This code doesn't work, but:
public virtual ICollection<SomeItem> items { get { return (ICollection<SomeItem>)items.Where(e => e.isVisible == true); } set { ;} }
I'd like to do something to that effect. So to get an ICollection filtered by a property of the collection's elements.
Sure, I could iterate through the elements, and get the right ones, put them in a new collection and return with that, but is there a nicer solution?
Perhaps what you're looking for is an Extension Method?
Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type.
public static class ExtensionMethods
{
public static ICollection<SomeItem> OnlyVisible(this ICollection<SomeItem) items) {
return items.Where(e => e.isVisible).ToList();
}
}
Note that Where returns an IEnumerable, which you cannot modify, so I call ToList() which essentially does everything in your last sentence.
You would then use it like this:
void Foo(ICollection<SomeItem> items) {
foreach (var i in items.OnlyVisible()) {
// Use i
}
}
Try:
items.Where(e => e.isVisible == true).ToList()
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.
When given an d you could be dealing with a fixed sequence like a list or array, an AST that will enumerate some external datasource, or even an AST on some existing collection. Is there a way to safely "materialize" the enumerable so that enumeration operations like foreach, count, etc. don't execute the AST each time?
I've often used .ToArray() to create this represenation but if the underlying storage is already a list or other fixed sequence, that seems like wasted copying. It would be nice if i could do
var enumerable = someEnumerable.Materialize();
if(enumberable.Any() {
foreach(var item in enumerable) {
...
}
} else {
...
}
Without having to worry that .Any() and foreach try to enumerate the sequence twice and without it unccessarily copying the enumerable.
Easy enough:
public static IList<TSource> Materialize<TSource>(this IEnumerable<TSource> source)
{
if (source is IList<TSource>)
{
// Already a list, use it as is
return (IList<TSource>)source;
}
else
{
// Not a list, materialize it to a list
return source.ToList();
}
}
Original answer:
Same as Thomas's answer, just a bit better according to me:
public static ICollection<T> Materialize<T>(this IEnumerable<T> source)
{
// Null check...
return source as ICollection<T> ?? source.ToList();
}
Please note that this tend to return the existing collection itself if its a valid collection type, or produces a new collection otherwise. While the two are subtly different, I don't think it could be an issue.
Edit:
Today this is a better solution:
public static IReadOnlyCollection<T> Materialize<T>(this IEnumerable<T> source)
{
// Null check...
switch (source)
{
case ICollection<T> collection:
return new ReadOnlyCollectionAdapter<T>(collection);
case IReadOnlyCollection<T> readOnlyCollection:
return readOnlyCollection;
default:
return source.ToList();
}
}
public class ReadOnlyCollectionAdapter<T> : IReadOnlyCollection<T>
{
readonly ICollection<T> m_source;
public ReadOnlyCollectionAdapter(ICollection<T> source) => m_source = source;
IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
public int Count => m_source.Count;
public IEnumerator<T> GetEnumerator() => m_source.GetEnumerator();
}
Check out this blog post I wrote a couple of years ago: http://www.fallingcanbedeadly.com/posts/crazy-extention-methods-tolazylist
In it, I define a method called ToLazyList that effectively does what you're looking for.
As written, it will eventually make a full copy of the input sequence, although you could tweak it so that instances of IList don't get wrapped in a LazyList, which would prevent this from happening (this action, however, would carry with it the assumption that any IList you get is already effectively memoized).
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
I have a class property exposing an internal IList<> through
System.Collections.ObjectModel.ReadOnlyCollection<>
How can I pass a part of this ReadOnlyCollection<> without copying elements into a new array (I need a live view, and the target device is short on memory)? I'm targetting Compact Framework 2.0.
Try a method that returns an enumeration using yield:
IEnumerable<T> FilterCollection<T>( ReadOnlyCollection<T> input ) {
foreach ( T item in input )
if ( /* criterion is met */ )
yield return item;
}
These foreach samples are fine, though you can make them much more terse if you're using .NET 3.5 and LINQ:
return FullList.Where(i => IsItemInPartialList(i)).ToList();
You can always write a class that implements IList and forwards all calls to the original list (so it doesn't have it's own copy of the data) after translating the indexes.
You could use yield return to create a filtered list
IEnumerable<object> FilteredList()
{
foreach( object item in FullList )
{
if( IsItemInPartialList( item )
yield return item;
}
}
Depending on how you need to filter the collection, you may want to create a class that implements IList (or IEnumerable, if that works for you) but that mucks about with the indexing and access to only return the values you want. For example
class EvenList: IList
{
private IList innerList;
public EvenList(IList innerList)
{
this.innerList = innerList;
}
public object this[int index]
{
get { return innerList[2*i]; }
set { innerList[2*i] = value; }
}
// and similarly for the other IList methods
}
How do the filtered elements need to be accessed? If it's through an Iterator then maybe you could write a custom iterator that skips the elements you don't want publicly visible?
If you need to provide a Collection then you might need to write your own Collection class, which just proxies to the underlying Collection, but prevents access to the elements you don't want publicly visible.
(Disclaimer: I'm not very familiar with C#, so these are general answers. There may be more specific answers to C# that work better)