I was looking for, but could not find, an idiomatic example for a generic class implementing IEnumerable, which does this: constructor takes start and interval, and GetEnumerator returns IEnumerator, which starts from starts and goes on forever returning items with the interval.
In other words, something like:
public class Sequence<T> : IEnumerable<T> where T :... {
public T start { get; private set; }
public T interval { get; private set; }
Sequence(T start, T interval)
{
...
}
IEnumerator<T> GetEnumerator()
{
for(T n = start ; ; n += interval) // does not compile
yield return n;
}
... what else?
}
There are lots of related questions, and small snippets, bits&pieces, but I have not been able to find a single nice example of a complete class, which does at least something similar enough. This is about how to implement this, so if there is existing class which does just this, it'd be good to know, but not an answer to the question.
So, actual question(s): Which is the most recent C# version that has introduced new features useful for this, and what is the ideal example code to do this with it?
Also if there are any common pitfalls, mistakes which inexperienced C# developers make related to this kind of a class, those would be good to know.
Update: since the exact thing I'm asking for seems impossible, I guess the next best thing is replacing interval with a lambda for getting the next item, or something like that.
Thanks to dlev for suggesting Func, I was originally just using a helper class.
public class Sequence<T> : IEnumerable<T>
{
public T Start { get; private set; }
public T Interval { get; private set; }
private Func<T, T, T> Adder { get; set; }
public Sequence(T start, T interval, Func<T,T,T> adder)
{
Start = start;
Interval = interval;
Adder = adder;
}
public IEnumerator<T> GetEnumerator()
{
for (T n = Start; ; n = Adder.Invoke(n, Interval))
yield return n;
}
IEnumerator IEnumerable.GetEnumerator()
{
return this.GetEnumerator();
}
}
You can then use it like this:
int i = 0;
foreach (int t in new Sequence<int>(3, 4, (int a, int b) => a + b))
{
if (i == 10)
{
break;
}
i++;
Console.WriteLine(t);
}
Alternatively, you could require that T implement some sort of Addable interface and call that Add method, but I think this is cleaner all around.
What you're describing is very similar to the Generate function of MoreLinq.
The implementation is simple enough:
public static IEnumerable<TResult> Generate<TResult>(TResult initial, Func<TResult, TResult> generator)
{
if (generator == null) throw new ArgumentNullException("generator");
return GenerateImpl(initial, generator);
}
private static IEnumerable<TResult> GenerateImpl<TResult>(TResult initial, Func<TResult, TResult> generator)
{
TResult current = initial;
while (true)
{
yield return current;
current = generator(current);
}
}
As an alternative to using delegates, you could use generic operators from MiscUtil. Using that, your code would look like this:
IEnumerator<T> GetEnumerator()
{
for(T n = start ; ; n = Operator.Add(n, interval))
yield return n;
}
Related
Imagine an extension like this..
public static Blah<T>(this IList<T> ra)
{
..
}
Imagine you want to make a note of the most recently-called one.
private static IList recent;
public static Blah<T>(this IList<T> ra)
{
recent = ra;
..
}
You actually can not do that:
error CS0266: Cannot implicitly convert type System.Collections.Generic.IList<T> to System.Collections.IList.
1- You can simply make recent an object and that seems to work fine, but it seems like a poor solution.
2- It seems if you do have recent as an IList, you can actually cast the "ra" to that...
recent = (System.Collections.IList)ra;
and it seems to work. Seems strange though?? So,
3- Actually, what type should recent be so that you don't have to cast to it?? How can you make recent the same type as ra? You can't say this ....
private static System.Collections.Generic.IList recent;
it's not meaningful. So what the heck is the type of "ra"? What should recent "be" so that you can simply say recent=ra ?
(I mention this is for Unity, since you constantly use generic extensions in Unity.)
4- Consider a a further difficulty the case if you want to have a Dictionary of them all.
private static Dictionary<object,int> recents = new Dictionary<object,int>();
I can really only see how to do it as an object.
USE CASE EXAMPLE.
Here's an extension you use constantly, everywhere, in game engineering,
public static T AnyOne<T>(this IList<T> ra)
{
int k = ra.Count;
if (k<=0) {Debug.Log("Warn!"+k);}
int r = UnityEngine.Random.Range(0,k);
return ra[r];
}
no problem so far. So,
explosions.AnyOne();
yetAnotherEnemyToDefeat = dinosaurStyles.AnyOne();
and so on. However. Of course, actual random selections feel bad; in practice what you want is a fairly non-repeating order, more like a shuffle. Usually the best thing to do with any list or array is shuffle them, and serve them in that order; perhaps shuffle again each time through. Simple example, you have 20 random sound effects roars , being for when the dino roars. Each time you need one, if you do this
roars.AnyOne();
its OK, but not great. It will sound sort of suck. (Most players will report it as "not being random" or "repeating a lot".) This
roars.NextOne();
is much better. So, NextOne() should, on its own, (a) if we're at the start shuffle the list, (b) serve it in that order, (c) perhaps shuffle it again each time you use up the list. {There are further subtleties, eg, try not to repeat any near the end/start of the reshuffle, but irrelevant here.}
Note that subclassing List (and/or array) would suck for many obvious reasons, it's a job for a simple self-contained extension.
So then, here's a beautiful way to implement NextOne() using a simple stateful extension.
private static Dictionary<object,int> nextOne = new Dictionary<object,int>();
public static T NextOne<T>(this IList<T> ra)
{
if ( ! nextOne.ContainsKey(ra) )
// i.e., we've never heard about this "ra" before
nextOne.Add(ra,0);
int index = nextOne[ra];
// time to shuffle?
if (index==0)
{
Debug.Log("shuffling!"); // be careful to mutate, don't change the ra!
IList<T> temp = ra.OrderBy(r => UnityEngine.Random.value).ToList();
ra.Clear(); foreach(T t in temp) ra.Add(t);
}
T result = ra[index];
++index;
index=index%ra.Count;
nextOne[ra] = index;
return result;
}
This is surely the perfect example of a "stateful extension".
Notice indeed, I just used "object".
I guess in a way, the fundamental question in this QA is, is it best to use the Dictionary of "object" there, or, would something else more typey be better? Really that's the question at hand. Cheers!
If you want a single globally most recent IList<T> where T potentially varies each time, then your only options are to use object or dynamic. Both require casting; the latter just casts automatically.
I think your confusion stems from thinking that IList<T> inherits IList - it doesn't:
public interface IList<T> : ICollection<T>, IEnumerable<T>, IEnumerable
So arguably you could do this, although I don't see any advantage really:
private static IEnumerable recent;
public static void Blah<T>(this IList<T> ra)
{
recent = ra;
...
}
The simplest, and most type-safe, solution is to store a separate value for each T:
private static class RecentHolder<T> {
public static IList<T> Value { get; set; }
}
public static Blah<T>(this IList<T> ra) {
RecentHolder<T>.Value = ra;
}
What is the “type” of a generic IList< T >?
The base type..
Console.WriteLine( new List<int>().GetType().BaseType);
System.Object
The Generic Type definition ...
Console.WriteLine( new List<int>().GetType().GetGenericTypeDefinition());
System.Collections.Generic.List`1[T]
And to expand on SLAKS Answer
Not really. In the absence of a separate common non-generic base class
You can also use interfaces. So you could do...
public interface IName
{
string Name { get; set; }
}
public class Person : IName
{
public string Name { get; set; }
}
public class Dog : IName
{
public string Name { get; set; }
}
Then you could
private static List<IName> recent;
public static Blah<T>(this List<IName> ra)
{
recent = ra;
..
}
and it won't matter if you put Dog or Person in the list.
OR
I can't believe I didn't think about this last night; LINQ to the rescue using object.
using System;
using System.Linq;
using System.Collections.Generic;
public class Program
{
private static class WonkyCache
{
private static List<object> cache = new List<object>();
public static void Add(object myItem)
{
cache.Add(myItem);
}
public static IEnumerable<T> Get<T>()
{
var result = cache.OfType<T>().ToList();
return result;
}
}
public static void Main()
{
WonkyCache.Add(1);
WonkyCache.Add(2);
WonkyCache.Add(3);
WonkyCache.Add(Guid.NewGuid());
WonkyCache.Add("George");
WonkyCache.Add("Abraham");
var numbers = WonkyCache.Get<int>();
Console.WriteLine(numbers.GetType());
foreach(var number in numbers)
{
Console.WriteLine(number);
}
var strings = WonkyCache.Get<string>();
Console.WriteLine(strings.GetType());
foreach(var s in strings)
{
Console.WriteLine(s);
}
}
}
Results:
System.Collections.Generic.List`1[System.Int32]
1
2
3
System.Collections.Generic.List`1[System.String]
George
Abraham
Try:
public static class StatefulRandomizer<T>
// Use IEquatable<T> for Intersect()
where T : IEquatable<T>
{
// this could be enhanced to be a percentage
// of elements instead of hardcoded
private static Stack<T> _memory = new Stack<T>();
private static IEnumerable<T> _cache;
public static void UpdateWith(IEnumerable<T> newCache)
{
_cache = newCache.ToList();
// Setup the stack again, keep only ones that match
var matching = _memory.Intersect(newCache);
_memory = new Stack<T>(matching);
}
public static T GetNextNonRepeatingRandom()
{
var nonrepeaters = _cache
.Except(_memory);
// Not familar with unity.. but this should make
// sense what I am doing
var next = nonrepeaters.ElementAt(UnityEngine.Random(0, nonrepeaters.Count()-1));
// this fast, Stack will know it's count so no GetEnumerator()
// and _cache List is the same (Count() will call List.Count)
if (_memory.Count > _cache.Count() / 2)
{
_memory.Pop();
}
_memory.Push(next);
return next;
}
}
I have been working on a list of records for some computations using a class that inherits from a List<> and implements some additional functions. Something like:
public class ComplexValue
{
public double value;
public string name;
private int type;
// More members here
}
public class ListOfComplexValues : List<ComplexValue>
{
public void UpdateValues(double parameter)
{
for (int i = 0; i < this.Count; i++)
{
this[i].value = SomeFunction(this[i].value, parameter);
}
}
}
It turns out that the computations I need to make, take one or more of those lists and generate combinations of them to produce new, much larger ones but for which I only need to keep the (double) value.
Given the size of those new lists I had to simplify them to be just lists of double because of memory issue and speed.
So I have a new class which also implements many of the functions that were implemented for the other. The code is the same, except that in one case I use this[i] and in the other this[i].value.
public class ListOfValues : List<double>
{
public void UpdateValues(double parameter)
{
for (int i = 0; i < this.Count; i++)
{
this[i] = SomeFunction(this[i], parameter);
}
}
}
Is there a way I could easily merge those two implementations, for instance ensuring that the ComplexValue class behaves like a double?
Unfortunately, it's not very clear from your question what it is you actually want to do. That is, in what way do you want "that the ComplexValue class behaves like a double"? What would the code look like, in which the ComplexValue class is used but behaves like a double?
As you're aware I'm sure, you cannot inherit double, because it's a value type. So polymorphism is not an option here, at least not on the value type itself. But if you are willing to take a step back and reevaluate what led you to the List<T>-derived classes in the first place, perhaps there are options that would work for you.
In my experience, it's very unusual for inheriting a collection type to be the correct approach; only when the subclass is doing genuine collection-oriented things, and only collection-oriented things, would subclassing a collection type be appropriate. Otherwise, it usually makes more sense to put the non-collection-oriented operations into some other class that has a collection type (like the List<T> object).
In any case (i.e. whether you compose or inherit), you could use some kind of accessor delegates to handle the disparity between collections of List<double> and of List<ComplexValue>.
For example (for the sake of clarity, I'm keeping the inherit approach here, even though IMHO composing is likely to be better):
class ListOfValues<T> : List<T>
{
private readonly Func<List<T>, int, double> _getter;
private readonly Action<List<T>, int, double> _setter;
public ListOfTValues(
Func<List<T>, int, double> getter, Action<List<T>, int, double> setter)
{
_getter = getter;
_setter = setter;
}
public void UpdateValues(double parameter)
{
for (int i = 0; i < this.Count; i++)
{
_setter(this, i, SomeFunction(_getter(this, i), parameter));
}
}
}
Then you could create your list object like this:
ListOfValues<double> doubleList = new ListOfValues<double>(
(list, i) => list[i], (list, i, value) => list[i] = value);
ListOfValues<ComplexValue> doubleList = new ListOfValues<ComplexValue>(
(list, i) => list[i].value, (list, i, value) => list[i].value = value);
Another alternative would be to do something similar to the above — i.e. provide accessor methods to map the collection type to the actual computational code — but to provide them as virtual method overrides rather than delegates:
abstract class ListOfValues<T> : List<T>
{
protected abstract double GetItemValue(int i);
protected abstract void SetItemValue(int i, double value);
public void UpdateValues(double parameter)
{
for (int i = 0; i < this.Count; i++)
{
SetItemValue(, i, SomeFunction(GetItemValue(i), parameter));
}
}
}
class ListOfDouble : ListOfValues<double>
{
protected override double GetItemValue(int i)
{
return this[i];
}
protected override void SetItemValue(int i, double value)
{
this[i] = value;
}
}
class ListOfComplexValue : ListOfValues<ComplexValue>
{
protected override double GetItemValue(int i)
{
return this[i].value;
}
protected override void SetItemValue(int i, double value)
{
this[i].value = value;
}
}
Note that all of the above could be implemented without inheriting the List<T> type. You can put the computation logic into a class that simply contains an instance of List<T> instead. Finally, note that all of the above still works even if you follow the advice that Jon provided in his first comment to your question (advice that I think is good and worth following).
Code updated
For fixing the bug of a filtered Interminable, the following code is updated and merged into original:
public static bool IsInfinity(this IEnumerable x) {
var it=
x as Infinity??((Func<object>)(() => {
var info=x.GetType().GetField("source", bindingAttr);
return null!=info?info.GetValue(x):x;
}))();
return it is Infinity;
}
bindingAttr is declared a constant.
Summary
I'm trying to implement an infinite enumerable, but encountered something seem to be illogical, and temporarily run out of idea. I need some direction to complete the code, becoming a semantic, logical, and reasonable design.
The whole story
I've asked the question a few hours ago:
Is an infinite enumerable still "enumerable"?
This might not be a good pattern of implementation. What I'm trying to do, is implement an enumerable to present infinity, in a logical and semantic way(I thought ..). I would put the code at the last of this post.
The big problem is, it's just for presenting of infinite enumerable, but the enumeration on it in fact doesn't make any sense, since there are no real elements of it.
So, besides provide dummy elements for the enumeration, there are four options I can imagine, and three lead to the StackOverflowException.
Throw an InvalidOperationException once it's going to be enumerated.
public IEnumerator<T> GetEnumerator() {
for(var message="Attempted to enumerate an infinite enumerable"; ; )
throw new InvalidOperationException(message);
}
and 3. are technically equivalent, let the stack overflowing occurs when it's really overflowed.
public IEnumerator<T> GetEnumerator() {
foreach(var x in this)
yield return x;
}
public IEnumerator<T> GetEnumerator() {
return this.GetEnumerator();
}
(described in 2)
Don't wait for it happens, throw StackOverflowException directly.
public IEnumerator<T> GetEnumerator() {
throw new StackOverflowException("... ");
}
The tricky things are:
If option 1 is applied, that is, enumerate on this enumerable, becomes an invalid operation. Isn't it weird to say that this lamp isn't used to illuminate(though it's true in my case).
If option 2 or option 3 is applied, that is, we planned the stack overflowing. Is it really as the title, just when stackoverflow is fair and sensible? Perfectly logical and reasonable?
The last choice is option 4. However, the stack in fact does not really overflow, since we prevented it by throwing a fake StackOverflowException. This reminds me that when Tom Cruise plays John Anderton said that: "But it didn't fall. You caught it. The fact that you prevented it from happening doesnt change the fact that it was going to happen."
Some good ways to avoid the illogical problems?
The code is compile-able and testable, note that one of OPTION_1 to OPTION_4 shoule be defined before compile.
Simple test
var objects=new object[] { };
Debug.Print("{0}", objects.IsInfinity());
var infObjects=objects.AsInterminable();
Debug.Print("{0}", infObjects.IsInfinity());
Classes
using System.Collections.Generic;
using System.Collections;
using System;
public static partial class Interminable /* extensions */ {
public static Interminable<T> AsInterminable<T>(this IEnumerable<T> x) {
return Infinity.OfType<T>();
}
public static Infinity AsInterminable(this IEnumerable x) {
return Infinity.OfType<object>();
}
public static bool IsInfinity(this IEnumerable x) {
var it=
x as Infinity??((Func<object>)(() => {
var info=x.GetType().GetField("source", bindingAttr);
return null!=info?info.GetValue(x):x;
}))();
return it is Infinity;
}
const BindingFlags bindingAttr=
BindingFlags.Instance|BindingFlags.NonPublic;
}
public abstract partial class Interminable<T>: Infinity, IEnumerable<T> {
IEnumerator IEnumerable.GetEnumerator() {
return this.GetEnumerator();
}
#if OPTION_1
public IEnumerator<T> GetEnumerator() {
for(var message="Attempted to enumerate an infinite enumerable"; ; )
throw new InvalidOperationException(message);
}
#endif
#if OPTION_2
public IEnumerator<T> GetEnumerator() {
foreach(var x in this)
yield return x;
}
#endif
#if OPTION_3
public IEnumerator<T> GetEnumerator() {
return this.GetEnumerator();
}
#endif
#if OPTION_4
public IEnumerator<T> GetEnumerator() {
throw new StackOverflowException("... ");
}
#endif
public Infinity LongCount<U>(
Func<U, bool> predicate=default(Func<U, bool>)) {
return this;
}
public Infinity Count<U>(
Func<U, bool> predicate=default(Func<U, bool>)) {
return this;
}
public Infinity LongCount(
Func<T, bool> predicate=default(Func<T, bool>)) {
return this;
}
public Infinity Count(
Func<T, bool> predicate=default(Func<T, bool>)) {
return this;
}
}
public abstract partial class Infinity: IFormatProvider, ICustomFormatter {
partial class Instance<T>: Interminable<T> {
public static readonly Interminable<T> instance=new Instance<T>();
}
object IFormatProvider.GetFormat(Type formatType) {
return typeof(ICustomFormatter)!=formatType?null:this;
}
String ICustomFormatter.Format(
String format, object arg, IFormatProvider formatProvider) {
return "Infinity";
}
public override String ToString() {
return String.Format(this, "{0}", this);
}
public static Interminable<T> OfType<T>() {
return Instance<T>.instance;
}
}
public IEnumerator<T> GetEnumerator()
{
while (true)
yield return default(T);
}
This will create an infinite enumerator - a foreach on it will never end and will just continue to give out the default value.
Note that you will not be able to determine IsInfinity() the way you wrote in your code. That is because new Infinity().Where(o => o == /*do any kind of comparison*/) will still be infinite but will have a different type.
As mentioned in the other post you linked, an infinite enumeration makes perfectly sense for C# to enumerate and there are an huge amount of real-world examples where people write enumerators that just do never end(first thing that springs off my mind is a random number generator).
So you have a particular case in your mathematical problem, where you need to define a special value (infinite number of points of intersection). Usually, that is where I use simple static constants for. Just define some static constant IEnumerable and test against it to find out whether your algorithm had the "infinite number of intersection" as result.
To more specific answer your current question: DO NOT EVER EVER cause a real stack overflow. This is about the nastiest thing you can do to users of your code. It can not be caught and will immediately terminate your process(probably the only exception is when you are running inside an attached instrumenting debugger).
If at all, I would use NotSupportedException which is used in other places to signal that some class do not support a feature(E.g. ICollections may throw this in Remove() if they are read-only).
If I understand correctly -- infinite is a confusing word here. I think you need a monad which is either enumerable or not. But let's stick with infinite for now.
I cannot think of a nice way of implementing this in C#. All ways this could be implemented don't integrate with C# generators.
With C# generator, you can only emit valid values; so there's no way to indicate that this is an infinite enumerable. I don't like idea of throwing exceptions from generator to indicate that it is infinite; because to check that it is infinite, you will have to to try-catch every time.
If you don't need to support generators, then I see following options :
Implement sentinel enumerable:
public class InfiniteEnumerable<T>: IEnumerable<T> {
private static InfiniteEnumerable<T> val;
public static InfiniteEnumerable<T> Value {
get {
return val;
}
}
public IEnumerator<T> GetEnumerator() {
throw new InvalidOperationException(
"This enumerable cannot be enumerated");
}
IEnumerator IEnumerable.GetEnumerator() {
throw new InvalidOperationException(
"This enumerable cannot be enumerated");
}
}
Sample usage:
IEnumerable<int> enumerable=GetEnumerable();
if(enumerable==InfiniteEnumerable<int>.Value) {
// This is 'infinite' enumerable.
}
else {
// enumerate it here.
}
Implement Infinitable<T> wrapper:
public class Infinitable<T>: IEnumerable<T> {
private IEnumerable<T> enumerable;
private bool isInfinite;
public Infinitable(IEnumerable<T> enumerable) {
this.enumerable=enumerable;
this.isInfinite=false;
}
public Infinitable() {
this.isInfinite=true;
}
public bool IsInfinite {
get {
return isInfinite;
}
}
public IEnumerator<T> GetEnumerator() {
if(isInfinite) {
throw new InvalidOperationException(
"The enumerable cannot be enumerated");
}
return this.enumerable.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator() {
if(isInfinite) {
throw new InvalidOperationException(
"The enumerable cannot be enumerated");
}
return this.enumerable.GetEnumerator();
}
}
Sample usage:
Infinitable<int> enumerable=GetEnumerable();
if(enumerable.IsInfinite) {
// This is 'infinite' enumerable.
}
else {
// enumerate it here.
foreach(var i in enumerable) {
}
}
Infinite sequences may be perfectly iterable/enumerable. Natural numbers are enumerable and so are rational numbers or PI digits. Infinite is the opposite of finite, not enumerable.
The variants that you've provided don't represent the infinite sequences. There are infinitely many different infinite sequences and you can see that they're different by iterating through them. Your idea, on the other hand, is to have a singleton, which goes against that diversity.
If you have something that cannot be enumerated (like the set of real numbers), then you just shouldn't define it as IEnumerable as it's breaking the contract.
If you want to discern between finite and infinite enumerable sequences, just crate a new interface IInfiniteEnumerable : IEnumerable and mark infinite sequences with it.
Interface that marks infinite sequences
public interface IInfiniteEnumerable<T> : IEnumerable<T> {
}
A wrapper to convert an existing IEnumerable<T> to IInfiniteEnumerable<T> (IEnumerables are easily created with C#'s yield syntax, but we need to convert them to IInfiniteEnumerable )
public class InfiniteEnumerableWrapper<T> : IInfiniteEnumerable<T> {
IEnumerable<T> _enumerable;
public InfiniteEnumerableWrapper(IEnumerable<T> enumerable) {
_enumerable = enumerable;
}
public IEnumerator<T> GetEnumerator() {
return _enumerable.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator() {
return _enumerable.GetEnumerator();
}
}
Some infinity-aware routines (like calculating the sequence length)
//TryGetCount() returns null if the sequence is infinite
public static class EnumerableExtensions {
public static int? TryGetCount<T>(this IEnumerable<T> sequence) {
if (sequence is IInfiniteEnumerable<T>) {
return null;
} else {
return sequence.Count();
}
}
}
Two examples of sequences - a finite range sequence and the infinite Fibonacci sequence.
public class Sequences {
public static IEnumerable<int> GetIntegerRange(int start, int count) {
return Enumerable.Range(start, count);
}
public static IInfiniteEnumerable<int> GetFibonacciSequence() {
return new InfiniteEnumerableWrapper<int>(GetFibonacciSequenceInternal());
}
static IEnumerable<int> GetFibonacciSequenceInternal() {
var p = 0;
var q = 1;
while (true) {
yield return p;
var newQ = p + q;
p = q;
q = newQ;
}
}
}
A test app that generates random sequences and tries to calculate their lengths.
public class TestApp {
public static void Main() {
for (int i = 0; i < 20; i++) {
IEnumerable<int> sequence = GetRandomSequence();
Console.WriteLine(sequence.TryGetCount() ?? double.PositiveInfinity);
}
Console.ReadLine();
}
static Random _rng = new Random();
//Randomly generates an finite or infinite sequence
public static IEnumerable<int> GetRandomSequence() {
int random = _rng.Next(5) * 10;
if (random == 0) {
return Sequences.GetFibonacciSequence();
} else {
return Sequences.GetIntegerRange(0, random);
}
}
}
The program output something like this:
20
40
20
10
20
10
20
Infinity
40
30
40
Infinity
Infinity
40
40
30
20
30
40
30
Is there a significant complexity difference between these two implementation or does the compiler optimize it anyway?
Usage:
for(int i = 0; i < int.MaxValue; i++)
{
foreach(var item in GoodItems)
{
if(DoSomethingBad(item))
break; // this is later added.
}
}
Implementation (1):
public IEnumerable<T> GoodItems
{
get { return _list.Where(x => x.IsGood); }
}
Implementation (2):
public IEnumerable<T> GoodItems
{
get { foreach(var item in _list.Where(x => x.IsGood)) yield return item; }
}
It appears that IEnumerable methods should always be implemented using (2)? When is one better than the other?
I just built an example program and then used ILSpy to examine the output assembly. The second option will actually generate an extra class that wraps the call to Where but adds zero value to the code. The extra layer the code must follow will probably not cause performance issues in most programs but consider all the extra syntax just to perform the same thing at a slightly slower speed. Not worth it in my book.
where uses yield return internally. You don't need to wrap it in another yield return.
You do _list.where(x => x.IsGood); in both. With that said, isn't it obvious which has to be the better usage?
yield return has its usages, but this scenario, especially in a getter, is not the one
The extra code without payload in "implementation 2" is the less evil here.
Both variants lead to undesirable creation of new object each time you call the property getter. So, results of two sequential getter calls will not be equal:
interface IItem
{
bool IsGood { get; set; }
}
class ItemsContainer<T>
where T : IItem
{
private readonly List<T> items = new List<T>();
public IEnumerable<T> GoodItems
{
get { return items.Where(item => item.IsGood); }
}
// ...
}
// somewhere in code
class Item : IItem { /* ... */ }
var container = new ItemsContainer<Item>();
Console.WriteLine(container.GoodItems == container.GoodItems); // False; Oops!
You should avoid this side-effect:
class ItemsContainer<T>
where T : IItem
{
private readonly List<T> items;
private readonly Lazy<IEnumerable<T>> goodItems;
public ItemsContainer()
{
this.items = new List<T>();
this.goodItems = new Lazy<IEnumerable<T>>(() => items.Where(item => item.IsGood));
}
public IEnumerable<T> GoodItems
{
get { return goodItems.Value; }
}
// ...
}
or make a method instead of property:
public IEnumerable<T> GetGoodItems()
{
return _list.Where(x => x.IsGood);
}
Also, the property is not a good idea, if you want to provide snapshot of your items to the client code.
Internally, the first version gets compiled down to something that looks like this:
public IEnumerable<T> GoodItems
{
get
{
foreach (var item in _list)
if (item.IsGood)
yield return item;
}
}
Whereas the second one will now look something like:
public IEnumerable<T> GoodItems
{
get
{
foreach (var item in GoodItemsHelper)
yield return item;
}
}
private IEnumerable<T> GoodItemsHelper
{
get
{
foreach (var item in _list)
if (item.IsGood)
yield return item;
}
}
The Where clause in LINQ is implemented with deferred execution. So there's no need to apply the foreach (...) yield return ... pattern. You're making more work for yourself, and potentially for the runtime.
I don't know if the second version gets jitted to the same thing as the first. Semantically, the two are distinct in that the first does a single round of deferred execution while the second does two rounds. On those grounds I'd argue that the second would be more complex.
The real question you need to ask is: When you're exposing the IEnumerable, what guarantees are you making? Are you saying that you want to simply provide forward iteration? Or are you stating that your interface provides deferred execution?
In the code below, my intent for is to simply provide forward enumeration without random access:
private List<Int32> _Foo = new List<Int32>() { 1, 2, 3, 4, 5 };
public IEnumerable<Int32> Foo
{
get
{
return _Foo;
}
}
But here, I want to prevent unnecessary computation. I want my expensive computation to be performed only when a result is requested.
private List<Int32> _Foo = new List<Int32>() { 1, 2, 3, 4, 5 };
public IEnumerable<Int32> Foo
{
get
{
foreach (var item in _Foo)
{
var result = DoSomethingExpensive(item);
yield return result;
}
}
}
Even though both versions of Foo look identical on the outside, their internal implementation does different things. That's the part that you need to watch out for. When you use LINQ, you don't need to worry about deferring execution since most operators do it for you. In your own code, you may wish to go with the first or second depending on your needs.
Is there a common way to pass a single item of type T to a method which expects an IEnumerable<T> parameter? Language is C#, framework version 2.0.
Currently I am using a helper method (it's .Net 2.0, so I have a whole bunch of casting/projecting helper methods similar to LINQ), but this just seems silly:
public static class IEnumerableExt
{
// usage: IEnumerableExt.FromSingleItem(someObject);
public static IEnumerable<T> FromSingleItem<T>(T item)
{
yield return item;
}
}
Other way would of course be to create and populate a List<T> or an Array and pass it instead of IEnumerable<T>.
[Edit] As an extension method it might be named:
public static class IEnumerableExt
{
// usage: someObject.SingleItemAsEnumerable();
public static IEnumerable<T> SingleItemAsEnumerable<T>(this T item)
{
yield return item;
}
}
Am I missing something here?
[Edit2] We found someObject.Yield() (as #Peter suggested in the comments below) to be the best name for this extension method, mainly for brevity, so here it is along with the XML comment if anyone wants to grab it:
public static class IEnumerableExt
{
/// <summary>
/// Wraps this object instance into an IEnumerable<T>
/// consisting of a single item.
/// </summary>
/// <typeparam name="T"> Type of the object. </typeparam>
/// <param name="item"> The instance that will be wrapped. </param>
/// <returns> An IEnumerable<T> consisting of a single item. </returns>
public static IEnumerable<T> Yield<T>(this T item)
{
yield return item;
}
}
Well, if the method expects an IEnumerable you've got to pass something that is a list, even if it contains one element only.
passing
new[] { item }
as the argument should be enough I think
In C# 3.0 you can utilize the System.Linq.Enumerable class:
// using System.Linq
Enumerable.Repeat(item, 1);
This will create a new IEnumerable that only contains your item.
Your helper method is the cleanest way to do it, IMO. If you pass in a list or an array, then an unscrupulous piece of code could cast it and change the contents, leading to odd behaviour in some situations. You could use a read-only collection, but that's likely to involve even more wrapping. I think your solution is as neat as it gets.
In C# 3 (I know you said 2), you can write a generic extension method which might make the syntax a little more acceptable:
static class IEnumerableExtensions
{
public static IEnumerable<T> ToEnumerable<T>(this T item)
{
yield return item;
}
}
client code is then item.ToEnumerable().
This helper method works for item or many.
public static IEnumerable<T> ToEnumerable<T>(params T[] items)
{
return items;
}
I'm kind of surprised that no one suggested a new overload of the method with an argument of type T to simplify the client API.
public void DoSomething<T>(IEnumerable<T> list)
{
// Do Something
}
public void DoSomething<T>(T item)
{
DoSomething(new T[] { item });
}
Now your client code can just do this:
MyItem item = new MyItem();
Obj.DoSomething(item);
or with a list:
List<MyItem> itemList = new List<MyItem>();
Obj.DoSomething(itemList);
Either (as has previously been said)
MyMethodThatExpectsAnIEnumerable(new[] { myObject });
or
MyMethodThatExpectsAnIEnumerable(Enumerable.Repeat(myObject, 1));
As a side note, the last version can also be nice if you want an empty list of an anonymous object, e.g.
var x = MyMethodThatExpectsAnIEnumerable(Enumerable.Repeat(new { a = 0, b = "x" }, 0));
I agree with #EarthEngine's comments to the original post, which is that 'AsSingleton' is a better name. See this wikipedia entry. Then it follows from the definition of singleton that if a null value is passed as an argument that 'AsSingleton' should return an IEnumerable with a single null value instead of an empty IEnumerable which would settle the if (item == null) yield break; debate. I think the best solution is to have two methods: 'AsSingleton' and 'AsSingletonOrEmpty'; where, in the event that a null is passed as an argument, 'AsSingleton' will return a single null value and 'AsSingletonOrEmpty' will return an empty IEnumerable. Like this:
public static IEnumerable<T> AsSingletonOrEmpty<T>(this T source)
{
if (source == null)
{
yield break;
}
else
{
yield return source;
}
}
public static IEnumerable<T> AsSingleton<T>(this T source)
{
yield return source;
}
Then, these would, more or less, be analogous to the 'First' and 'FirstOrDefault' extension methods on IEnumerable which just feels right.
This is 30% faster than yield or Enumerable.Repeat when used in foreach due to this C# compiler optimization, and of the same performance in other cases.
public struct SingleSequence<T> : IEnumerable<T> {
public struct SingleEnumerator : IEnumerator<T> {
private readonly SingleSequence<T> _parent;
private bool _couldMove;
public SingleEnumerator(ref SingleSequence<T> parent) {
_parent = parent;
_couldMove = true;
}
public T Current => _parent._value;
object IEnumerator.Current => Current;
public void Dispose() { }
public bool MoveNext() {
if (!_couldMove) return false;
_couldMove = false;
return true;
}
public void Reset() {
_couldMove = true;
}
}
private readonly T _value;
public SingleSequence(T value) {
_value = value;
}
public IEnumerator<T> GetEnumerator() {
return new SingleEnumerator(ref this);
}
IEnumerator IEnumerable.GetEnumerator() {
return new SingleEnumerator(ref this);
}
}
in this test:
// Fastest among seqs, but still 30x times slower than direct sum
// 49 mops vs 37 mops for yield, or c.30% faster
[Test]
public void SingleSequenceStructForEach() {
var sw = new Stopwatch();
sw.Start();
long sum = 0;
for (var i = 0; i < 100000000; i++) {
foreach (var single in new SingleSequence<int>(i)) {
sum += single;
}
}
sw.Stop();
Console.WriteLine($"Elapsed {sw.ElapsedMilliseconds}");
Console.WriteLine($"Mops {100000.0 / sw.ElapsedMilliseconds * 1.0}");
}
As I have just found, and seen that user LukeH suggested too, a nice simple way of doing this is as follows:
public static void PerformAction(params YourType[] items)
{
// Forward call to IEnumerable overload
PerformAction(items.AsEnumerable());
}
public static void PerformAction(IEnumerable<YourType> items)
{
foreach (YourType item in items)
{
// Do stuff
}
}
This pattern will allow you to call the same functionality in a multitude of ways: a single item; multiple items (comma-separated); an array; a list; an enumeration, etc.
I'm not 100% sure on the efficiency of using the AsEnumerable method though, but it does work a treat.
Update: The AsEnumerable function looks pretty efficient! (reference)
Although it's overkill for one method, I believe some people may find the Interactive Extensions useful.
The Interactive Extensions (Ix) from Microsoft includes the following method.
public static IEnumerable<TResult> Return<TResult>(TResult value)
{
yield return value;
}
Which can be utilized like so:
var result = EnumerableEx.Return(0);
Ix adds new functionality not found in the original Linq extension methods, and is a direct result of creating the Reactive Extensions (Rx).
Think, Linq Extension Methods + Ix = Rx for IEnumerable.
You can find both Rx and Ix on CodePlex.
I recently asked the same thing on another post
Is there a way to call a C# method requiring an IEnumerable<T> with a single value? ...with benchmarking.
I wanted people stopping by here to see the brief benchmark comparison shown at that newer post for 4 of the approaches presented in these answers.
It seems that simply writing new[] { x } in the arguments to the method is the shortest and fastest solution.
This may not be any better but it's kind of cool:
Enumerable.Range(0, 1).Select(i => item);
Sometimes I do this, when I'm feeling impish:
"_".Select(_ => 3.14) // or whatever; any type is fine
This is the same thing with less shift key presses, heh:
from _ in "_" select 3.14
For a utility function I find this to be the least verbose, or at least more self-documenting than an array, although it'll let multiple values slide; as a plus it can be defined as a local function:
static IEnumerable<T> Enumerate (params T[] v) => v;
// usage:
IEnumerable<double> example = Enumerate(1.234);
Here are all of the other ways I was able to think of (runnable here):
using System;
using System.Collections.Generic;
using System.Linq;
public class Program {
public static IEnumerable<T> ToEnumerable1 <T> (T v) {
yield return v;
}
public static T[] ToEnumerable2 <T> (params T[] vs) => vs;
public static void Main () {
static IEnumerable<T> ToEnumerable3 <T> (params T[] v) => v;
p( new string[] { "three" } );
p( new List<string> { "three" } );
p( ToEnumerable1("three") ); // our utility function (yield return)
p( ToEnumerable2("three") ); // our utility function (params)
p( ToEnumerable3("three") ); // our local utility function (params)
p( Enumerable.Empty<string>().Append("three") );
p( Enumerable.Empty<string>().DefaultIfEmpty("three") );
p( Enumerable.Empty<string>().Prepend("three") );
p( Enumerable.Range(3, 1) ); // only for int
p( Enumerable.Range(0, 1).Select(_ => "three") );
p( Enumerable.Repeat("three", 1) );
p( "_".Select(_ => "three") ); // doesn't have to be "_"; just any one character
p( "_".Select(_ => 3.3333) );
p( from _ in "_" select 3.0f );
p( "a" ); // only for char
// these weren't available for me to test (might not even be valid):
// new Microsoft.Extensions.Primitives.StringValues("three")
}
static void p <T> (IEnumerable<T> e) =>
Console.WriteLine(string.Join(' ', e.Select((v, k) => $"[{k}]={v,-8}:{v.GetType()}").DefaultIfEmpty("<empty>")));
}
For those wondering about performance, while #mattica has provided some benchmarking information in a similar question referenced above, My benchmark tests, however, have provided a different result.
In .NET 7, yield return value is ~9% faster than new T[] { value } and allocates 75% the amount of memory. In most cases, this is already hyper-performant and is as good as you'll ever need.
I was curious if a custom single collection implementation would be faster or more lightweight. It turns out because yield return is implemented as IEnumerator<T> and IEnumerable<T>, the only way to beat it in terms of allocation is to do that in my implementation as well.
If you're passing IEnumerable<> to an outside library, I would strongly recommend not doing this unless you're very familiar with what you're building. That being said, I made a very simple (not-reuse-safe) implementation which was able to beat the yield method by 5ns and allocated only half as much as the array.
Because all tests were passed an IEnumerable<T>, value types generally performed worse than reference types. The best implementation I had was actually the simplest - you can look at the SingleCollection class in the gist I linked to. (This was 2ns faster than yield return, but allocated 88% of what the array would, compared to the 75% allocated for yield return.)
TL:DR; if you care about speed, use yield return item. If you really care about speed, use a SingleCollection.
The easiest way I'd say would be new T[]{item};; there's no syntax to do this. The closest equivalent that I can think of is the params keyword, but of course that requires you to have access to the method definition and is only usable with arrays.
Enumerable.Range(1,1).Select(_ => {
//Do some stuff... side effects...
return item;
});
The above code is useful when using like
var existingOrNewObject = MyData.Where(myCondition)
.Concat(Enumerable.Range(1,1).Select(_ => {
//Create my object...
return item;
})).Take(1).First();
In the above code snippet there is no empty/null check, and it is guaranteed to have only one object returned without afraid of exceptions. Furthermore, because it is lazy, the closure will not be executed until it is proved there is no existing data fits the criteria.
To be filed under "Not necessarily a good solution, but still...a solution" or "Stupid LINQ tricks", you could combine Enumerable.Empty<>() with Enumerable.Append<>()...
IEnumerable<string> singleElementEnumerable = Enumerable.Empty<string>().Append("Hello, World!");
...or Enumerable.Prepend<>()...
IEnumerable<string> singleElementEnumerable = Enumerable.Empty<string>().Prepend("Hello, World!");
The latter two methods are available since .NET Framework 4.7.1 and .NET Core 1.0.
This is a workable solution if one were really intent on using existing methods instead of writing their own, though I'm undecided if this is more or less clear than the Enumerable.Repeat<>() solution. This is definitely longer code (partly due to type parameter inference not being possible for Empty<>()) and creates twice as many enumerator objects, however.
Rounding out this "Did you know these methods exist?" answer, Array.Empty<>() could be substituted for Enumerable.Empty<>(), but it's hard to argue that makes the situation...better.
I'm a bit late to the party but I'll share my way anyway.
My problem was that I wanted to bind the ItemSource or a WPF TreeView to a single object. The hierarchy looks like this:
Project > Plot(s) > Room(s)
There was always going to be only one Project but I still wanted to Show the project in the Tree, without having to pass a Collection with only that one object in it like some suggested.
Since you can only pass IEnumerable objects as ItemSource I decided to make my class IEnumerable:
public class ProjectClass : IEnumerable<ProjectClass>
{
private readonly SingleItemEnumerator<AufmassProjekt> enumerator;
...
public IEnumerator<ProjectClass > GetEnumerator() => this.enumerator;
IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();
}
And create my own Enumerator accordingly:
public class SingleItemEnumerator : IEnumerator
{
private bool hasMovedOnce;
public SingleItemEnumerator(object current)
{
this.Current = current;
}
public bool MoveNext()
{
if (this.hasMovedOnce) return false;
this.hasMovedOnce = true;
return true;
}
public void Reset()
{ }
public object Current { get; }
}
public class SingleItemEnumerator<T> : IEnumerator<T>
{
private bool hasMovedOnce;
public SingleItemEnumerator(T current)
{
this.Current = current;
}
public void Dispose() => (this.Current as IDisposable).Dispose();
public bool MoveNext()
{
if (this.hasMovedOnce) return false;
this.hasMovedOnce = true;
return true;
}
public void Reset()
{ }
public T Current { get; }
object IEnumerator.Current => this.Current;
}
This is probably not the "cleanest" solution but it worked for me.
EDIT
To uphold the single responsibility principle as #Groo pointed out I created a new wrapper class:
public class SingleItemWrapper : IEnumerable
{
private readonly SingleItemEnumerator enumerator;
public SingleItemWrapper(object item)
{
this.enumerator = new SingleItemEnumerator(item);
}
public object Item => this.enumerator.Current;
public IEnumerator GetEnumerator() => this.enumerator;
}
public class SingleItemWrapper<T> : IEnumerable<T>
{
private readonly SingleItemEnumerator<T> enumerator;
public SingleItemWrapper(T item)
{
this.enumerator = new SingleItemEnumerator<T>(item);
}
public T Item => this.enumerator.Current;
public IEnumerator<T> GetEnumerator() => this.enumerator;
IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();
}
Which I used like this
TreeView.ItemSource = new SingleItemWrapper(itemToWrap);
EDIT 2
I corrected a mistake with the MoveNext() method.
I prefer
public static IEnumerable<T> Collect<T>(this T item, params T[] otherItems)
{
yield return item;
foreach (var otherItem in otherItems)
{
yield return otherItem;
}
}
This lets you call item.Collect() if you want the singleton, but it also lets you call item.Collect(item2, item3) if you want