Are multiple iterators (for a single class or object) possible in C# .NET? If they are, give me some simple examples.
Sorry if the question is not understandable and please make me clear.
You could certainly create different iterators to traverse in different ways. For example, you could have:
public class Tree<T>
{
public IEnumerable<T> IterateDepthFirst()
{
// Iterate, using yield return
...
}
public IEnumerable<T> IterateBreadthFirst()
{
// Iterate, using yield return
...
}
}
Is that the kind of thing you were asking?
You could also potentially write:
public class Foo : IEnumerable<int>, IEnumerable<string>
but that would cause a lot of confusion, and the foreach loop would pick whichever one had the non-explicitly-implemented GetEnumerator call.
You can also iterate multiple times over the same collection at the same time:
foreach (Person person1 in party)
{
foreach (Person person2 in party)
{
if (person1 != person2)
{
person1.SayHello(person2);
}
}
}
It's not really clear if you mean that you can implement more than one iterator for a class, or if you can use more than one iterater for a class at a time. Either is possible.
You can have as many iterators as you like for a class:
public class OddEvenList<T> : List<T> {
public IEnumerable<T> GetOddEnumerator() {
return this.Where((x, i) => i % 2 == 0);
}
public IEnumerable<T> GetEvenEnumerator() {
return this.Where((x, i) => i % 2 == 1);
}
}
You can have as many instances of an iterator for a class active at the same time as you like:
foreach (int x in list) {
foreach (int y in list) {
foreach (int z in list) {
...
}
}
}
One option would be to implement the Strategy pattern:
Create separate IEnumerator classes for each traversal strategy.
Create a private attribute in the collection that stores the current strategy (with a default).
Create a SetStrategy() method that changes that private attribute to the selected concrete strategy.
Override GetEnumerator() to return an instance of the current strategy.
Of course, this means two threads trying to set the strategy at the same time could interfere, so if sharing the collection between threads is important, this isn't the best solution.
A straight Iterator pattern would also work, which is what I believe Jon Skeet is suggesting in his first example, but you lose the syntactic sugar of being able to use foreach.
Related
I have a class that stores a string list, I would like to make this class usable in a foreach statement, so I found these two interfaces and I tried to implement them.
public class GroupCollection : IEnumerable, IEnumerator
{
public List<string> Groups { get; set; }
public int Count { get { return Groups.Count; } }
int position = -1;
}
public IEnumerator GetEnumerator()
{
return (IEnumerator)this;
}
public object Current
{
get
{
try
{
return new Group(Groups[position]);
}
catch (IndexOutOfRangeException)
{
throw new InvalidOperationException();
}
}
}
public bool MoveNext()
{
position++;
return position < Groups.Count;
}
public void Reset()
{
position = 0;
}
I'm iterating through a GroupCollection variable twice:
foreach (GroupCollection.Group in groups) // where groups is a GroupCollection
{
}
foreach (GroupCollection.Group in groups)
{
}
// where Group is a nested class in GroupCollection.
When it is at the first foreach it works well (count is 1 at this time). I don't modify anything, and when it goes to the second foreach it doesn't go into the loop. I went through the code line by line in debugging mode and found out that the reset is not called after the first foreach. So should I manually call reset after the foreach? Isn't there a nicer way to do this?
I don't modify anything
Yes you do - your MoveNext() modifies the state of the class. This is why you shouldn't implement both IEnumerable and IEnumerator in the same class. (The C# compiler does for iterator blocks, but that's a special case.) You should be able to call GetEnumerator() twice and get two entirely independent iterators. For example:
foreach (var x in collection)
{
foreach (var y in collection)
{
Console.WriteLine("{0}, {1}", x, y);
}
}
... should give you all possible pairs of items in a collection. But that only works when the iterators are independent.
I went through the code line by line in debugging mode and found out that the reset is not called after the first foreach.
Why would you expect it to? I don't believe the specification says anything about foreach calling Reset - and that's a good job, as many implementations don't really implement it (they throw an exception instead).
Basically, you should make your GetEnumerator() method return a new object which keeps the mutable state of the "cursor" over your data. Note that the simplest way of implementing an iterator in C# is usually to use an iterator block (yield return etc).
I'd also strongly encourage you to implement the generic interfaces rather than just the non-generic ones; that way your type can be used much more easily in LINQ code, the iterator variable in a foreach statement can be implicitly typed appropriately, etc.
Reset is not called at the end of a foreach loop - you could do that in the GetEnumerator call, or just return the enumerator for the List:
public IEnumerator GetEnumerator()
{
return Groups.GetEnumerator;
}
Note that with the yield keyword there is almost no need to implement IEnumerator or IEnumerable explicitly:
public IEnumerator<string> GetEnumerator()
{
foreach(string s in Groups)
yield return s;
}
I have a function that accepts an Enumerable. I need to ensure that the enumerator is evaluated, but I'd rather not create a copy of it (e.g. via ToList() or ToArray()) if it is all ready in a List or some other "frozen" collection. By Frozen I mean collections where the set of items is already established e.g. List, Array, FsharpSet, Collection etc, as opposed to linq stuff like Select() and where().
Is it possible to create a function "ForceEvaluation" that can determine if the enumerable has deffered execution pending, and then evaluate the enumerable?
public void Process(IEnumerable<Foo> foos)
{
IEnumerable<Foo> evalutedFoos = ForceEvaluation(foos)
EnterLockedMode(); // all the deferred processing needs to have been done before this line.
foreach (Foo foo in foos)
{
Bar(foo);
}
}
public IEnumerable ForceEvaluation(IEnumerable<Foo> foos)
{
if(??????)
{ return foos}
else
{return foos.ToList()}
}
}
After some more research I've realized that this is pretty much impossible in any practical sense, and would require complex code inspection of each iterator.
So I'm going to go with a variant of Mark's answer and create a white-list of known safe types and just call ToList() anything not on that is not on the white-list.
Thank you all for your help.
Edit*
After even more reflection, I've realized that this is equivalent to the halting problem. So very impossible.
Something that worked for me way :
IEnumerable<t> deffered = someArray.Where(somecondition);
if (deffered.GetType().UnderlyingSystemType.Namespace.Equals("System.Linq"))
{
//this is a deffered executin IEnumerable
}
You could try a hopeful check against IList<T> or ICollection<T>, but note that these can still be implemented lazily - but it is much rarer, and LINQ doesn't do that - it just uses iterators (not lazy collections). So:
var list = foos as IList<Foo>;
if(list != null) return list; // unchanged
return foos.ToList();
Note that this is different to the regular .ToList(), which gives you back a different list each time, to ensure nothing unexpected happens.
Most concrete collection types (including T[] and List<T>) satisfy IList<T>. I'm not familiar with the F# collections - you'd need to check that.
I would avoid it if you want to make sure it is "frozen". Both Array elements and List<> can be changed at any time (i.e. infamous "collection changed during iteration" exception). If you really need to make sure IEnumerable is evaluated AND not changing underneath your code than copy all items into your own List/Array.
There could be other reasons to try it - i.e. some operations inside run time do special checks for collection being an array to optimize them. Or have special version for specialized interface like ICollection or IQueryable in addition to generic IEnumerable.
EDIT: Example of collection changing during iteration:
IEnumerable<T> collectionAsEnumrable = collection;
foreach(var i in collectionAsEnumrable)
{
// something like following can be indirectly called by
// synchronous method on the same thread
collection.Add(i.Clone());
collection[3] = 33;
}
If it is possible to use a wrapper in your case, you could do something like this
public class ForceableEnumerable<T> : IEnumerable<T>
{
IEnumerable<T> _enumerable;
IEnumerator<T> _enumerator;
public ForceableEnumerable(IEnumerable<T> enumerable)
{
_enumerable = enumerable;
}
public void ForceEvaluation()
{
if (_enumerator != null) {
while (_enumerator.MoveNext()) {
}
}
}
#region IEnumerable<T> Members
public IEnumerator<T> GetEnumerator()
{
_enumerator = _enumerable.GetEnumerator();
return _enumerator;
}
#endregion
#region IEnumerable Members
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
#endregion
}
Or implement the force method like this if you want to evaluate in any case
public void ForceEvaluation()
{
if (_enumerator == null) {
_enumerator = _enumerable.GetEnumerator();
}
while (_enumerator.MoveNext()) {
}
}
EDIT:
If you want to ensure that the enumeration is evaluated only once in any case, you could change GetEnumerator to
public IEnumerator<T> GetEnumerator()
{
if (_enumerator == null) }
_enumerator = _enumerable.GetEnumerator();
}
return _enumerator;
}
I am developing a collection class, which should implement IEnumerator and IEnumerable.
In my first approach, I implemented them directly. Now I have discovered the yield keyword, and I have been able to simplify everything a whole lot substituting the IEnumerator/IEnumerable interfaces with a readonly property Values that uses yield to return an IEnumerable in a loop.
My question: is it possible to use yield in such a way that I could iterate over the class itself, without implementing IEnumerable/IEnumerator?
I.e., I want to have a functionality similar to the framework collections:
List<int> myList = new List<int>();
foreach (int i in myList)
{
...
}
Is this possible at all?
Update: It seems that my question was badly worded. I don't mind implementing IEnumerator or IEnumerable; I just thought the only way to do it was with the old Current/MoveNext/Reset methods.
You won't have to implement IEnumerable<T> or IEnumerable to get foreach to work - but it would be a good idea to do so. It's very easy to do:
public class Foo : IEnumerable<Bar>
{
public IEnumerator<Bar> GetEnumerator()
{
// Use yield return here, or
// just return Values.GetEnumerator()
}
// Explicit interface implementation for non-generic
// interface; delegates to generic implementation.
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}
The alternative which doesn't implement IEnumerable<T> would just call your Values property, but still providing a GetEnumerator() method:
public class Foo
{
public IEnumerator<Bar> GetEnumerator()
{
// Use yield return here, or
// just return Values.GetEnumerator()
}
]
While this will work, it means you won't be able to pass your collection to anything expecting an IEnumerable<T>, such as LINQ to Objects.
It's a little-known fact that foreach will work with any type supporting a GetEnumerator() method which returns a type with appropriate MoveNext() and Current members. This was really to allow strongly-typed collections before generics, where iterating over the collection wouldn't box value types etc. There's really no call for it now, IMO.
You could do somthing like this, but why? IEnumerator is already simple.
Interface MyEnumerator<T>
{
public T GetNext();
}
public static class MyEnumeratorExtender
{
public static void MyForeach<T>(this MyEnumerator<T> enumerator,
Action<T> action)
{
T item = enumerator.GetNext();
while (item != null)
{
action.Invoke(item);
item = enumerator.GetNext();
}
}
}
I'd rather have the in keyword and I wouldn't want to rewrite linq.
How can I make ComponentTraversal.GetDescendants() better using LINQ?
Question
public static class ComponentTraversal
{
public static IEnumerable<Component> GetDescendants(this Composite composite)
{
//How can I do this better using LINQ?
IList<Component> descendants = new Component[]{};
foreach(var child in composite.Children)
{
descendants.Add(child);
if(child is Composite)
{
descendants.AddRange((child as Composite).GetDescendants());
}
}
return descendants;
}
}
public class Component
{
public string Name { get; set; }
}
public class Composite: Component
{
public IEnumerable<Component> Children { get; set; }
}
public class Leaf: Component
{
public object Value { get; set; }
}
Answer
I edited Chris's answer to provide a generic extension method that I've added to my Common library. I can see this being helpful for other people as well so here it is:
public static IEnumerable<T> GetDescendants<T>(this T component, Func<T,bool> isComposite, Func<T,IEnumerable<T>> getCompositeChildren)
{
var children = getCompositeChildren(component);
return children
.Where(isComposite)
.SelectMany(x => x.GetDescendants(isComposite, getCompositeChildren))
.Concat(children);
}
Thanks Chris!
Also,
Please look at LukeH's answer at http://blogs.msdn.com/b/wesdyer/archive/2007/03/23/all-about-iterators.aspx . His answer provides a better way to approach this problem in general, but I did not select it because it was not a direct answer to my question.
There are often good reasons to avoid (1) recursive method calls, (2) nested iterators, and (3) lots of throwaway allocations. This method avoids all of those potential pitfalls:
public static IEnumerable<Component> GetDescendants(this Composite composite)
{
var stack = new Stack<Component>();
do
{
if (composite != null)
{
// this will currently yield the children in reverse order
// use "composite.Children.Reverse()" to maintain original order
foreach (var child in composite.Children)
{
stack.Push(child);
}
}
if (stack.Count == 0)
break;
Component component = stack.Pop();
yield return component;
composite = component as Composite;
} while (true);
}
And here's the generic equivalent:
public static IEnumerable<T> GetDescendants<T>(this T component,
Func<T, bool> hasChildren, Func<T, IEnumerable<T>> getChildren)
{
var stack = new Stack<T>();
do
{
if (hasChildren(component))
{
// this will currently yield the children in reverse order
// use "composite.Children.Reverse()" to maintain original order
// or let the "getChildren" delegate handle the ordering
foreach (var child in getChildren(component))
{
stack.Push(child);
}
}
if (stack.Count == 0)
break;
component = stack.Pop();
yield return component;
} while (true);
}
var result = composite.Children.OfType<Composite>().SelectMany(child => child.GetDescendants()).Concat(composite.Children);
return result.ToList();
When doing a translation from imperitive syntax to LINQ, it is usually pretty easy to take the translation one step at a time. Here is how this works:
This is looping over composite.Children, so that will be the collection we apply LINQ to.
There are two general operations occuring in the loop, so lets do one of them at a time
The "if" statement is performing a filter. Normally, we would use "Where" to perform a filter, but in this case the filter is based on type. LINQ has "OfType" built in for this.
For each child composite, we want to recursively call GetDescendants and add the results to a single list. Whenever we want to transform an element into something else, we use either Select or SelectMany. Since we want to transform each element into a list and merge them all together, we use SelectMany.
Finally, to add in the composite.Children themselves, we concatenate those results to the end.
I don't know about better, but I think this performs the same logic:
public static IEnumerable<Component> GetDescendants(this Composite composite)
{
return composite.Children
.Concat(composite.Children
.Where(x => x is Composite)
.SelectMany(x => x.GetDescendants())
);
}
It might be shorter, but there is nothing wrong with what you have. As I said above, this is supposed to perform the same thing and I doubt that the performance of the function is improved.
This is a good example for when you might want to implement an iterator. This has the advantage of lazy evaluation in a slightly more readable syntax. Also, if you need to add additional custom logic then this form is more extensible
public static IEnumerable<Component> GetDescendants(this Composite composite)
{
foreach(var child in composite.Children)
{
yield return child;
if(!(child is Composite))
continue;
foreach (var subChild in ((Composite)child).GetDescendants())
yield return subChild;
}
}
Does dot net have an interface like IEnumerable with a count property? I know about interfaces such as IList and ICollection which do offer a Count property but it seems like these interfaces were designed for mutable data structures first and use as a read only interface seems like an afterthought - the presence of an IsReadOnly field and mutators throwing exceptions when this property is true is IMO ample evidence for this.
For the time being I am using a custom interface called IReadOnlyCollection (see my own answer to this post) but I would be glad to know of other alternative approaches.
The key difference between the ICollection family and the IEnumerable family is the absence of certainty as to the count of items present (quite often the items will be generated/loaded/hydrated as needed) - in some cases, an Enumerable may not ever finish generating results, which is why the Count is missing.
Deriving and adding a Count is possible depending on your requirements, but it goes against this spirit, which is the purpose of ICollection - a collection of stuff that's all there.
Another way might be to use the System.Linq.Enumerable.Count method, i.e.
using System.Linq;
class X
{
void Y(IEnumerable<int> collection)
{
int itemCount = collection.Count();
}
}
or use the (System.Linq.Enumerable) .ToList() to pull all the items from the enumerator into a Collection and work from there.
(Also to answer your comment before having 50 rep:- the ".Count()" bit is a call to an extension method on the extension class System.Linq.Enumerable - the extension method is available on all things that derive from IEnumerable because the code has a "using System.Linq" which brings the extension methods in all classes in that namespace into scope - in this case its in the class Enumerable. If you're in VS, pressing F12 will bring you to the definition of S.L.Enumerable. BTW C# In Depth is a fantastic book for learning LINQ properly - its a page turner thats really helps you get the whole picture compared to learning the bits of LINQ piece by piece)
As of .Net 4.5, there are two new interfaces for this: IReadOnlyCollection<T> and IReadOnlyList<T>.
IReadOnlyCollection<T> is IEnumerable<T> with a Count property added, IReadOnlyList<T> also adds indexing.
It sounds like you really just want ReadOnlyCollection<T> - expose it as IList<T>, but by wrapping the original list like this you just get a read-only wrapper with an appropriate count.
Taking into consideration some of the comments I have decided to go with a wrapper class implementing a custom interface...
interface IReadOnlyCollection<T> : IEnumerable<T>
{
int Count { get; }
}
//This can now be not misused by downcasting to List
//The wrapper can also be used with lists since IList inherits from ICollection
public class CollectionWrapper<T> : IReadOnlyCollection<T>
{
public CollectionWrapper(ICollection<T> collection)
{
_collection = collection;
}
public int Count
{
get
{
return _collection.Count;
}
}
public IEnumerator<T> GetEnumerator()
{
return (IEnumerator<T>)_collection.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
return (IEnumerator)((IEnumerable)_collection).GetEnumerator();
}
////////Private data///////
ICollection<T> _collection;
}
class Program
{
static void Main(string[] args)
{
List<int> list = new List<int>();
list.Add(1);
list.Add(2);
list.Add(3);
list.Add(4);
CollectionWrapper<int> collection = new CollectionWrapper<int>(list);
Console.WriteLine("Count:{0}", collection.Count);
foreach (var x in collection)
{
Console.WriteLine(x);
}
foreach (var x in (IEnumerable)collection)
{
Console.WriteLine(x);
}
}
}
Thanks all for your suggestions.
Edit: Now cannot be misused by downcasting to List (or whatever).
IList can return IsReadOnly as true, which marks the collection as readonly. Other than that I'm afraid I don't know of anything fitting.
Since it's an interface, you would have to implement the Count property yourself, why don't you create a new interface that inherits IEnumerator and add a Count property?
IList or ICollection would be the way to go, if you want to use the standard interfaces.
Note that you can "hide" methods required by the interface if you don't want them in your class's public interface -- for example, since it's meaningless to add things to a readonly collection you can do this:
void ICollection<DataType>.Add(DataType item)
{
throw new NotSupportedException();
}
public DataType this[int index]
{
get { return InnerList[index]; }
}
DataType IList<DataType>.this[int index]
{
get { return this[index]; }
set { throw new NotSupportedException(); }
}
etc.
An array can be cast to an IList, which makes the IList ReadOnly == true :)
You can get .Count on IEnumerable with an extension method if you add a reference to System.Linq (in 3.5 anyway).
As Jon Skeet mentions, you're much better off using System.Collections.ObjectModel.ReadOnlyCollection instead of creating your own wrapper class.
Then you can implement your sample as follows:
class Program {
static void Main(string[] args) {
List<int> list = new List<int>();
list.Add(1);
list.Add(2);
list.Add(3);
list.Add(4);
ReadOnlyCollection<int> collection = new ReadOnlyCollection<int>(list);
Console.WriteLine("Count:{0}", collection.Count);
foreach (var x in collection) {
Console.WriteLine(x);
}
foreach (var x in (IEnumerable)collection) {
Console.WriteLine(x);
}
}
}