TravelFeaturesVm = new List<TravelFeaturesVm>();
}
}
public class WarrantyFeaturesVm : AvailableFeatureVm
{
}
public class TravelFeaturesVm : AvailableFeatureVm
{
}
public class AvailableFeatureVm
{
public Guid FeatureId { get; set; }
public bool HasFeature { get; set; }
public string Name { get; set; }
}
private IList<AvailableFeature> FillAvailableFeatures(IEnumerable<AvailableFeatureVm> avaliableFeaturesVm)
{
List<AvailableFeature> availableFeatures = new List<AvailableFeature>();
foreach (var f in avaliableFeaturesVm)
{
if (f.HasFeature)
{
// nhibernate call to Load<>()
AvailableFeature availableFeature = featureService.LoadAvaliableFeatureById(f.FeatureId);
availableFeatures.Add(availableFeature);
}
}
return availableFeatures;
}
Now I am returning IList for the simple fact that I will then add this to my domain model what has a property like this:
public virtual IList<AvailableFeature> AvailableFeatures { get; set; }
The above is an IList itself as this is what seems to be the standard to use with nhibernate. Otherwise I might have returned IEnumberable back but not sure. Still, I can't figure out what the user would 100% need(that's where returning a concrete has an advantage over).
Edit 2
I was also thinking what happens if I want to do pass by reference in my method?
private void FillAvailableFeatures(IEnumerable<AvailableFeatureVm> avaliableFeaturesVm, IList<AvailableFeature> toFill)
{
foreach (var f in avaliableFeaturesVm)
{
if (f.HasFeature)
{
// nhibernate call to Load<>()
AvailableFeature availableFeature = featureService.LoadAvaliableFeatureById(f.FeatureId);
toFill.Add(availableFeature);
}
}
}
would I run into problems with this? Since could they not pass in an array(that has a fixed size)? Would it be better maybe for a concrete List?
There are three questions here: what type should I use for a formal parameter? What should I use for a local variable? and what should I use for a return type?
Formal parameters:
The principle here is do not ask for more than you need. IEnumerable<T> communicates "I need to get the elements of this sequence from beginning to end". IList<T> communicates "I need to get and set the elements of this sequence in arbitrary order". List<T> communicates "I need to get and set the elements of this sequence in arbitrary order and I only accept lists; I do not accept arrays."
By asking for more than you need, you (1) make the caller do unnecessary work to satisfy your unnecessary demands, and (2) communicate falsehoods to the reader. Ask only for what you're going to use. That way if the caller has a sequence, they don't need to call ToList on it to satisfy your demand.
Local variables:
Use whatever you want. It's your method. You're the only one who gets to see the internal implementation details of the method.
Return type:
Same principle as before, reversed. Offer the bare minimum that your caller requires. If the caller only requires the ability to enumerate the sequence, only give them an IEnumerable<T>.
The most practical reason I've ever seen was given by Jeffrey Richter in CLR via C#.
The pattern is to take the basest class or interface possible for your arguments and return the most specific class or interface possible for your return types. This gives your callers the most flexibility in passing in types to your methods and the most opportunities to cast/reuse the return values.
For example, the following method
public void PrintTypes(IEnumerable items)
{
foreach(var item in items)
Console.WriteLine(item.GetType().FullName);
}
allows the method to be called passing in any type that can be cast to an enumerable. If you were more specific
public void PrintTypes(List items)
then, say, if you had an array and wished to print their type names to the console, you would first have to create a new List and fill it with your types. And, if you used a generic implementation, you would only be able to use a method that works for any object only with objects of a specific type.
When talking about return types, the more specific you are, the more flexible callers can be with it.
public List<string> GetNames()
you can use this return type to iterate the names
foreach(var name in GetNames())
or you can index directly into the collection
Console.WriteLine(GetNames()[0])
Whereas, if you were getting back a less specific type
public IEnumerable GetNames()
you would have to massage the return type to get the first value
Console.WriteLine(GetNames().OfType<string>().First());
IEnumerable<T> allows you to iterate through a collection. ICollection<T> builds on this and also allows for adding and removing items. IList<T> also allows for accessing and modifying them at a specific index. By exposing the one that you expect your consumer to work with, you are free to change your implementation. List<T> happens to implement all three of those interfaces.
If you expose your property as a List<T> or even an IList<T> when all you want your consumer to have is the ability to iterate through the collection. Then they could come to depend on the fact that they can modify the list. Then later if you decide to convert the actual data store from a List<T> to a Dictionary<T,U> and expose the dictionary keys as the actual value for the property (I have had to do exactly this before). Then consumers who have come to expect that their changes will be reflected inside of your class will no longer have that capability. That's a big problem! If you expose the List<T> as an IEnumerable<T> you can comfortably predict that your collection is not being modified externally. That is one of the powers of exposing List<T> as any of the above interfaces.
This level of abstraction goes the other direction when it belongs to method parameters. When you pass your list to a method that accepts IEnumerable<T> you can be sure that your list is not going to be modified. When you are the person implementing the method and you say you accept an IEnumerable<T> because all you need to do is iterate through that list. Then the person calling the method is free to call it with any data type that is enumerable. This allows your code to be used in unexpected, but perfectly valid ways.
From this it follows that your method implementation can represent its local variables however you wish. The implementation details are not exposed. Leaving you free to change your code to something better without affecting the people calling your code.
You cannot predict the future. Assuming that a property's type will always be beneficial as a List<T> is immediately limiting your ability to adapt to unforeseen expectations of your code. Yes, you may never change that data type from a List<T> but you can be sure that if you have to. Your code is ready for it.
Short Answer:
You pass the interface so that no matter what concrete implementation of that interface you use, your code will support it.
If you use a concrete implementation of list, another implementation of the same list will not be supported by your code.
Read a bit on inheritance and polymorphism.
Here's an example: I had a project once where our lists got very large, and resulting fragmentation of the large object heap was hurting performance. We replaced List with LinkedList. LinkedList does not contain an array, so all of a sudden, we had almost no use of the large object heap.
Mostly, we used the lists as IEnumerable<T>, anyway, so there was no further change needed. (And yes, I would recommend declaring references as IEnumerable if all you're doing is enumerating them.) In a couple of places, we needed the list indexer, so we wrote an inefficient IList<T> wrapper around the linked lists. We needed the list indexer infrequently, so the inefficiency was not a problem. If it had been, we could have provided some other implementation of IList, perhaps as a collection of small-enough arrays, that would have been more efficiently indexable while also avoiding large objects.
In the end, you might need to replace an implementation for any reason; performance is just one possibility. Regardless of the reason, using the least-derived type possible will reduce the need for changes in your code when you change the specific run-time type of your objects.
Inside the method, you should use var, instead of IList or List. When your data source changes to come from a method instead, your onlySomeInts method will survive.
The reason to use IList instead of List as parameters, is because many things implement IList (List and [], as two examples), but only one thing implements List. It's more flexible to code to the interface.
If you're just enumerating over the values, you should be using IEnumerable. Every type of datatype that can hold more than one value implements IEnumerable (or should) and makes your method hugely flexible.
Using IList instead of List makes writing unit tests significantly easier. It allows you to use a 'Mocking' library to pass and return data.
The other general reason for using interfaces is to expose the minimum amount of knowledge necessary to the user of an object.
Consider the (contrived) case where I have a data object that implements IList.
public class MyDataObject : IList<int>
{
public void Method1()
{
...
}
// etc
}
Your functions above only care about being able to iterate over a list. Ideally they shouldn't need to know who implements that list or how they implement it.
In your example, IEnumerable is a better choice as you thought.
It is always a good idea to reduce the dependencies between your code as much as possible.
Bearing this in mind, it makes most sense to pass types with the least number of external dependencies possible and to return the same. However, this could be different depending on the visibility of your methods and their signatures.
If your methods form part of an interface, the methods will need to be defined using types available to that interface. Concrete types will probably not be available to interfaces, so they would have to return non-concrete types. You would want to do this if you were creating a framework, for example.
However, if you are not writing a framework, it may be advantageous to pass parameter with the weakest possible types (i.e. base classes, interfaces, or even delegates) and return concrete types. That gives the caller the ability to do as much as possible with the returned object, even if it is cast as an interface. However, this makes the method more fragile, as any change to the returned object type may break the calling code. In practice though, that generally isn't a major problem.
You accept an Interface as a parameter for a method because that allows the caller to submit different concrete types as arguments. Given your example method LogAllChecked, the parameter someClasses could be of various types, and for the person writing the method, all might be equivalent (i.e. you'd write the exact same code regardless of the type of the parameter). But for the person calling the method, it can make a huge difference -- if they have an array and you're asking for a list, they have to change the array to a list or v.v. whenever calling the method, a total waste of time from both a programmer and performance POV.
Whether you return an Interface or a concrete type depends upon what you want to let your callers do with the object you created -- this is an API design decision, and there's no hard and fast rule. You have to weigh their ability to make full use of the object against their ability to easily use a portion of the objects functionality (and of course whether you WANT them to be making full use of the object). For instance, if you return an IEnumerable, then you are limiting them to iterating -- they can't add or remove items from your object, they can only act against the objects. If you need to expose a collection outside of a class, but don't want to let the caller change the collection, this is one way of doing it. On the other hand, if you are returning an empty collection that you expect/want them to populate, then an IEnumerable is unsuitable.
Here's my answer in this .NET 4.5+ world.
Use IList<T> and IReadonlyList<T>,
instead of List<T>, because ReadonlyList<T> doesn't exist.
IList<T> looks so consistent with IReadonlyList<T>
Use IEnumerable<T> for minimum exposure (property) or requirement (parameter) if foreach is the only way to use it.
Use IReadonlyList<T> if you also need to expose/use Count and [] indexer.
Use IList<T> if you also allow callers to add/update/delete elements
because List<T> implements IReadonlyList<T>, it doesn't need any explicit casting.
An example class:
// manipulate the list within the class
private List<int> _numbers;
// callers can add/update/remove elements, but cannot reassign a new list to this property
public IList<int> Numbers { get { return _numbers; } }
// callers can use: .Count and .ReadonlyNumbers[idx], but cannot add/update/remove elements
public IReadOnlyList<int> ReadonlyNumbers { get { return _numbers; } }
I have a class called DataStructures where I have a set of public static data structures that store objects. To add an object to a data structures is an involved process requiring a number of checks to be carried out, processes to be remembered and data to be rearranged. In another class called Foo, I need to add objects to the data structures.
I was thinking I can do this by making a method called ObjectFeed which would take an object and the object's label as parameters. The label would tell the method which of the data structures the object should be added to. I would also have a method called addObject which would take the object to append and the appropriate target data structure as parameters:
Public Class DataStructures
{
public static List<obj> object1Storage = new List<obj>();
public static List<obj> object2Storage = new List<obj>();
...
}
Public Class Foo
{
public void ObjectFeed(/* PARAMETERS */)
{
//Code that generates an object called inspectionObject
//inspection object has an associated enum Type
if(objectType == Type.Type1)
{
addObject(inspectionObject, DataStructures.object1Storage);
}
if(objectType == Type.Type2)
{
addObject(inspectionObject, DataStructures.object2Storage);
}
...
}
private void addObject(obj inspectionObject, List<obj> objStorage)
{
objStorage.Add(inspectionObject);
//And a lot more code
}
}
Passing a public data structure as a parameter to a method that can just as well access that data structure directly doesn't feel correct. Is there a more clever and less intuitive way of doing this?
Edit:
In the example I originally contrived, the ObjectFeed method served no apparent purpose. I rewrote the method to look more like a method from the real world.
Where is the object type coming from? Passing a string value as a type of something is very rarely a good idea. Consider different options:
Create an enum for these values and use this. You can always parse it from string or print it to string if you need to.
Maybe it makes sense to have a couple of specific methods: FeedObjectType1(object obj), etc.? How often will these change?
Its really difficult to give you a definite answer without seeing the rest of the code.
Exposing public static lists from your DataStructures class is in most cases not a good design. To start with I would consider making them private and providing some methods to access the actual functionality that is needed. I would consider wrapping the lists with the addObject method, so that you don't have to pass the list as an argument. But again I am not sure if it makes sense in your case.
You seem to use DataStructures like some kind of global storage. I don't know what you store in there so I'm going to assume you have good reasons for this global storage.
If so, I would replace each list with a new kind of object, which deals with additions of data and does the checks relevant for it.
Something like:
interface IObjectStorage
{
void Add(object obj);
void Remove(object obj);
}
Each object storage type would derive from this and provide their own logic. Or it could derive from Collection<T> or something similar if collection-semantics makes sense. As your example is right now, I can't see the use for ObjectFeed, it serves as a fancy property accessor.
Selecting which property to access through a string sounds iffy to me. It is very prone to typos; I would rather use Type-objects available from any object in C# through the GetType-method or typeof() construct.
However. The whole setup feels a bit wrong to me, DataStructures et al.
First, testing your static class will be hard. I would pass around these stores to the types that need them instead. Replacing them with other stuff will also be hard, using interfaces will at least not tie you to a concrete implementation, but what if you want to use another location to store the objects in other code? Your static class is no longer relevant and you'll need to change a lot of code.
Maybe these things are out of your control, I don't know, the example code is a bit vague in that sense.
As pointed out in other answers:
The public static Lists are bad practice
Since the addObject method is the same for every data structure, it should be implemented as a data structure accessor.
To this end, I moved the instantiation of the data structures into Foo and moved the addObject method from Foo to a new class called StorageLibrary that more accurately represents the data structure architecture.
private class StorageLibrary
{
private List<obj> storedObjects = new List<obj>();
public void addObject(obj inspectionObject)
{
storedObjects.Add(inspectionObject);
//And a lot more code
}
}
public class Foo : StorageLibrary
{
//Declaration of libraries
public static StorageLibrary storage1 = new StorageLibrary();
public static StorageLibrary storage2 = new StorageLibrary();
...
private void ObjectFeed(/* PARAMATERS */)
{
//generate objects
if (objectType == Type.Type1)
{
storage1.addObject(inspectionObject);
}
if (objectType == Type.Type2)
{
storage2.addObject(inspectionObject);
}
...
}
}
Prior to C# generics, everyone would code collections for their business objects by creating a collection base that implemented IEnumerable
IE:
public class CollectionBase : IEnumerable
and then would derive their Business Object collections from that.
public class BusinessObjectCollection : CollectionBase
Now with the generic list class, does anyone just use that instead? I've found that I use a compromise of the two techniques:
public class BusinessObjectCollection : List<BusinessObject>
I do this because I like to have strongly typed names instead of just passing Lists around.
What is your approach?
I am generally in the camp of just using a List directly, unless for some reason I need to encapsulate the data structure and provide a limited subset of its functionality. This is mainly because if I don't have a specific need for encapsulation then doing it is just a waste of time.
However, with the aggregate initializes feature in C# 3.0, there are some new situations where I would advocate using customized collection classes.
Basically, C# 3.0 allows any class that implements IEnumerable and has an Add method to use the new aggregate initializer syntax. For example, because Dictionary defines a method Add(K key, V value) it is possible to initialize a dictionary using this syntax:
var d = new Dictionary<string, int>
{
{"hello", 0},
{"the answer to life the universe and everything is:", 42}
};
The great thing about the feature is that it works for add methods with any number of arguments. For example, given this collection:
class c1 : IEnumerable
{
void Add(int x1, int x2, int x3)
{
//...
}
//...
}
it would be possible to initialize it like so:
var x = new c1
{
{1,2,3},
{4,5,6}
}
This can be really useful if you need to create static tables of complex objects. For example, if you were just using List<Customer> and you wanted to create a static list of customer objects you would have to create it like so:
var x = new List<Customer>
{
new Customer("Scott Wisniewski", "555-555-5555", "Seattle", "WA"),
new Customer("John Doe", "555-555-1234", "Los Angeles", "CA"),
new Customer("Michael Scott", "555-555-8769", "Scranton PA"),
new Customer("Ali G", "", "Staines", "UK")
}
However, if you use a customized collection, like this one:
class CustomerList : List<Customer>
{
public void Add(string name, string phoneNumber, string city, string stateOrCountry)
{
Add(new Customer(name, phoneNumber, city, stateOrCounter));
}
}
You could then initialize the collection using this syntax:
var customers = new CustomerList
{
{"Scott Wisniewski", "555-555-5555", "Seattle", "WA"},
{"John Doe", "555-555-1234", "Los Angeles", "CA"},
{"Michael Scott", "555-555-8769", "Scranton PA"},
{"Ali G", "", "Staines", "UK"}
}
This has the advantage of being both easier to type and easier to read because their is no need to retype the element type name for each element. The advantage can be particularly strong if the element type is long or complex.
That being said, this is only useful if you need static collections of data defined in your app. Some types of apps, like compilers, use them all the time. Others, like typical database apps don't because they load all their data from a database.
My advice would be that if you either need to define a static collection of objects, or need to encapsulate away the collection interface, then create a custom collection class. Otherwise I would just use List<T> directly.
It's recommended that in public API's not to use List<T>, but to use Collection<T>
If you are inheriting from it though, you should be fine, afaik.
I prefer just to use List<BusinessObject>. Typedefing it just adds unnecessary boilerplate to the code. List<BusinessObject> is a specific type, it's not just any List object, so it's still strongly typed.
More importantly, declaring something List<BusinessObject> makes it easier for everyone reading the code to tell what types they are dealing with, they don't have to search through to figure out what a BusinessObjectCollection is and then remember that it's just a list. By typedefing, you'll have to require a consistent (re)naming convention that everyone has to follow in order for it to make sense.
Use the type List<BusinessObject> where you have to declare a list of them. However,
where you return a list of BusinessObject, consider returning IEnumerable<T>, IList<T> or ReadOnlyCollection<T> - i.e. return the weakest possible contract that satisfies the client.
Where you want to "add custom code" to a list, code extension methods on the list type. Again, attach these methods to the weakest possible contract, e.g.
public static int SomeCount(this IEnumerable<BusinessObject> someList)
Of course, you can't and shouldn't add state with extension methods, so if you need to add a new property and a field behind it, use a subclass or better, a wrapper class to store this.
I've been going back and forth on 2 options:
public class BusinessObjectCollection : List<BusinessObject> {}
or methods that just do the following:
public IEnumerable<BusinessObject> GetBusinessObjects();
The benefits of the first approach is that you can change the underlying data store without having to mess with method signatures. Unfortunately if you inherit from a collection type that removes a method from the previous implementation, then you'll have to deal with those situations throughout your code.
You should probably avoid creating your own collection for that purpose. It's pretty common to want to change the type of data structure a few times during refactorings or when adding new features. With your approach, you would wind up with a separate class for BusinessObjectList, BusinessObjectDictionary, BusinessObjectTree, etc.
I don't really see any value in creating this class just because the classname is more readable. Yeah, the angle bracket syntax is kind of ugly, but it's standard in C++, C# and Java, so even if you don't write code that uses it you're going to run into it all the time.
I generally only derive my own collection classes if I need to "add value". Like, if the collection itself needed to have some "metadata" properties tagging along with it.
I do the exact same thing as you Jonathan... just inherit from List<T>. You get the best of both worlds. But I generally only do it when there is some value to add, like adding a LoadAll() method or whatever.
You can use both. For laziness - I mean productivity - List is a very useful class, it's also "comprehensive" and frankly full of YANGNI members. Coupled with the sensible argument / recommendation put forward by the MSDN article already linked about exposing List as a public member, I prefer the "third" way:
Personally I use the decorator pattern to expose only what I need from List i.e:
public OrderItemCollection : IEnumerable<OrderItem>
{
private readonly List<OrderItem> _orderItems = new List<OrderItem>();
void Add(OrderItem item)
{
_orderItems.Add(item)
}
//implement only the list members, which are required from your domain.
//ie. sum items, calculate weight etc...
private IEnumerator<string> Enumerator() {
return _orderItems.GetEnumerator();
}
public IEnumerator<string> GetEnumerator() {
return Enumerator();
}
}
Further still I'd probably abstract OrderItemCollection into IOrderItemCollection so I can swap my implementation of IOrderItemCollection over in the future in (I may prefer to use a different inner enumerable object such as Collection or more likley for perf use a Key Value Pair collection or Set.
I use generic lists for almost all scenarios. The only time that I would consider using a derived collection anymore is if I add collection specific members. However, the advent of LINQ has lessened the need for even that.
6 of 1, half dozen of another
Either way its the same thing. I only do it when I have reason to add custom code into the BusinessObjectCollection.
With out it having load methods return a list allows me to write more code in a common generic class and have it just work. Such as a Load method.
As someone else pointed out, it is recommended not to expose List publicly, and FxCop will whinge if you do so. This includes inheriting from List as in:
public MyTypeCollection : List<MyType>
In most cases public APIs will expose IList (or ICollection or IEnumerable) as appropriate.
In cases where you want your own custom collection, you can keep FxCop quiet by inheriting from Collection instead of List.
If you choose to create your own collection class you should check out the types in System.Collections.ObjectModel Namespace.
The namespace defines base classes thare are ment to make it easier for implementers to create a custom collections.
I tend to do it with my own collection if I want to shield the access to the actual list. When you are writing business objects, chance is that you need a hook to know if your object is being added/removed, in such sense I think BOCollection is better idea. Of coz if that is not required, List is more lightweight. Also you might want to check using IList to provide additional abstraction interface if you need some kind of proxying (e.g. a fake collection triggers lazy load from database)
But... why not consider Castle ActiveRecord or any other mature ORM framework? :)
At the most of the time I simply go with the List way, as it gives me all the functionality I need at the 90% of the time, and when something 'extra' is needed, I inherit from it, and code that extra bit.
I would do this:
using BusinessObjectCollection = List<BusinessObject>;
This just creates an alias rather than a completely new type. I prefer it to using List<BusinessObject> directly because it leaves me free to change the underlying structure of the collection at some point in the future without changing code that uses it (as long as I provide the same properties and methods).
try out this:
System.Collections.ObjectModel.Collection<BusinessObject>
it makes unnecessary to implement basic method like CollectionBase do
this is the way:
return arrays, accept IEnumerable<T>
=)