Related
I am still learning the basics of C# and found a task where you have to implement the method below, it is supposed to return the same sequence of strings but uppercase, and I assume by sequence it means an array.
But I am also somehow supposed to use IEnumerable for this.
I thought IEnumerable was an interface, how is it a type and a parameter in that method I am supposed to add logic into?
I searched and found that return type IEnumerable means it has to return something that can implement IEnumerable, but the parameters still confuse me, how do I use them to return uppercase? Do I use a foreach ?
using System;
using System.Collections.Generic;
namespace EnumerableTask
{
public class EnumerableManipulation
{
/// <summary> Transforms all strings to upper case.</summary>
/// <param name="data">Source string sequence.</param>
public IEnumerable<string> GetUppercaseStrings(IEnumerable<string> data)
{
}
}
You can use Linq Select() which would return an IEnumerable. ie:
public IEnumerable<string> GetUppercaseStrings(IEnumerable<string> data)
{
return data.Select(d => d.ToUpper());
}
EDIT: You can check Linq documentation or my blog starting from the oldest post (Feb 2009) at:
FoxSharp
You can also use foreach and yield:
public IEnumerable<string> GetUppercaseStrings(IEnumerable<string> data)
{
foreach (var item in data)
yield return item?.ToUpper();
}
IEnumerable<T> is a generic interface. It represents a collection of objects of type T. In your case, data is a collection of string that can be enumerated.
Enumerate a IEnumerable
To enumerate through the 'data' and acccess the elements one after the others, there is several possibilities. Here are two examples.
GetEnumerator()
GetEnumerator() gives you a way to enumerate through the IEnumerable<T>. enumerator.Current allows you to access the current element and enumerator.MoveNext() gives you a way to move to the next element in the collection.
At first use call MoveNext() method to place the enumerator at the first element of the IEnumerable<T>. While there is still elements to enumerate, MoveNext() will return true. When the end is reached, MoveNext() returns false.
Here is an example of how it is typically used.
IEnumerator<string> enumerator = data.GetEnumerator();
while(enumerator.MoveNext())
{
string text = data.Current;
/* Do something */
}
enumerator.Dispose();
foreach
Using foreach takes care of all the complexity of the enumerator process. It is in fact recommended to use foreach instead of directly manipulating the enumerator (see Microsoft Docs, in Remarks).
foreach(string text in data)
{
/* Do something */
}
Return a IEnumerable
Because IEnumerable<T> is an interface, you cannot "construct" it as you would with a class or a struct. Hence to return a IEnumerable<T>, you have to instanciate an actual class or a struct that implements this interface. For example:
public IEnumerable<string> Foo()
{
List<string> list = new List<string>();
/* Do something on the list*/
return list;
}
or
public IEnumerable<string> Foo(int count)
{
string[] array = new string[count];
/* Do something on the array*/
return array;
}
You can do this because List<T> and Array implement the interface IEnumerable<string>.
I am trying to process some list with a functional approach in C#.
The idea is that I have a collection of Tuple<T,double> and I want to change the Item 2 of some element T.
The functional way to do so, as data is immutable, is to take the list, filter for all elements where the element is different from the one to change, and the append a new tuple with the new values.
My problem is that I do not know how to append the element at the end. I would like to do:
public List<Tuple<T,double>> Replace(List<Tuple<T,double>> collection, T term,double value)
{
return collection.Where(x=>!x.Item1.Equals(term)).Append(Tuple.Create(term,value));
}
But there is no Append method. Is there something else?
I believe you are looking for the Concat operator.
It joins two IEnumerable<T> together, so you can create one with a single item to join.
public List<Tuple<T,double>> Replace(List<Tuple<T,double>> collection, T term,double value)
{
var newItem = new List<Tuple<T,double>>();
newItem.Add(new Tuple<T,double>(term,value));
return collection.Where(x=>!x.Item1.Equals(term)).Concat(newItem).ToList();
}
It seems that .NET 4.7.1 adds Append LINQ operator, which is exactly what you want. Unlike Concat it takes a single value.
By the way, if you declare a generic method you should include type parameter(s) after its name:
public List<Tuple<T, double>> Replace<T>(List<Tuple<T, double>> collection, T term, double value)
{
return collection.Where(x => !x.Item1.Equals(term))
.Append(Tuple.Create(term, value))
.ToList();
}
LINQ is not for mutation.
Functional programming avoid mutation.
Thus:
public IEnumerable<Tuple<T,double>> Extend(IEnumerable<Tuple<T,double>> collection,
T term,double value)
{
foreach (var x in collection.Where(x=>!x.Item1.Equals(term)))
{
yield return x;
}
yield return Tuple.Create(term,value);
}
If you're willing to use an additional package, check out MoreLinq, available on Nuget. This provides a new overload to the Concat-Function:
public static IEnumerable<T> Concat<T>(this IEnumerable<T> head, T tail);
This function does exactly what was asked for, e.g. you could do
var myEnumerable = Enumerable.Range(10, 3); // Enumerable of values 10, 11, 12
var newEnumerable = myEnumerable.Concat(3); // Enumerable of values 10, 11, 12, 3
And, if you like LINQ, you will probably like a lot of the other new functions, too!
Additionally, as pointed out in a discussion on the MoreLinq Github-page, the function
public static IEnumerable<TSource> Append<TSource>(this IEnumerable<TSource> source, TSource element);
with a different name but the same functionality is available in .NET Core, so it might be possible that we will see it for C# in the future.
This should do what you want (although it uses mutation inside, it feels functional from a callers perspective):
public List<Tuple<T, double>> Replace(List<Tuple<T, double>> collection, T term, double value) {
var result = collection.Where(x => !x.Item1.Equals(term)).ToList();
result.Add(Tuple.Create(term, value));
return result;
}
A alternative way to do it is to use "map" (select in LINQ):
public List<Tuple<T, double>> Replace(List<Tuple<T, double>> collection, T term, double value) {
return collection.Select(x =>
Tuple.Create(
x.Item1,
x.Item1.Equals(term) ? value : x.Item2)).ToList();
}
But it might give you different results than your original intention. Although, to me, that's what I think when I see a method called Replace, which is, replace-in-place.
UPDATE
You can also create what you want like this:
public List<Tuple<T, double>> Replace(List<Tuple<T, double>> collection, T term, double value) {
return collection.
Where(x => !x.Item1.Equals(term)).
Append(Tuple.Create(term, value)).
ToList();
}
Using Concat, as mentioned by Oded:
public static class EnumerableEx {
public static IEnumerable<T> Append<T>(this IEnumerable<T> source, T item) {
return source.Concat(new T[] { item });
}
}
One way is to use .Concat(), but you need to have a enumerable rather than a single item as the second argument. To create an array with a single element does work, but is combersome to write.
It is better to write an custom extension method to do so.
One method is to create a new List<T> and add the items from the first list and then the items from the second list. However, it is better to use the yield-keyword instead, so you do not need to create an list and the enumerable will be evaluated in a lazy fashion:
public static class EnumerableExtensions
{
public static IEnumerable<T> Concat<T>(this IEnumerable<T> list, T item)
{
foreach (var element in list)
{
yield return element;
}
yield return item;
}
}
The closest answer I could find came from this post and is:
return collection.Where(x=>!x.Item1.Equals(term)).Concat(new[]{Tuple.Create(term,value)});
I have a class which has two HashSet<String> collections as private members. Other classes in my code would like to be able to iterate over those HashSets and read their contents. I don't want to write a standard getter because another class could still do something like myClass.getHashSet().Clear(); Is there any other way to expose the elements of my HashSets to iteration without exposing the reference to the HashSet itself? I'd love to be able to do this in a way that is compatible with for-each loops.
Assuming you're using .NET 3.5, one alternative to writing the yield code yourself is to call a LINQ method. For example:
public IEnumerable<string> HashSet
{
get { return privateMember.Select(x => x); }
}
or
public IEnumerable<string> HashSet
{
get { return privateMember.Skip(0); }
}
There are various LINQ operators which could be used like this - using Skip(0) is probably the most efficient, as after the initial "skip 0 values" loop, it's probably just the foreach/yield return loop shown in the other answers. The Select version will call the no-op projection delegate for each item yielded. The chances of this difference being significant are astronomically small, however - I suggest you go with whatever makes the code clearest to you.
Expose a IEnumerable<T> property:
public IEnumerable<whatevertype> MyHashSet {
get {
return this.myHashSet;
}
}
Of course, the user of this code can cast that IEnumerable<T> to a HashSet<T> and edit elements, so to be on the safe side (while hurting performance), you can do:
public IEnumerable<whatevertype> MyHashSet {
get {
return this.myHashSet.ToArray();
}
}
or:
public IEnumerable<whatevertype> MyHashSet {
get {
foreach(var item in this.myHashSet) {
yield return item;
}
}
}
A more performant method of protection, but less convenient to the caller, is to return an IEnumerator<T>:
public IEnumerator<whatevertype> GetMyHashSetEnumerator() {
return this.myHashSet.GetEnumerator();
}
Add a method/property like this to avoid exposing the actual container:
public IEnumerable EnumerateFirst()
{
foreach( var item in hashSet )
yield return item;
}
You can also use the Select method to create a wrapper than can't be cast back to HashSet<T>:
public IEnumerable<int> Values
{
get { return _values.Select(value => value);
}
This avoids iterating over _values twice, as you would with .ToArray(), but keeps the implementation to a single clean line.
You may also provide a sequence like this:
public IEnumerable<string> GetHashSetOneValues()
{
foreach (string value in hashSetOne)
yield return value;
}
This method may then be called within a foreach loop:
foreach (string value in myObject.GetHashSetOneValues())
DoSomething(value);
This might be quite a bit too late to the party but the easiest way today would be to use Linq. Instead of writing
public IEnumerable<string> GetValues()
{
foreach(var elem in list)
yield return elem;
}
you can write
public IEnumerable<string> GetValues() => list;
Make your getter expose the HashSet as IEnumerable.
private HashSet<string> _mine;
public IEnumerable<string> Yours
{
get { return _mine; }
}
If the generic type is mutable, then that can still be modified, but no items can be added or removed from your HashSet.
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
A coworker asked me today how to add a range to a collection. He has a class that inherits from Collection<T>. There's a get-only property of that type that already contains some items. He wants to add the items in another collection to the property collection. How can he do so in a C#3-friendly fashion? (Note the constraint about the get-only property, which prevents solutions like doing Union and reassigning.)
Sure, a foreach with Property. Add will work. But a List<T>-style AddRange would be far more elegant.
It's easy enough to write an extension method:
public static class CollectionHelpers
{
public static void AddRange<T>(this ICollection<T> destination,
IEnumerable<T> source)
{
foreach (T item in source)
{
destination.Add(item);
}
}
}
But I have the feeling I'm reinventing the wheel. I didn't find anything similar in System.Linq or morelinq.
Bad design? Just Call Add? Missing the obvious?
No, this seems perfectly reasonable. There is a List<T>.AddRange() method that basically does just this, but requires your collection to be a concrete List<T>.
Try casting to List in the extension method before running the loop. That way you can take advantage of the performance of List.AddRange.
public static void AddRange<T>(this ICollection<T> destination,
IEnumerable<T> source)
{
List<T> list = destination as List<T>;
if (list != null)
{
list.AddRange(source);
}
else
{
foreach (T item in source)
{
destination.Add(item);
}
}
}
Since .NET4.5 if you want one-liner you can use System.Collections.Generic ForEach.
source.ForEach(o => destination.Add(o));
or even shorter as
source.ForEach(destination.Add);
Performance-wise it's the same as for each loop (syntactic sugar).
Also don't try assigning it like
var x = source.ForEach(destination.Add)
cause ForEach is void.
Edit: Copied from comments, Lippert's opinion on ForEach.
Remember that each Add will check the capacity of the collection and resize it whenever necessary (slower). With AddRange, the collection will be set the capacity and then added the items (faster). This extension method will be extremely slow, but will work.
Here is a bit more advanced/production-ready version:
public static class CollectionExtensions
{
public static TCol AddRange<TCol, TItem>(this TCol destination, IEnumerable<TItem> source)
where TCol : ICollection<TItem>
{
if(destination == null) throw new ArgumentNullException(nameof(destination));
if(source == null) throw new ArgumentNullException(nameof(source));
// don't cast to IList to prevent recursion
if (destination is List<TItem> list)
{
list.AddRange(source);
return destination;
}
foreach (var item in source)
{
destination.Add(item);
}
return destination;
}
}
The C5 Generic Collections Library classes all support the AddRange method. C5 has a much more robust interface that actually exposes all of the features of its underlying implementations and is interface-compatible with the System.Collections.Generic ICollection and IList interfaces, meaning that C5's collections can be easily substituted as the underlying implementation.
You could add your IEnumerable range to a list then set the ICollection = to the list.
IEnumerable<T> source;
List<item> list = new List<item>();
list.AddRange(source);
ICollection<item> destination = list;
Or you can just make an ICollection extension like this:
public static ICollection<T> AddRange<T>(this ICollection<T> #this, IEnumerable<T> items)
{
foreach(var item in items)
{
#this.Add(item);
}
return #this;
}
Using it would be just like using it on a list:
collectionA.AddRange(IEnumerable<object> items);
Agree with some guys above and Lipert's opinion.
In my case, it's quite often to do like this:
ICollection<int> A;
var B = new List<int> {1,2,3,4,5};
B.ForEach(A.Add);
To have an extension method for such operation a bit redundant in my view.